EPITHELIUM
Author: Dr. R. Menaka
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EPITHELIUM- CELLULAR COMPONENTS AND INTERCELLULAR MATRIX
TYPES OF EPITHELIUM
SIMPLE SQUAMOUS (Pavement Epithelium)
This consists of flat-scale like or plate like squamous cells.
The cells rest on the lamina properia and their free surface shows delicate.
Shape: The cells are spindle shaped. Thin at ends and thicker in the center.
Nucleus: The nucleus is situated in the center of the cell. Shape of the nucleus is ovoid or sperical.
Note: a). If simple squamous epithelium lined the body cavities, it is called Mesothelium.
The mesothelium together with an underlying structural layer or of connective tissue, forms the serous membrane
e.g: Pleura, Peritoneum and Pericardium
b). If simple squamous epithelium lines the herat chambers, known as Endocarium. If simple squamous epithelium lines the all blood vessels and lymphatic vessels, it is known as Endothelium.
Additional Note: The simple squamous epithelium also lines the membranous labyrinth of the internal ear, bowman's capsule and tubules and alveoli of the lungs.
SIMPLE CUBOIDAL
The single layer of cells rest on the basement membrane.
Shape: Cuboidal i.e. Cubical
e.g: i. Kidney- Prominent brush border is seen in the proximal convoluted tubules, thyroid gland, acinar ducts of exocrine glands, mammary glands, on the surface of the ovary where it is called germinal epithelium, the inner surface of the lens and the pigment cell layer of the retina.
SIMPLE COLUMNAR
The single layer of cells rest on the basement membrane.
Nucleus: Oval shape and situated at the basal part of the cell.
e.g: Stomach, intestine, interlobar and interlobualr ducts of many glands, uterus, vas deferens etc.
Note: In certain locations the simple columnar cells shows some modifications.
for example: a. Simple columnar epithelium of small intestine shows the fine striations, which has been identified as microvilli. This microvilli is observed through EM.
b. A common variation is in the form of goblet cells. e.g. Goblet cells are found in intestine and respiratory passages. In ordinary H & E staining mucin is under stained. So cell appears as empty.
c. In certain locations, the simple columnar epithelium shows on their free surface motile and non-motile cilia. Pseudo Stratified Columnar Epithelium. Trachea
e.g: Motile cilia: Trachea and larger bronchi
Non-motile cilia also known as sterocilia e.g: Male reproductive system- urethra membranous and penile parts), epididymis, ductus deferens, receptor cells in the internal ear.
TRANSITIONAL EPITHELIUM
The transitional epithelium is seen in the mucous membrane of the excretory passages of the urinary system. The cells are divided in to three groups
a. The basal polyhedral cells
b. The intermediate pear shaped or pyriform cells
c. Flattened or umbrella like surface squamous cells.
STRATIFIED CUBOIDAL & COLUMNAR
a. Stratified Cuboidal epithelium is commonly rare.
The top cells are cuboidal type.
e.g. The layers of epithelium is found in the following areas.
duct of sweat glands, and Cell lining antral cavity of the ovarian follicles.
b. Stratified Cuboidal epithelium is commonly rare.
The top cells are columnar type.
e.g. The layers of epithelium is found in the following areas.
i. Larger excretory ducts of some glands, palpebral conjunctiva of the horse and dog.
STRATIFIED SQUAMOUS - Three to five distinct cell layers are present in stratified squamous epithelium.
1. Stratum basale - basal layer
2. Stratum spinosum - spinous or prickle layer
3. Stratum granulosum - granular layer
4. Stratum lucidum - clear layer
5. Stratum corneum - horny layer
Stratified squamous epithelium can be divided in to two type
1. Non-Keratinized
These layers are found, where the surface of the epithelium remains moist,
Superficial layers of cells are living and nuclei can be seen in them.
e.g. Mouth, Tongue, Pharynx, Esophagus, Vagina and cornea.
2. Keratinized
These layers are found, where the surface of the epithelium remains dry,
This epithelial layer is subjected to the friction, as a result of friction the most superficial layers are constantly being removed and are replaced by proliferation of cells from the basal or germinal layer. Due to this physiological significance mitotic figure is very frequent.
Superficial layers of cells are dead and lose their nuclei.
These cells contain a substance known as keratin, which is a water resistant protein that forms a protective barrier against the destructive forces of the environment.
The keratinized stratified squamous epithelium forms epidermis, which is the outmost layer of the skin. The epidermis is composed of two primary cell types based on origin,
a. The Kerotinocytes -str. basale, str. spinosum, str. granulosum, str. lucidum and str. corneum.
b. The Non-Kerotinocytes - Melanocytes, Merkel cells and Langerhans cells
NEUROEPITHELIUM -Denotes the specialized epithelial cells in certain locations, which are concerned in reception of sensory stimuli.