EMBRYOLOGY

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Embryology deals with the formative history of animals from the time of fertilization of the female germ cell or ovum to the adult condition. The phenomena of individual development are comprehended under the term ontogeny while those of racial development under the term phylogeny. The ontogeny of an organism exhibits stages similar to those of the evolutionary series as expressed by successively higher forms in racial history. This fact is at the basis of the biogenetic law or theory, which states that, “ ontogeny recapitulates phylogeny.”

To the student of medicine, the study of embryology is of great practical value as it supplies a comprehensive and rational explanation of the many facts of anatomy which are otherwise meaningless or anomalous. The study of embryology is necessary to interpret rudimentary structures, variations, anomalies and monstrosities and also it helps to understand the origin of certain tumors and other pathological changes in tissues. From its theoretical study embryology is the key which helps to unlock the secrets of heredity; determination of sex and organic evolution. A general conception of how man and other animals develop from a single cell by orderly and logical processes should share in the cultural background of every educated mind.

A multi cellular embryo begins as a fertilized egg or zygote. Further development depends on (i) cell proliferation (ii) growth (iii) cell movements which lead to the production of body form or morphogenesis (iv) cell specialization through changes in shape, form structure, and function to produce tissues that is Histogenesis (v) tissue combination to form organs-organogenesis and (vi) functional adaptation and correlation to produce working organism- integration.

OOGENESIS

During foetal period, ova arise by proliferation of the germinal epithelium which encloses the ovary. These cells sink into the ovary and continue to multiply as oogonia. In the next stage, late in the foetal life, these oogonia come to be enclosed birth; the formation of oogonia comes to be a halt due to the tunica albuginea forming a complete and perfect capsule to the ovary. The germinal epithelium is thus cut off from the ovary. Until puberty there is no advance in development of the primary Grafiaan follicles. Thereafter the follicles begin to develop in size.

A primary Grafian follicle of an oogonium has single layer of follicular cells. Through growth the oogonium increases in size and at the end of the period, it forms a primary oocyte. Concurrent with the enlargement of the oogonium, the follicular cells proliferate and form a stratified epithelium. When the oogonium is full grown, the follicular cells have arranged themselves into several layers of cells. Irregular spaces appear between the layers of cells and these coalesce to form a cleft. This cavity- the antrum folliculi grows on gradually increasing and is filled up with a fluid secreted by follicular cell- the liquor folliculi. As the fluid accumulates the oogonium has a more eccentric position within the follicle. It is finally anchored to a mound of follicular cells- the membrana granulose by the liquor folliculi. During this development the follicles move at first towards the medulla of the ovary, but in the final stages of the growth, the follicles again move towards the cortex and the surface of the ovary forming elevations.

Maturation: This process of meiosis consists of two divisions between which the nucleus of the cell is not reconstituted as in ordinary mitosis. One of the divisions is still further peculiar so that chromosomes of each pair pass unsplit into each daughter cell. Thus the primary oocyte first divides into two unequal cells- a large secondary oocyte and the first polar body. Each of these now containing only half the numbers of chromosomes. The first polar body divides into two smaller cells.

As a result of this reduction division or meiosis four cells are formed each with a reduced number of chromosomes, that is a single set of chromosomes replacing the duplicate set of the oogonium and primary oocyte. Another peculiarity is that the division of cytoplasm is very u8nequal so that the end products are one large ripe ovum and three rudimentary ova known as polar bodies. The ovum loses the centrosome at the end of second division. The ovum is nearly spherical in form, has a thick cell wall- (the zona pellucida), nucleus- (the germinal spot). It has no centrosome. The ovum is the largest in size of animals cells.

SPERMATOGENESIS

The germ cells of the male develop within the testis cords of the embryo, which grow from the germinal epithelium that covers the sex gland. These epithelial strands are not hollowed out till puberty. Two types of cell are recognizable in such a thick wall of the tube. (1) The Sustentacular cells which act as supporting cells; and (2) the male germ cells in various stages of development arranged in layers. All the germ cells are decendants of the primitive stem cells which, after numerous divisions become spermatogonia. Some of the spermatogonia remain as stem cells and others enter a period of growth at the end of which they are termed the primary spermatocytes, containing the full number of chromosomes typical for the species. Next follow two cell divisions that accomplish maturations. Each primary spermatocyte divides into two secondary spermatocytes and each of these divides into two spermatids. During these divisions the size of the cells greatly diminishes and the number of chromosomes is reduced to one half and the spermatids become attached to the Sertoli cells. They become gradually transformed into spermatozoa by losing most of their cytoplasm and developing the tail.

Significance of maturation division meiosis:

It prevents accumulation of chromatin and a numerical duplication of chromosomes from generation to generation and it affords a mechanism for the selection and recombination of the genes of unique character and thereby ensures variations. It also provides for sex determination of the individuals.

COMPARISON OF AN OVUM AND A SPERMATOZOON

Ovum Sperm

Large (as it contains a large amount of nutriment) Small

Spheroidal Elongated

Inactive Flagellate and mobile

Has egg envelop None

Has XX chromosomes Has XY chromosomes

No centrosome Has centrosome

Ovulation:- The discharge of the ovum from the follicle comprises ovulation. Commonly there is seasonal period during which ovulation occurs. When the follicle is ripe, it makes a protuberance in the ovarian wall. By rupture of the wall of the Grafian follicle and the capsule of the ovary, the liquor folliculi is liberated carrying with it the ovum and the discus proligerous. The ovum passes into the fimbriated end of the Fallopian tube with sets up a current of fluid by the ciliary movement for attraction of the ovum. The further descent of the ovum is due to muscular action of the uterine tube.

Time of ovulation:

Cow: 14 hour after end of “oestrus period”

Heifer: 11 hours before ”

Mare: 24 hours before ”

Ewe: 18 to 40 hours after onset of “oestrus period”

Sow: Between 36 to 48 hours after beginning of “oestrus period”

Bitch: 1 to 3 days after acceptance of the male during “oestrus period” of the cycle.

After ovulation, the Grafian follicle is transformed into a new structure, the corpus luteum. Its luteal cells are derived from the cells of the stratum granulosum and they function as a ductless gland. Its subsequent history depends on weather fertilization has taken place or not. If fertilization has not taken place, this body is termed corpus luteum of menstruation. It reaches its full size in 10 days and afterwards it degenerates and is transformed into a corpus albicans. When however fertilization has taken place, the corpus luteum continues to grow and is termed the corpus luteum of pregnancy. It functions as an endocrine origin organ by preventing further ovulation during pregnancy and by assisting successful implantation of the embryo in the uterine mucous membrane and in inducing growth of mammary glands. In animals it is very essential for successful gestation and is supposed to degenerate during the latter half of the gestation period.

Coitus and semination: The purpose of coitus is to introduce the semen into the vagina. The deposition of semen is called semination. The volume of semen ejected at a time by a bull is about 8c.c. containing about 9600 million sperms. The forward movement of the human sperm is about 1.5 to 3 m.m. a minute and is aided by the muscular contractions of the cervix uteri. The bovine sperm takes about 6 to 9 hours to reach the infundibulum of the uterine tube. The site of the fertilization in farm and most other mammals is the lower portion of the ampulla of the uterine tube.

FERTILIZATION:-

The head of the sperm penetrates the ovum and the resultant fusion of the two cells constitutes fertilization. After penetration, the head rotates and proceeds towards the centre of the ovum. The two pronuclei of the ovum and sperm approach, lose their nuclear membranes and the chromatin is split up into chromosomes. The centrosome of the sperm now divides into two and appears between the two chromosome groups. Cell division is thus initiated. The original number of chromosomes characteristic of the species is thus restored in fertilization.

CLEAVAGE AND ORIGIN OF GERM LAYERS IN AMPHIOXUS

The fertilized ovum enters on a cell division. The initial period i.e. the development of a new multicellular individual is called cleavage period. The process of cellular division without growth is called cleavage. During this period the zygote is split up into a number of smaller cells called the blastomeres. This cell division goes an until the cells are of normal size. By the 16 to 32 cell, stage the cells are crowed together into a compact mass within the Zona pellucida. The embryo is now known as morula. Soon the blastomeres arrange themselves in a circular manner forming a blastula around a central space the blastocoele. This marks the end of the clveage period.

Gastrulation is the process by which the three germ layers come to occupy their characteristic position in the embryo.

Gastrulation is brought about by the indentation of the lower wall of the blastula and deepening of this indentation till the upper and lower walls meet and obliterate the blastocoele. The two layers thus formed are the ectoderm and endoderm. The cavity of the gastrula is the archenteron or primitive gut and the entrance into the cavity is the blastopore bounded by the two lips.

The gastrula becomes elongated and tubular by cellular proliferation from the lips of the blastopore where the two layers meet. The dorsal lip on the blastopore is very active and new cells are added to both the layers but it is more in the roof and it is from this the third germ layer the mesoderm and also the notochord arise. As a result, the early endoderm is pushed forwards by the cells arising from the dorsal lip of the blastopore. The newly formed cells from the roof of the archenteron and is in the form cells above the gut from the mesoderm or middle germ layer. Of the three folds, the central one forms the notochord and the other two form the lateral sheets of mesoderm.

CLEAVAGE AND ORIGIN OF GERM LAYERS IN THE CHICK.

The ovum of the chick contains a large quantities of yolk which is at the bottom. Over it is the nucleus surrounded by a thin layer of cytoplasm. In this ovum cleavage is not complete as the yolk never divides. It is confined only to dorsal part of the ovum involving the cytoplasm and nucleus. As a result, a layer of cells from a cap over the yolk with a little space between it and the yolk. This layer of cells is the germinal disc or blastoderm and the space is the blastocoele. This corresponds to the blastula of amphioxus.

The grastula is formed by a process of involution of the blastoderm. The growing caudal part of the blastoderm rolls in tucks itself under and the cells thus turned beneath, continue to divide and spread until they constitute a new inner layer the endoderm the outer layer forming the ectoderm. The region where the inturning as occurred and where the ecto and endoderm meet represents the dorsal lip of the blastopore of the amphioxus. The space between the endoderm and yolk represents archenteron. This stage corresponds to the gastrula of the amphioxus. At the close of gastrulation, an opaque median band appears caudally on the surface of blastoderm. This is at first merely a short linear thickening of ectoderm but it rapidly elongates and acquires a knob at its anterior end called the primitive knot. The thickening is the primitive streak. A shallow groove, the primitive groove, presently courses length-ways along the middle of the streak and ends close the primitive knot as the primitive pit.

It has been said that the endoderm arises by a rolling under of the ectoderm on itself at the caudal end of the blastoderm. As the blastoderm continues to expand, it is belived that a middle point on this margin (dorsal lip) remains relatively fixed where the more rapidly growing edges of the margin on each side are of necessity carried caudally. Thus a crecentic margin is transformed into two parallel lips, bordering a longitudinal slit. The lips of the slit fuse even as they are developed, thereby giving rise to a primitive streak. The adjacent cells on the surface of the blastoderm move medially, enter the thickening primitive streak and leave it as mesodermal cells. Due to the migration of cells from the primitive streak on both sides a middle pouch like invagination occurs on the median line of the streak giving rise to the primitive groove bounded by the primitive folds. From the primitive knot a column of cells proceeds forwards to form the head process which becomes the notochord.

CLEAVAGE AND ORIGIN OF GERM LAYERS IN MAMMALS

In mammals, the cleavage is complete. Fluid begins to collect in the intercellular spaces of morula and the blastula is a hollow spherical vesicle lined externally by the blastomeres forming the trophoblast and an inner cell mass attached to the trophoblast at one end. The inner cell mass delaminates into an inner layer of ectoderm. The endoderm forms the roof of the yolk sac and the two layers together are spoken of as the embryonic disc. The primitive streak appears caudally on the upper face of the disc and proliferation of the cells in this area produces the cells of mesoderm which leave it and spread out in all directions between the two germ layers. This forms the middle germ layer or mesoderm and this differentiates into the axial cord- notochord and two lateral sheets of mesoderm which splits tangentially into two layers- somatic and splanchnic layers- to enclose the coelom or body cavity. The ectoderm and the somatic mesoderm together are spoken of as the somatopleurae or body wall and the3 endoderm and splanchnic mesoderm splanchnoplurae or gut wall.

FOETAL MEMBRANES IN CHICK AND MAMMALS

In the development of a higher vertebrates embryo like those of chick and mammals, only part of the cleavage cell mass forms the actual embryo, whereas other parts lie outside the embryonic territory and are called extraembryonic. The extraembryonic parts form foetal membranes- auxillary organs to protect the embryo and provide for its nutrition and excretion. All thesew membranes eventually are discarded. The foetal membranes include yolk sac, amnion, chorion and allantois. The placenta is a distinctive “membrane” that is developed only in higher mammals (plancentalia) chiefly from the chorion, allantois and the uterine lining. The umbilical cord connects the foetus and its placenta.

The blastoderm, originally a small disc soon spread by peripheral growth and covers the entire egg surface. But only the most central part of it is directly concernedin the formation of fissures all round. The extra embryonic blastoderm furnishes the foetal membranes. The blastoderm consists of somatopleurae and splanchnopleurae separated by a coelom.

Yolk sac: As the embryo enlarges, its circular connection with the extra embryonic blastoderm grows at a slower rate. This produces an apparent construction of the splanchnopleurae that unites the rapidly elongating gut with the yolk sac. The region of construction soon lengthens into tubular yolk stalk and the yolk overlaid an enveloped by the extraembryonic splanchnopleurae constitutes the yolk sac. The vitelline blood vessels arising in the splanchnic mesodermal wall ramify on the surface of the yolk sac, and through them yolk substance is absorbed and conveyed to the embryo during the incubation period. Shortly before hatching, the shrunken yolk sac slips through the navel into the body cavity. The yolk sac is a very important structure in the chick as it supplies the yolk necessary for the development of the chick till it hatches. In mammals, it is vestigial but is important as the earliest organ in which angioblasts first make their appearance.

Amnion and chorion: These two foetal membranes are synchronous in development. They are concentric layers which arise by foldings of the extra embryonic somatopleure. The earliest fold to appear is the head fold located in the front of the head of the embryo. The second is the tail fold just behind the embryo. As these crecentic folds arch higher and higher are draw over the embryo like hoods, their lateral extensions also unite in lateral folds which flank the embryo on the sides gradually to complete a circular fold which closes in, from all sides as a purse is closede by drawing its strings.The final stage is brought about by fusion of similar parts of the somatopleurae-ectoderm with ectoderm and mesoderm with mesoderm, the result being production of two completely separate compound membranes- an inner amnion of ectoderm internally and mesoderm externally and an outer chorion of ectoderm externally and mesoderm internally.The amniotic sacs contain fluid within which the embryo is suspended. The embryo is thus protected from drying and is able to maintain its shape free from distortion and muscle fibres developing in the mesodermal layer produce rhythmic contractions which agitate the embryo gently and prevent adhesions. The chorion lies next to the shell membrane enclosing the other layers and separated from them by the extra-embryonic coelom.

Allantois:- This is a temporary sac for storage of urine of the foetus. It arises as an outgrowth of the ventral part of the hind gut and its wall is made up of an external layer of splanchnic mesoderm and internal layer of endoderm. It soon grows into the extra embryonic coelom and connected to the hind gut by the allantoic stalk. It completely fills up the extra embryonic coelom and lines the entire shell. Fusion of the outer wall of the sac with the overlying chorion produces a common membrane in contact with the porous shell. Blood vessels ramify in this common mesodermal layer-allanto chorion constituting the respiratory organ for exchange of gases.

PLACENTATION IN HIGHER MAMMALS

Placentation includes the events of implantation and development of placenta.

Implantation: - At the 8 to 16 cell stage, the embryo has reached the uterus, in about 2 to 5 ½ days in most of the species of mammals. However, the pig embryo enters the uterus relatively in the 4 celled stages, while in the bitch, trhe time taken is 6 to 8 days. The blastocyst enlarges and fills up the uterine lumen. Zona pellucida is shed and the free living embryo is nourished by uterine secretion. The embryo is said to be implanted when it is fixed in position and does not float freely in the uterine lumen. In farm mammals the attachment to the uterine wall of the blastocyst in loose prior to formation of placenta. Implantation takes place about 11 to 40 days in cow and 10 to 22 days in sheep, after coitus.

PLACENTA

Placenta is the intimate apposition or fusion of extra embryonic membranes to the maternal tissue for the physiological processes of respiration, nutrition and excretion. In addition placenta protects the embryo.

(Extra embryonic) foetal membrane:-

Yolk Sac: - The endoderm lines the inner border of the blastocyst, converting the blastocoele into the yolk sac cavity.

Amnion and Chorion:- The newly formed chorionic sac in the early stages encloses a large cavity- the extra embryonic coelom (exo- coelom) which lies between chorion and amnion. In later stages the amniotic membrane fuses with chorion in the cow, ewe and sow to form and amniochorionic membrane.

Allantois:- The allantois grows into the exocoelom and surrounds the amnion. During this period the yolk sac regresses. The allantois which is highly vascularised by foetal vessels, fuses with the chorion bringing foetal vessels to the periphery of chorionic sac and thus into closer apposition with the maternal tissue. This is the formation of the chorio allantoic membrane.

The compound allantochorionic membrane together with the uterine tissue is designated as chorioallantoic placenta and is classified on the basis of its form and also the number of layers of tissue separating foetal and maternal vascular system, viz degree of placental union.

The chorion which is beset with vascular villi that are in intimate as ociation with the endometrium, constitutes the placenta which subserves the functions of nutrition, respiration, excretion and protection of the embryo.

The placenta is of two varieties- deciduate in which the foetal part of the placenta is very closely associated with the maternal part and hence part of endometrium also is shed off after parturition, e.g., man and primates; (2) nondeciduates in which the association between the part is not so clopse and hence the maternal endometrium is not shed, e.g., all domestic mammals.

The non deciduate placenta is of three types:-

(1) Cotyledonary:- the chorionic villi are grouped in well marked prominent projection called cotyledons which are separated by stretches of smooth chorion and these foetal cotyledons are received into the maternal caruncles e.g., ruminants.

(2) Diffuse: - The chorionic villi are diffusely scattered all over the chorion e.g. .mare and sow.

(3) Zonary: - The chorio0nic villi occupy a girdle-like band about the middle of the chorionic sac and these are received into crypts of endometrium around a circular zone which is shed off at parturition. Hence it is partly deciduate, e. g., carnivores.

DEGREES OF PLACENTAL UN ION:

All mammals, above the egg laying monotremes have placenta, but the degree of development and the intimacy of placental union vary. The ascending series with the names of the different types of placental structures follows in a brief form.

STAGES IN PRENATAL DEVELOPMENT

 TWINS AND TWINING

Twins are either identical twins or flaternal twins. In uniparous animals usually only one ovum is fertilized at a time. If two individuals are derived from a single fertilized egg, each member acquires the same chromosomal heritage and hence the similarly between the twins is so striking, physical, mental and other characters. They are identical true or duplicate twins. They are always of the same sex.

Two or more individuals, derived due to independent ripening of an equal number of more of ova and fertilization, have the same degree of family resemblance, as between brothers and sisters of different ages. They can be of the same or opposite sexes. They are flaternal twins.

In animals, e.g. the cow there is identical twinning. Also in bovines, a condition called freemartin is recorded, which is the female fraternal twin of a male. It is sterile, as the internal genital organs are of the male type and the external genital organs are of the female type. This is caused by placental anastomosis and the influence of the male hormone which is earlier to appear in circulation.

TERATOLOGY

Teratology is the study of abnormalities. The causal factors responsible for faultydevelopment are of two kinds- intrinsic and extrinsic. Under the former, heredity is one in which the anomalies are predetermined in the parental germplasms subsequently transferred to the offspring. Under the extrinsic factors, trauma, external violence, physical factors as Roengten rays, chemical factors at decrease of oxygen etc., affect the embryo. The fundamental point of the whole problem of monstrosities has thus become reduced to developmental inhibition or arrest.

ORGANOGENESIS

 The germ layers formed as a result of gastrulation are the source material for development of all organ rudiments of the embryo. The germ layers become progressively subdivided into groups of cells which are called primary organ rudiments or primordial (primordium singular). The functional part of an organ arises from primordium of one germ layer only (in some cases two germ layers) while the other two germ layers contribute to other structures like, covering epithelium, or capsule, muscle and nerve. So when we say an organ is derived from say endoderm we mean the development of its functional part only, but we have to remember that the other germ layers also contribute to make it an intergrated organ for carrying out its physiological processes.

DEVELOPMENT OF HEAD AND NECK

The cephalic end of an embryo is composed of two proteins almost from the start. One is neural in nature and includes brain, eyes and internal ears and the other is the facial or visceral part containing the upper ends of the alimentary and respiratory tracts. In young embryos, the neural portion is much larger.

The construction of the face and neck is closely bound up with the history of the brachial arches. These are bar-like ridges separated by grooves which appear on the lateral surface of the embryonic head during the fourth week. These correspond to the gill arches of fishes and in amniotic mammals they never assume a respiratory function but become transformed into various permanent structures of the adult. Usually they develop five pairs of brachial arches separated by four external ectodermal grooves- brachial grooves. Subjacent to these grooves the endoderm of the pharynx pushes aside the mesenchyme and buldes outwards to constitutes the pharyngeal pouches. The ectoderm and endoderm of each groove and pouch meet and unite to form thin plates. These rupture sometimes to make temporary opening reminiscent of the gill slit condition of fishes. Failure of these opening to close constitutes brachial fistulae.

The first brachial arch on each side bifurcates into an upper maxillary and a lower mandibular process, for the development of the face. The second and third arches contribute to the formation of the neck.

DEVELOPMENT OF THE FACE

The face is developed by the fronto-nasal process of the neuro- cranium and the first pair of brachial arches. At the early stage, the expansive fronto-nasal process represents much of the front of head. A thickening of ectoderm on each side of this process forms the olfactory placode. The first brachial arch divides into the maxillary and mandibular processes. The two mandibular processes unite in front, to form the lower jaw. Each olfactory placode develops into the olfactory pit which deepens and is soon bounded by prominent lateral and medial, nasal processes and it communicates with the mouth cavity. By unequal growth, the lower portion of each median nasal process buldges below to from the globular processes. The two globular process then unite with each other. The maxillary and lateral nasal processes then fuse closing the oculo nasal salcus and thus complete the socket. Each ( median nasal ) globular process then unites with the maxillary process of its side to form the upper jaw. The lateral nasal process becomes the sides and wings of the nose, the maxillary processes furnish the cheek regons and the upper part of the fronto-nasal process forms the forehead. The line of fusion of the two median nasal (globular) processes is the philtrum. The rima oris bifurcation of the maxillary and mandibular processes is reduced markedly by the establishment of the lips and cheeks. The chin is caused by the fused mandibular processes.

ANOMALIES:

ECTODERMAL DERIVATIVES

Note:- The ectodermal contribution to the oral, nasal and cavities and to the teeth, tongue, plate and salivary glands are first described here.

The primary tissue of the entire digestive systems is the endoderm. That part of the endoderm which underlines the embryonic disc becomes the gut endoderm. When the embryonic disc begins to expand rapidly, the gut endoderm pushes itself into the head end and then into the hind end and takes the form of two internal blind tubes- the foregut and the hindgut. The intermediate region which opens into the yolk sac is midgut. At each end, the gut comes into contact with the ectoderm and fuses with it forming the faryngeal or oral membrane, forms the floor of the external depression- the stomodeum or primitive buccal cavity bounded by the frontonasal, maxillary and mandibular processes. It is ectodermal in origin. The oral membrane ruptures during the fourth week; thus the oral fossa and foregut merge.

The caudal end of the hind gut forms the cloaca. At an early stage the allantois is developed from the hind gut. The cloaca begins to divide by the development of uro-rectal septum into a dorsal rectum and a ventral bladder and urogenital sinus, and by the end of the seventh week the cloacal membrane divides into an upper anal and urethral membranes rupture and each of the new canals, the rectum and urogenital sinus acquires its individual opening to the exterior. The end of the hindgut is lined for a short distance with ectoderm and this portion forms the anal canal.

ANOMALIES:

After the rupture of the oral membrane it is impossible to determine the exact junction of ectoderm and endoderm in the mouth cavity. The terminal sulcus on the root of the tongue marked by the presence of circumvallate papillae is the only demarcation. The stomodeal part is lined with ectodermal epithelium and the pharyngeal part with endodermal epithelium.

The hypophysis cerebri or pituitary gland is an ectodermal derivativies and an endocrine gland of double origin. One part, glandular in nature is derived from the epithelium of the stomodeum, while the other not so plainly secretory, is a specialized extension brain.

A dorsal evagination-the Rathke`s pouch from the roof of the buccal cavity just in front of the oral membrane forms the epithelial part of the gland. The puoch infundibulum, an invagination from the floor of the forebrain vesicle. The infundibulum forms the non-epithelial part of the gland. At about four months, the cranial wall of the pouch thickens greatly and differentiates into pars anterior and the caudal wall remains thin and forms pars intermedia. Very early in development, a pair of buds detach from the pouch and encircle the infundibulum and form pars tuberalis. The original lumen of the Rathke`s pouch becomes reduced to the oblique cleft between the pars anterior and pars intermedia in the adult and is filled with a glairy structure composed of nerve fibers, nuroglial tissue and certain spindle shaped cells and this forms the pars nervosa of the gland.

THE CHEEKS

The cheeks come into existence by a progressive reduction in size of the primitive mouth slit through fusion of the jaws. The muscles differentiate from the mesoderm of the first branchial arch.

THE LIPS

The lips are separated from the gums by the formation of labial lamina which grows from the ectoderm covering of the primitive jaw into the mesoderm under it .It is a long curved thick band of epithelium and progressive disintegration of the central cells causes each plate to split ints two sheets, the anterior forming the lips and the posterior, the gums. The groove- the labial groove develops into the vestibule of the mouth. Another groove develops behind it-the labio-gingival groove-which separates the tip of the tongue from the gums.

THE SALIVARY GLANDS

They are ectodermal in origin. The primordium arises as epithelial bud which grows by branching into a system of solid ducts whose end twigs, round out into spherical masses of cells which form the secretory acini. Cannalisation of the ducts through the disintegration of central cells and specialization of the acinar cells complete the epithelial differentiation. The mesenchyme, in which this primordium lies furnishes the capsule and interlobular connective tissue and vessels.

The primordium of the parotid gland first appears at the angle of the mouth. This grows away from the labial grooves, elongates itself from the parent epithelium and forms a tube. This tube grows backward to the region of the ear and develops into the body of the gland, while the stem of the tube, becomes the parotid duct. The primordial for the submaxillary and sublingual glands are in the labiogingival groove between the lower jaw and tongue on either side, on the floor of the buccal cavity, and they develop like the parotid primordium.

ANOMALIES

THE PALATE

The primordial of the palate are two shelf –like folds- the lateral palatine-processes that are given off from the maxillary processes which grow towards the median line of the mouth cavity. They unite with each other and then with the nasal septum and median palatine processes. Bone appears in the anterior part of the fused lateral palatine processes and this constitutes the hard palate. Behind, ossification fails and in this region, the structure persists as soft palate. The median palatine processes do not contribute to palate, but form the premaxillary portion of upper jaw.

ANOMALY

Cleft palate: - Failure of fusion of the lateral palatine processes.

THE NASAL CAVITY

The olfactory pits gradually extend backwards in the form of two blind sacs, over the roof of the anterior part of the primitive mouth cavity and is separated from it only by membrane. The membrane ruptures during the seventh week creating the two posterior nares.

THE TEETH

Teeth are derived both from ectoderm and mesoderm. A tooth is a highly modified connective tissue papilla that has undergone a peculiar ossification into dentine and caged by hard enamel, elaborated from the epidermis. In addition the base is encrusted by the cement a bony deposit. The enamel is from ectoderm and the dentine, pulp and cement are from mesoderm.

The earliest indication of the development of teeth is the appearance of a dental groove on the surface of the gum about the seventh week of the embryo. From this groove a lamina, the dental lamina projects from the bottom of the groove into the underlying mesoderm. From this lamina a series of knob like thickenings-the enamel organs appear at definite intervals as many in number as there are milk teeth. In the third mouth, the underlying mesoderm forms a dense papilla-the dental papilla and this meets the enamel organ which forms a cup-shaped covering over the papilla. The rudiment of a future tooth is thus laid.

The enamel organ gradually becomes a double walled sac composed of an outer convex wall and inner concave wall enclosing in between a stellate reticulum the enamel pulp. The walls are made up of columnar cells and the cells of inner layer are taller and constitute the ameloblasts which produce the enamel at their free surfaces. The cells of the outer wall form the circular covering of the unworn tooth. The enamel substance arises as a circular secretion from the ends of the ameloblasts. Calsification of these`Tomes processes` is secondary and this completes the formation of enamel prisms.

At the end of the fourth month, the superficial cells of the dental papilla arrange themselves in a definite layer that simulates columnar epithelium. These specialized connective tissue cells are called “ odontoblasts” which deposite their exoplasm on their free surface a dentine and withdraw themselves into the underlying mesoderm. As they withdraw they leave a process of cytoplasm the dentine fibre in the midest of the column of exoplasm. Calcium salts are deposited around these processes in the dentinal tubules. Thus dentine is formed. The odontoblasts continuously form the dentine and sink deeper into the dental papilla which becomes a very narrow structures and which persists as the dental pulp occupying the root canal of the tooth.

The mesoderm surrounding the root of the developing tooth becomes condensed and highly vascular to form the dental sac. The elements of the dental sac are the cementoblasts which deposite their exoplasm in the form of a uniform matrix and this gets ossified by deposition of calcium salts. These cells occupy the lacunae and canaliculi in this ossified matrix.

The tooth thus formed erupt on the gum. Its crown is covered by the cuticular membrane of Nasmyth which is the persisting outer layer of the enamel organ. The temporary tooth is connected to the dental lamina by an epithelial sheath from which the permanent tooth grows in much the same way and pushes the temporary tooth out of its alveolus and the sametime, the root of the temporary tooth is absorbed by giant cells.

BRANCHIAL ARCHES

The branchial arches in mammals give rise to the neck, face, external ear various artries, muscles, cartilages and bones. On the floor of the pharynx, they contribute in the formaton of the tongue.

THE TONGUE

The tongue develops from the ventral ends of the branchial arches. It consists of two parts- oral and pharyngeal. The oral part is the body occuping the mouth cavity and arises from the mandibular arches in front of the oral membrane and hence is covered by ectodermal epithelium. It bears papillae and is concerned with mastication.The pharyngeal part is the root which develops from the second third and fourth arches and is coverd by endodermal epithelium. It is concerned with swallowing. The junction between ectoderm and endoderm is in front of the row of vallate papillae whearas the body and root are demarcated by a V- shaped terminal sulcus, behind them.

The body arises from three primordial- a median,triangular tuberculum impar and a pair of lateral swelling from the first arch. At the same time the root arises from a median primordium-copula from the union of the base of second arches. Copula is encroached by the third and fourth arches.

Anamalies:-

The lateral walls of the pharynx present a series of period sacculation that bulge outwards towards the ectodermal grooves. Five sets are formed but the last pair are rudimentary and merge in the fourth pair. Each pouch develops a dorsal and a ventral wing. Each pouch in its outwards expansion, pushes aside the mesenchyme, comes in contact with the overlying ectoderm, fuses with it and forms a closing plate. The first and second pouches open into a broad lateral expansion of the pharnx. The third and fourth grow outwards and communicate with the pharynx through narrow ducts.

The first pharyngeal pouch retains its lumen and differentiates into the Eustachian tube and the tympanic cavity or the middle ear. The overlying ectodermal groove forms the external acoustic meatus and the drum of the ear.

The second pouch is greatly reduced and becomes the fossa and covering epithelium of palatine tonsil.

The third, fourth and fifth loose all traces of lumen and give rise to the ductless glands thyroid, thymus and parathyroid.

THE THYMUS

Towards the end of the sixth week, each third pharyngeal pouch shows a pronounced ventral sacculation and this is set free during the next week. This primordium is at first hollow but soon becomes a solid epithelial bar. The lower ends of the two primordial unite and are attached to the pericardium and sink along with it to their permanent position in the thorax. During descent, the most cranial parts become drawn out. The original epithelium is soon infiltrated with lymphocytes and lobulation completes the process of morphogenesis. The thymus enlarges, remains till puberty and after that, regresses.

THE THYROID

The main mass of the thyroid gland develops from the floor of the pharynx. An ectodermal pocket in the midplane about the level of the first pharyngeal pouches is the thyroid diverticulum which soon becomes a solid mass attached to the pharynx by a narrow neck- the thyroglossal duct. This duct atrophies and the thyroid primordium is converted into a two lobed structure and settles in a transverse position with a lobe on either side of the trachea. Later on, two bodies- the ultimobranchial bodies from the fifth pouch come into contact with main mass and fuse with it. Sometimes accessory thyroid may be derived from the detached portion of the main primordium.

THE PARATHYROIDS

The dorsal wings of the third and fourth pouches thicken into solid masses cell and form the primordial of the parathyroids. These are soon set free from the pharynx and are drawn down along with the migrating thymic primordial which deposit them in the lobes of the thyroid.

THE PHARYNX

This is a funnel shaped muscular sac which is the residual product after the transformation of its floor and side walls.

THE DIGESTIVE TUBE

The digestive tube consists of an internal tube of endoderm which is the primary tissue that becomes the epithelial lining and an investing layer of splanchnic mesoderm that specialises into the connective tissue, muscle and peritoneum. The primitive gut is made up of three points- the foregut, midgut and hind gut. These three regions are supplied respectively by the coelic, anterior mesenteric and posterior mesenteric arteries.

By unequal growth, the foregut differentiate into a narrow tube, the oesophagus and a wider tube, the stomach, followed by the narrow portion which form the upper part of the duodenum. The stomach is formed by certain changes in shape and orientation. First, the entire organ increases in length; the dorsal border grows faster than the ventral and hence produces the convex greater curvature and a concave lesser curvature: the fundus arises as a local bulge near its cranial end; the dorsal mesentry expands more rapidly than the ventral mesentry and this produses rotation of the organ through 90 degrees so that the greater curvature is on the left and lesser curvature is on the right; the surface becomes dorsal and ventral; the enlarging liver pushes the cranial end of the organ to the left and hence the stomach extends obliqily across the abdomen from left to right.

The intestine is a simple tube begning in the stomach and ending in the cloaca. The midgut loses its connection with the yolk sac early and at this stage is in the form of loop around the anterior mesenteric artery as its axis. This loop has cranial and caudal limbs and it undergoes, torsion- an anticlock-wise rotation as one views the embryo from the ventral side, about the anterior mesenteric artery as its axis. Due to this, the cranial limb goes to the right and behind, and the caudal limb is taken forwards and to the left. Now the gut begins to elongate rapidly and the loop herniates into the umbilical cord. This protrusion constitutes a temporary but normal umbilical hernia, Extensive coiling takes place and the caudal limb presents a bulged caecum. The cranial limb grows rapidly and the caudal limb at a much slower rate. By this time, the abdomen has increased sufficiently in size and hence the intestine is withdrawn into the abdomen. The small intestine is the first t re-enter the abdomen.

The original cranial limb of the primitive intestinal loop forms the duodenum, the jejunum and ileum.

The original caudal limb of the loop forms the caecum and the first part of the colon. The hind gut forms the terminal part of the colon. The rectum is derived from the subdivision of the cloaca. The endodermal lining by proliferation obliterates the lumen of the gut in the early stages which later on gets canalised and the lumen gets restored. The lining develops villi throughout but later the large intestine loses the villi.

ANOMALIES

THE LIVER

The liver is an endodermal derivatives. It is developed from a ventral diverticulum of the endodermal epithelium lining the posterior part of the foregut which develops later into the duodenum. This hepatic diverticulum consists of a cranial portion which differentiates into the glandular tissue and bile duct and a caudal portion that becomes the gall bladder and cystic duct. The hepatic diverticulum forces its way into the mass of splanchnic mesoderm which later forms most of diaphragm and which at this stage, is termed the septum transversum.

From the cranial end of the hepatic diverticulum, epithelial laminae arise and proliferate in the septum transversum, The paired vitelline veins flanking the gut send branches into this region and the result is a mutual and intimate inter-growth of tortuous liver cords and sinusoidal channels. The diverticulum in the meanwhile elongates and differentiates into the duct system. The main portion of the diverticulum forms the ductus choledochus and hepatic duct. From the expanding end of hepatic duct, the larger intrahepatic ducts budfrom these arise the small interlobular ducts. The regularlity of the system of branching of the vitelline (portal) vein is responsible for the creation of the characteristic hepatic lobules from the parenchyma. The gall bladder and systic duct develop from the caudal portion of the hepatic diverticulum. The septum transversum furnishes the peritoneal covering and the connective tissue framework.

THE PANCREAS

The pancreas is also an endodermal derivatives. The two outpocketings from the endodermal lining of the gut represent the primordial of the pancreas. These buds arise on the opposite sides of duodenum one on the dorsal side in front of the hepatic diverticulum and the other on the ventral side in the angle between the gut and the diverticulum. Unequal growth of the duodenal wall shifts the bile duct dorsally and the ventral primordium of the pancreas is thus shifted dorsal and is brought very near the dorsal primordium. During further development, the two primordial fuse.

By the proliferation of the chief ductular system, terminal and side buds appear and develop into the acini. Some of the epithelial buds loose connection with the ducts and develop into islets Langerhans. The development of the gland from the ducts is similar to that of salivary glands.

THE SPLEEN:

NOTE: Spleen development is described here for the sake of convenience.

The spleen is a mesodermal derivative. It is developed in the dorsal mesogestrium. At first an accumulation of mesenchymal cells is seen just beneath the surface of the peritoneal epithelium. This mass increase in size and projects above the omental surface as several hillocks which soon merge. The part of the dorsal mesogastrium which is attached to the spleen becomes reduced to narrow band-the gastrosplenic omentum. The mesenchyme is soon vascularised and the capsule trabeculate and pulp cords are differentiated. Sometimes partially subdivided or multiple spleen can be observed due to the absence of complete union of the original hillocks.

THE RESPIRATORY SYSTEM

 The epithelium of respiratory tree is derived from endoderm.

The primordium of this system is the so-called laryngotracheal groove which runs lengthways in the floor of the foregut just behind the last pair of pharyngeal grooves. On the ventral surface the endoderm projects as a ridge. This ridge is the primordium for caudal part of larynx, trachea, in front and caudally the bronchi and lungs. A lateral groove appears on each side at the junction of the ridge and the oesophagus, which becomes deeper and thus separates the oesophagus from the trachea and lung bud. The ridge extends forwards upto the level of the fourth branchial arches. The mesenchyme of the fourth and fifth arches give rise to the thyroid, cricoid, epiglottic and arytenoids cartilages and laryngeal muscles, and the primitive opening of the pharynx into the trachea forms the glottis.

The trachea elongates and bifurcates into the bronchial buds-two on the right side and one on the left. The bronchial buds elongate and branch and the tubular system in each pulmonary lobe becomes bush-like, the splanchnic mesoderm within which the early respiratory tree has developed is later named as the mediastinum and the mesenchyme furnishes the connective tissue and cartilage plates of the tracheal and bronchial wall. Into this grow the blood vessels and nerve fibres.

A serious anomaly in connecton with the development of this system is a fistula between the trachea and oesophagus, due to incomplete separation of the larngotracheal groove from the gut.

MESODERMAL DERIVATIVES

THE URO GENITAL SYSTEM

The urinary and genital systems both arise from the mesoderm, both drain into a common urogenital sinus and hence these two systems are intimately associated in their origin, development and relation.

THE URINARY ORGANS:

Nowhere in the development of the individual can be found a better example of the principle of recapitulation than in the development of kidneys of mammals. The earliest excretory organ is the pronephros (functional in adult amphioxus). In fishes and amphibians, the pronephros is replaced in the adult condition by the mesonephros. This is present in the embryos of reptiles, birds and mammals but the final permanent kidney in these classes is the metanephros. All the three types of kidneys are formed in succession, one behind the other during development of a mammal.

The kidney arises from the mesoderm of the intermediate cell mass or nephrotome which lies next to the mesodermal somites. The series of nephrotomes from the urino-genital ridge which splits up into an lateral nephric and a medial genital ridge, which project into the coelom from the dorsal wall.

The pronephros arises as a series of about seven simple paired pronephric tubules arranged segmentally. One end of each tube opens into the coelom and the other end into a longitudinal excretory duct-the pronephric duct opening into the cloaca. The arterial tuft projecting into the coelom is the external glomerulus separated from it by the splanchnopleure which serves to filter the waste products from the blood into the coelom. These degenerate early in foetal life.

The mesonephros or wolffian body is larger and it consists of about 30 tubes which are longer and complicated, situated about the middle of the ridge. The mesonephric tubules are tortuous and are connected to the pronphric duct which is now termed as the mesonephric or Wolffian duct. At one end each tubules is invaginated like a gastrula Bowman`s capsule and is associated with a glomerulus (internal glomerulus) and at the other end, with the duct. These tubules degenerate about the fourth mouth.

The permanent kidney or metanephros arises far behind in the body. As in the case of mesonephros, the final kidney consists of an aggregate of tubules which drain into a common duct. Also like mesonephros, the metanephros is of double origin; but in this instance, the boundary lies midway of the uriniferous tubules themselves. Thus the inert system of drainage ducts (ureter, pelvis, calyces, papillary ducts and straight collecting tubules) is derived from a bud- ureteric bud- growing off the mesonephric duct. On the other hand, the secretory unit, the nephron (Bowman`s capsule, both convoluted tubules, and henle`s loop) differentiate from the substance of the nephrogenous cord (metanephrogenic mass). The collecting (drainage duct) and secretory tubules then unite secondarily to complete a continuous uriniferous tubules. In structure and function, therefore, these two components remain as different as was their origin.

The mesonephric duct makes a sharp bend before opening into the cloaca. Here the ureteric primordium appears as a hollow outgrowth from the mesonephric duct and this grows forwards. The proximal , rapidally elongating portion of this invagination forms the ureters, while the blind end expands into the renal pelvis. This ureteric bud pushes itself into the caudal most portion of the nephrogenous cord which forms a cap over the ureteric bud. The ureteric bud differentiate into primary, secondary and tertiary ect., about twelve generations of collecting tubules- the calyces majore, calyces minore, papillary ducts and collecting tubules making up a large part of the medulla of the kidney. The metanephrogenous tissue differentiates into the secretory tubules and the capsule and intestinal tissue and connection is established between the secretory and excretory portions of the tubules. Soon the metanephros migrates cephalad to the sublumbar protion.

ANOMALIES

THE GENITAL SYSTEM

The genital organs are differentiated when they first appear. Either by gross or microscopic appearance and hence during this “indifferent period” the term gonad is applied to them. The gonad appears within a thickening called the genital ridge on the medial aspect the urino-genital ridge. The ventral surface of the genital ridge shows rapid proliferation of the mesothelium which becomes thickened and many layered. The gonad consists of a superficial germinal epithelium and an internal epithelial blastemal mass. The primordial germ cells from the yolk sac endoderm migrate by way of the dorsal mesentery of the gut and locate themselves in the epithelium of the genital ridge.

In the male, genital glands as they increase in size, shorten into more compact organs and begin t show branched and anastomosing cords of cells- the tests cords. These arise from the superficial germinal epithelium which dips into the interior of the mesenchyme forming cords of cells. These cords differentiate into the tubuli contorti mesenchyme forms the interstitial tissue and the mediastinum testis. When the migration of the germ cells is complete the mesenchyme, surrounding the glands forms the tunica albuginea and cuts off the germinal epithelium, which becomes mesothelium. Certain cells of the mesenchymal stroma transform themselves into large cells lying in the intertubular connective tissue forming the interstitial cells.

In the female, the gonad does not exhibit any distinctive ovarian features until several weeks after the gonad of the male has declared itself as a testis. Then the blastemal mass shows clusters of small indifferent cells and one or more primordial germ cells. Soon primary cortex beneath the germinal epithelium and primary medulla internally. Later the ovary enlarges by addition of a new (secondary) definitive cortex upon the original blastemal mass. In the primary medulla and cortex, the earliar ova decline, regress and are replaced by vascular fibrous stroma- the definitive medulla. In the definite cortex, develop ova and the primary follicles, Later vesicular (Graafian) follicles develop during the active sexual years from the primary follicles. It is to be remembered that while the primary medulla and primary cortex correspond to the male component of a gonad, the definitive cortex of the gonad is a distinctive female characteristic.

GENITAL DUCT

The male does not elaborate any ducts intended primary for its own purposes. With the degeneration of the mesonephros it merely appropriates the mesonephric ducts and some of the mesonephric tubules and converts them into genital canals (mesonephric duct becomes, the duct of epididymis, ductus deferens and ureters and pelvis while the cranial group of mesonephric tubules become efferent ductules of epididymis. Seminal vesicle arises from the mesonephric duct).

In the female, the mesonephric ducts mostly degenetrate, except for ureters, pelvis and some vestigial remanents .

THE MULLERIAN DUCT

Both sexes develop a pair of female ducts of Muller. But it`s fate in the two sexes is different, as described below:

Female:-

These ducts arise by formation of a groove in the thickened epithelium of the urino-genital ridge which is lateral to the mesonephros near its craniaql end. This extreme cranial end of the groove remains open like a trumpet while caudally the lips of the groove fuse into a tube. In the most posterior part, the two Mullerian ducts approach each other, reach the dorsal wall of the urogenital sinus fuse and end blindly at the Muller`s tubercle- a median projection in the dorsal wall of the cloaca, caused by the opening of the mesonephric ducts into it. This fused region is the first indication of the uterus and vagina and the unfused cranial parts serve as uterine tubes. The fused part by unequal growth of its walls, becomes transformed into the fimbriated end of the fallopian tube. The Muller`s tubercle becomes the site of hymen.

Male:-

The same primordial also develop but remain rudimentary. Degeneration of the ducts occurs in the third month and only the extreme cranial end is spared which remains as a vestige-the appendix testis. The portion which for the uterus and vagina persists in a rudimentary form as the uterus masculinus. The Muller`s tubercle is represented by the colliculus seminalis.

THE MESONEPHRIC OR WOLFFIAN DUCTS

Male: Some of the cranial degenerating mesonephric tubules, unite with the somniferous tubules and form the efferent ducts of the epididymis. The caudal group of degenerating mesonephric tubules highly convoluted to form the duct of epididymis and lower portion forms the vas deferens.

The primordial of the seminal vesicle arise from the terminal part of the mesonephric ducts and so are mesodermal origin. The prostate and Couper`s glands arises from the pelvic urethera which is endodermal in origin. The penile part of the urethra is ectodermal in origin.

Female:-

The cranial group of mesonephric tubules persist as the epoophoron and the caual group as the paroophoron. The greater part of the mesonephric duct atropies and the distal porton persists and Gartner`s canals in the wall of the vagina.

THE EXTERNAL GENITALIA

Embryos of six weeks present a conical genital tubercle in the mid-ventral line between the umbilical cord and tail. Its caudal slope bears a shallow urethral groove. Flanked on either side by slightly elevated urethral folds. The genital tubercle elongates into a cylindrical phallus whose tip is rounded into glans. Lateral to the phallus on either side appears a round ridge-the labio-scrotal swelling. Rupture of the urethral membrane in the floor of the urethral groove provides an external opening for the urino-genital sinus. So far the development is identical in both the sexes.

In the male, the phallus elongates to become the penis. The edges of the urethral groove progressively fold together in a distal direction to transform an open urino-genital sinus into the tubular urethra within the penis. The fused edges from the raphe. The scrotal swelling shift laterally and behind and develop the scrotum. Each half of the scrotum is separated from its mate by a septum and a superficial raphe.

In the female, the phallus lags in development and becomes the clitoris. The shorter urethral groove remains as the vestibule and the urethral folds constitute the labra minora. The primitive labio-scrotal swelling enlarge and fuse below the anus in the dorsal commissure, while the swellings enlarge to form the labia majora.

ANOMALIES:

THE VASCULAR SYSTEM

 Both the blood vessels and cells arise from mesenchyme. The earliest formative tissue of this kind is the angioblast which differentiates in the mesodermal wall of the yolk sac where groups of angioblasts from the blood islands of Pander, originally solid, these soon hollow out and in this process peripheral cells become arranged as endothelium and central cells from blood cells. From the wall of the yolk sac, the blood vessels extend into the body of the embryo and spread into the different parts.

HAEMOPOIESIS

The first site of blood formation is in the area vasculosa of the wall of the yolk sac. Here between the extraembryonic splanchnic mesoderm and endoderm the blood island of Pander develop. These are solid masses of mesenchymal cells united by cellular strands to form a network. As they become differentiated the outer cells join to form the tubular wall of blood vessels, while the inner cells are separated by the secretion of plasma between them and become primitive blood cells, which multiply rapidly by mitosis. Formation of primitive blood cells also occurs for some time by the detachment of cells from the endothelial wall. These cells die out during embryonic life and do not form definitive erythrocytes. Other primitive blood cells remain as colourless proliferating haemocytoblasts. Blood formation is soon transferred from the yolk sac to the body of the embryo-where it is continued by the body mesenchyme, endothelium, liver, spleen, thymus, lymph nodes and bone marrow.

The haemacytoblasts or stem cells are large amoeboid cells with a large pale spherical nucleus and basophilic cytoplasm. All the definitive blood cells develop from this stem or blast cell. According to monophyletic theory, a common stem cell gives rise to all the types of blood elements, both red and white. The polyphyletic theory holds that there are two stem cells- one stem cell for erythrocytes and granular leucocytes and the other for non-granular leucocytes.

The haemocytoblast differentiates by the gradual elaboration of haemoglobin consequent to transformation of basophilic cytoplasm to the acidophilic cytoplasm, nuclear condensation and reduction in size of the cell. The successive stages are proerythro-erythroblasts-normoblasts. The normoblasts (which have a size about equal to the erythrocytes, a small dense nucleus and an eosinophilic cytoplasm) lose their nuclei by extrusion and become erythrocytes.

The haemocytoblast also gives origin to the differentiating granular leucocytes termed myelocytes. These elaborates within their cytoplasm specific kinds of granules, giving rise to neutrophil, acidophil and basophil mylocytes. After a number of mitotic divisions, these mylocytes lose the capacity for division, the nucleus becomes indented and these cells enter the blood stream, as the different type of granular leucocytes.

The lymphocytes are produced in the lymphatic tissue by differentiation of haemocytoblasts into large lymphocytes. The large lymphocytes give rise to small lymphocytes.

The monocytes are belived by some to arise by the transformation of lymphocytes in the sinusoids of spleen, liver and bone marrow. Others are derived from the lining cells of sinusoids or from monoblasts in the bone marrow.

The megakaryocytes are giant cells derived from haemocytoblast and occur in the embryo, in the haemopoietic organs (red marrow, liver, spleen) and their processes extand through the walls of sinuses and by constriction and segmentation of these, platelets are formed.

THE HEART

The heart is a specialized blood vessel with a lumen and thick muscular walls. In mammals, it arises from a cardiogenic plate located in front of the head in the splanchnic mesoderm and bebeath the pericardial coelom and in the mesoderm beneath the foregut. The earliest indication of the cardiac primordial is the formation of two endothelial tubes which fuse in front and later behind, to form a single tube. The internal endothelial tube becomes the endocardium and the mesodermal layer surrounding it forms the myocardium and epicardium. It is suspended by the dorsal mesocardium from the gut which is also soon lost.

At this stage, this single tube, by constrictions, present three regions-the paired atria receiving the veins, the single ventricle and the single bulbus continued in front by the ventral aortae. A sinus venosus is soon demarcated from the atrium. Due to the greater rate of the growth of the heart tube than the pericardial cavity it, is soon bent into a simple spiraled S.A. continuation of this growth process drops the bulboventricular loop still further backward and downward. By development of septa, indicated externally by grooves, the heart presents the two ventricles and the aorta and pulmonary artery. The interatrial septum presents the foramen ovale.

The two atria are separated by the development of the septum primum and the septum secundum which fuse to form the interatrial septum. The ventricular primordium is subdivided into two ventricles by the development of the interventricular septum. The bulbus is split into the aorta and the pulmonary artery by the development of two endocardial septa which fuse.

THE ARTERIES

The aorta is dilated at its origin forming the aortic sac from which the ventral aortae ascend and continue as right and left dorsal aortae. The dorsal aortae unite behind to form a single descending aorta.

From the aortic sac, five pairs of aortic arches arise and connect themselves to the dorsal aortae. The pulmonary artery divides into right and left pulmonary arteries which connect themselves by the sixth aortic arches with the dorsal aortae.

The first and second aortic arches of both sides degenerate. The dorsal aortae between the third and fourth arches degenerate, thus establishing continuity between the third arches and the anterior segments of the dorsal aortae. The third aortic arch gives off a sprout- the external carotid artery, after which it is continued as the internal carotid artery. The common stem of the third arch before the sprout arises, is the common carotid artery. The fourth arches are transformed into different structures on both sides. The right arch becomes the right subclavin (right axillary), the left subclavian artery (left axillary). The fifth arches degenerate.The sixth arches (pulmonary arches) on the right side degenerates, whereas on the left side it persists as the ductus arteriosus. The vagi detach the the recurrent laryngeal nerves which pass round the sixth arches. During the migration of the heart downwards, the left recurrent laryngeal nerve, hooks around the sixth arch (ductus arteriosus) and is dragged down where as the right nerve lags behind and is hooked around the right branchial. The right dorsal aorta between the sixth arch and the descending aorta degenerates. Thus the main details of arteries in the adult are laid down.

In the domestic animals the right branchial and the right common carotid shift themselves on the arch of the aorta and fuse with the left common and the left branchial.

The dorsal aorta gives off the following branches:-

The dorsal aorta terminates by giving off two umbilical arteries after which it is continued by the middle sacral artery. The external iliac arteries are sprouts from the umbilical arteries. A new connecting trunk from the junction of the umbilical artery with aorta, becomes the internal iliac.

 THE VEINS

All the veins of the body develop as modifications of the ground plan present in the developing embryo.

The plan may be outlined briefly as consisting of:-

The proximal ends of vitelline and umbilical veins are absorbed by the developing liver to from the portal system. The mesenteric veins draining the venous blood from the gut, entering the liver, persists as the portal vein.

The anterior cardinal veins drains the venous blood from the head and neck. The posterior, sub and supracardinal veins drain the venous blood from the posterior parts of the body.

The cranial segment of the anterior cardinal is transformed into the venous sinuses of the dura mater and the cervical segment persists as the internal jugular form which sprouts the external jugular. The thoracic segments of the anterior cardinals unite to form the anterior vena cava.

The posterior cardinals degenerates without contributing anything except a small proximal part of the azygos vein. The subcardinals unite and are transformed into the posterior vena cava. The supra-cardinals unite and are transformed into the posterior vena cava. The supra-cardinals unite and are transformed into azygos vein on the right and vena hemiazygos on the left side.

THE FOETAL CIRCULATION

 Oxygenated blood, returning the placenta, enters the embryo by the large umbilical vein and is conveyed to the liver. From here it is conveyed to the posterior venacava for the most part directly through the ductus venosus and a certain extent through the hepatic veins. The impure blood of the portal vein and posterior venacava contaminates only partially the large volume of oxygenated blood from the placenta. Insharp contrst, the blood returning from the anterior venacava is very poor in oxygen. In the heart, the blood coming from the anterior venacava is directed in to the right ventricle and through the pulmonary artery it leaves the heart. Some of this reaches the lung, the lungs is mostly conveyed through the ductus arteriousus to the posterior aorta. On the other hand, the blood from the posterior venacava entering the right atrium passes mostly to the left atrium through the foramen ovale and reaches the left ventricle. From here it is pumped in to the aorta. Thus the blood reaching the heart substance through the coronary arteries and the head and neck through the bicarotid trunk arteries contains comparatively more oxygen then that reaches distributed to the other parts of the body through the poaterior aorta. The umbilical arteries arising from the aorta, transport a large volume of this blood to the placenta for oxygenation.

At birth, the lungs become functional and placental circulation ceases. This throws some foetal vessels in to disuse. The umbilical vessels pass into sudden and complete dis use, the arteries become transformed in to lateral ligaments of the bladder an dthe vein forms ligamnetum teres of liver. The ductus venosus also atrophies and is transformed in to the fiberous ligamnetum venosum embedded in the wall of the liver.

The ductus arteriousus is transformed in to the liogamnetum arteriousum and the foramen ovale is obliterated and the site is marked permanently by the foassa ovalis.

THE MUSCULAR SYSTEM

The muscular system consists of specialise dcells – the muscle fibers which are of three varieties, skeletal, plain and cardiac. These are differentiated by from the myoblast which are derived from the mesoderm. The smoothe muscle fibers of iris and sweat gland are ectodermal.the muscles of head and neck develop from the mechenchyme of the brachial arches. The skeletal muscle originate in the myotomes. The visceral and cardiac musculature is developed from the splanchnic mesoderm that surrounds the gut and heart.

THE SKELETAL SYSTEM

The skeletal system is a mesoderm derivatives. Bone always develops by a transformation of the embryonic or adult connective tissue. When development of bone takes place in cartilage, the method of ossification is termed onchonral ossification. When it occurs in membrane, the process is termed intramembranous but essentially the processes is the same in botth. Bone matrix is deposited through the activity of specalised connective tissue cells are called osteoblasts. The only difference between the two types of ossification is that the former occurs at first in cartilage which is destroyed replaced and substituted by bone. Hence bones are formed in this way are also known as substitution bones, for example long bone sof limbs. The flat bones of the cranium or membrane bones.

Intramembranous ossification:

In an area where bone is going to develop, the mesenchymal cells are connected with one another by their processes and delicate bundles of white fibers run in all direction between them. In between the cells and bundles is asemi fluid intracellular substance. At one or more central point, ossification by appearance of osteoblast which promptly deposit the bone matrix in the foerm of acidophil bars or spicules which unite in to a mesh work of trabeculae. As the matrix is laid down some cells remain imprisoned in the meshes as osteoblats or bone producing cells which takes up calcium salts from the blood and deposit the same in the network of acidophil bars. Thus converting them in to plates of bone. Thus ossification starts from the centre and proceeds towards the pheripery. As this proceeds the surrounding mesenchyme furnishes more and more of membrane for ossification and this result in an increase in size of bone. The osteblast at the periphery arrange theselves in a continous layer an dthese also produce parallel plates of bone at the periphery. The mesenchyme in the region forms a fibrous membrane enclosing the bone – Periostium.

Bone formed in the process has in spongy character and concsits of irregular plates of bone which branched and arise with one another and in the meshes is enclosed embryonic connective tissue rich in blood vessels forming the red marrow.

ENDOCHONDRAL OSSIFICATION

Most of the bones in the body are pierced by a temporary cartilaginous model of the same shape as the definite bone and the chief peculiarity of the method is the preliminary destruction of the cartilage and its replacement by bone, which develops exactly as in intra-membranous ossification. So bones formed by this method are called replacement or substitution bones.

In the hyaline cartilage which is to be substituted by bones, the cartilage cells enlarge in the centre and become arranged in characteristic radial rows and some lime is deposited in their matrix. The cartilage cells and part of the matrix then disintegrate forming primary marrow cavities. This destruction is accompanied by a simultaneous invasion of this region by the vascular connective tissue, rich in cells derived from the perichondrium. The cells perform the function of osteoblasts of depositing lime salts in the cartilage matrix and this results in the formation of bars of calcified matrix. Bone thus formed is spongy in nature consisting of network of bony plates enclosing the primary marrow spaces containing blood vessels and connective tissue corpuscles and this constitutes the red marrow. In a progressive manner the cartilage undergoes invasion, destruction and replacement by spongy bone.

The spongy bone at the center is destroyed and absorbed by giant cells called osteoclasts. Thus a large marrow cavity is created in the center of the bone. Simultaneously the perichondrium at the periphery functions as periosteum and deposists by the activity of the cells of the osteogenic layers, periosteal bone. This process of formation of spongy bone in the centre and periosteal bone at the periphery, its destruction and absorption by osteoclasts at the centre goes on, resulting in increased dimension of the marrow cavity and circumferential growth of bone.

The centre of ossification appearing in the shaft of a long bone of the foetus in the primary centre of ossification. Sometime between birth and puberty, centers of ossification appear at the epiphyses also. At the junction of the shaft and the epiphyses there are two plates of cartilages, one above and other below termed the epiphyseal cartilages. These supply more and more of cartilage for the shaft and the epiphyses for formation of new bone and this process results in an increase in length of the bone. When the bone has reached its adult stage, both in its lengths and circumference, the epiphyseal cartilages are ossified and further growth stops.

Reconstruction takes place in periosteal bone to form the Haversian systems and the outer and inner circumferential lamellae. The osteoclasts destroy and removeperiosteal bone forming Haversian canals. Osteoblasts arrange themselves around them and deposit their exoplasm in concentric layers which are later ossified by deposition of calcium salts.Some of the osteoblasts are engulfed in the lamellae and occupy lacunae and canaliculi. Others withdraw and arrange themselves at the periphery and deposit periosteal lamellae. The cells towards the medullary cavity form the endosteal lamellae. Thus through reconstruction, compact bone is formed outside enclosing cancellated or spongy inside, around the medullary cavity.

THE BONES OF THE SKULL

The earliest indication of the skull is a mass of dense mesenchyme enveloping the cranial end of the notochord and it extends forwards into the nasal region. Laterally it becomes continuous with the neuro-cranium. The mesoderm covering and protecting the brain and interiorly with the mesodermal cores of branchial arches.Condrification sets in during the seventh week and it is confined chiefly to the base of the skull-the basioccipital and sphenoidal cartilages. The rest of the sides and roof is connective tissue membrane. The chondro cranium hence refers to the base and neuro cranium to the roof and lateral walls. Ossification, of chondrocranium begins during the third month and forms the occipital, sphenoid, ethmoid and petrous and mastoid, parts of the temporal bone. The parietals, squamous and tympanic parts of temporal, vomer, nasal, lacrimal. Malar are all developed in membrane.

The first brachial arch divides into an upper maxillary and a lower mandibular process. The Meckel`s cartilage is the mesechymal core of the mandibular process which becomes enveloped by surrounding mesenchyme and finally disappears except at the proximal portion where it is transformed into the spheno-mandibular ligament, malleus and incus. The adult mandible develops in membrane.

The maxillary process under goes similar degeneration and the mesoderm surrounding the original cartilage develops into the maxilla and palatine bones.

The mesoderm of the second branchial arch (Reichert`s cartilage) contributes the stapes and styloid of the temporal bone and its lower part furnishes the small corna of the hyoid.

The cartilage of third branchial arch forms the styloid cornu and the body of the hyoid. The fourth branchial arch differentiates into cuneiform and thyroid cartilages of the larynx. The fifth branchial arch furnishes the arytenoid and cricoid cartilages.

THE VERTEBRAL COLUMN

The primitive axial support of all vertebrates is the notochord or chordadorsalis. It is a transient structure in mammals and consists of a cylindrical rod of mesodermal cells from the Seesel`s pocket to the tip of the tail. The notochord degenerates soon except at the intervertebral discs within which it persists as the wollen mucoid nuclei pulposi.

The axial skeleton differentiates from the mesenchyme that traces its origin to serially arranged pairs of mesodermal segments-the somites. The ventro-medial part of a somite develops into the sclerotome and cells proliferate from this mass and migrate towards the notochord. The sclerotomes are destined to from the vertebrae and ribs.

Each sclerotome presents an anterior loose and a posterior dense part. The anterior part of a sclerotome fuses with the posterior part of the sclerotome in front of it, and these recvontituted sclerotomes from the primordial of the definitive vertebra. From each of these sclerotomes growth takes place in three direction

Following this blastemal stage condrification sets in during the seventh rib and ossification during the tenth rib. When the atlas forms its body it is soon appropriated by the excess as the Dens

In the thoracic region the costal processes enlarge and form ribs. Here the original union of costal processes with the vertebra is replaced by a joint (costo central and costo transverse articulation). Condrification and ossification follow to form the ribs. The costal processe3s remain diminutive and are fused with the transverse processes in the cervical and lumber region.

The sternum arises from a pair of mesenchymal bands on the ventro ventral aspect of the bodywall. these unite progressively from before backwards and obtain connection with the ribs and the cartilages. The mesenchyme of the sternum undergoes condrification at 9 wks and ossification at about 5 months. The segmentation of the sternum is a feature secondarily acquired.

The bones of the appendicular skeleton are substitution bones developed from the mesenchyme of limb buds.

ECTODERMAL DERIVATIVES

 THE NERVOUS SYSTEM

The entire nervous system and the organs of special sense are derived from ectoderm. The basis of the entire nervous system is a thickened band of ectoderm the neural plate along the middorsal line of the embryo. This plate becomes stratified and by unequal growth the plate is folded into a nervous groove bounded by neural fold. The groove deepens and the folds meet above, thus forming a neural tube.Along the line of junction of the neural plate with the ectoderm,a longitudinal band of cells appear on each side. This is the neural crest from which are derived the ganglion cells of the cranial, spinal and autonomic ganglia. The neural tube gives rise to the central nervous system.

The cranial end of the neural tube presents two constriction, separating three primary brain vesicles which are very much enlarged-the forebrain, midbrain and hindbrain vesicles. The forebrain forms the rhinencephalon, corpora striata, cerebral cortex, epithalamus, thalamus, metathalamus and hypothalamus and optic vesicles.The hind brain specialises into the crura cerebri and corpora quadrigemina. The rest of the neural tube develops into the spinal cord. The cavity of the forebrain vesicles is modified into the paired lateral ventricles and the third ventricle. The cavity of the midbrain vesicle remains as the cerebral aqueduct. The cavity of the hindbrain vesicle constitutes the fourth ventricle. The lumen of the rest of the neural tube forms the central canal of the spinal cord.

PERIPHERAL NERVOUS SYSTEM

The afferent fibers develop from the neuroblasts located in the neural crest. The efferent fibre develop from the neuroblasts in the basal plate of the neural tube. The afferent and efferent fibers include both cerebrospinal and autonomic series.

THE SUPRARENAL GLANDS

The gland is developed from two germ layers-the mesoderm and medulla from neural ectoderm. The cortex is derived from the coelomic epithelium at the root of the dorsal mesentery medial to the urinogenital ridge. From the primitive ganglia of the celiac plexus neuroblasts enter the cortical primordium to form the medulla of the gland.

THE ORGANS OF SPECIAL SENSE

Lamellated corpuscles are differentiated from a mass of mesenchymal cells clustering around a nerve termination. These multiply, flattenand give rise to concentric fibrous lamellae, the Pacinian corpuscles. A tectile corpuscle originates with a looping plexus of terminal nerve fibres located just beneath the epidermis and this plexus becomes encapsulated along with a cluster of mesenchymal cells.

Local thickening of lingual epithelium present taste buds. The basal cells get elongatedand extend towards the surface and specialise into taste cells ending in hairlike tips, while others become columnar supporting cells. The gustatory cells are connected to the nerve terminations. Taste buds are derived both from endoderm and ectoderm.

The first indication of the olfactory epithelium is the thickening of ectoderm on the ventro-lateral surfaces of the head olfactory placodes. These are coverted into olfactory pits which elongate and form the nasal cavities. Rupture of the plate at the deep end of the olfactory sac produces the primitive choanae or posterior nares.

THE EYE

The eye derived from three sources:-

From the floor of the forebrain on each side a small diverticulum develops as the optic vesicles. This vesicles elongates t from the optic stalk and it terminates by widening out to form the optic cup. The optic cup is formed by indentation of its distal wall brought about by rapid marginal growth. The optic stalk forms optic nerve and the cup forms the anterior wall forming the nervous portion and the posterior wall, pigmentary layer.

Meanwhile the surface ectoderm overlying the optic vesicle thickens into a lens placode and the this pockets inwards to produce a lens vesicle which occupies the cavity of the optic cup. The lens vesicle has a layer of cubical cells in front forming th epithelium of the lenss and th cells of the back wall from elongated lens fibres. The mesoderm sourrounding th ptic cup forms the sclera, cornea and the chorioid. The vitreous humour is a secretory body from both retina and lens. Later probably an invading mesenchyme also contributes to its structure.

 THE INTERNAL EAR

It is derived from ectoderm. A thickened ectodermal plate- the auditory placode located on each side of the hindbrain becomes the auditory pit which soon closes into the otocysts.These sink into the mesoderm and get detached from the ectoderm. From the otocyte the membranous labyrinth and neuroepithelium are derived. The contributions of ectoderm to the oral, nasal and anal cavities to the teeth tongue palate and salivary glands have been described already.

THE SKIN

The integument has a double origin. The epidermis is derived from ectoderm and the dermis or corium from the mesoderm. From the ectoderm are developed nails, hoof, horn, hair sebaceous glands, sweat glands, mammary glands and arrector pilli muscle.

An ectodermal thickening milk line or mammary ridge extends on either side as longitudinal bands between the bases of limb buds. The primordial band is developed in the pectoral region (primates, elephants) inguinal region (ungulates) or throughout (axilla to groin region) in carnivores and pigs. The primordium of each gland becomes lens shaped, then globular and then lobed. From these, solid cords bud inwards into the corium. These primary solid milk ducts branch and terminate in secretory acini. Lumen is acquired by canalization of the solid ducts and acini.

THE TRUNK AND APPENDAGES

LIMB BUDS AND TAIL BUD

Trunk is formed by the rolling lateroventrally of the somatopleure.

Appendages:-

The limbs are outgrowths from somatopleure.Upper (fore) limb bud forms very early in development. After some time the lower (hind) limb bud and tail bud form.

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