Give an account of the process of oogenesis.

Outline:

·        Ovarian structure

·        Prepubertal development

·        Phases of follicular development:

- primary follicle

- secondary follicle

- Graafian follicle

·        Ovulation

Essay:

            Oogenesis, the production of the female gametes, oocytes, occurs in the ovaries. The ovary is divided into a central medulla and a peripheral cortex. The medulla is occupied largely by twisting blood vessels and a dense connective tissue stroma. The cortex contains numerous ovarian follicles in various stages of development. In each follicle there is an epithelium of variable thickness enclosed by a basement membrane. The follicular epithelial cells, in turn, surround a single oocyte, which may undergo development that eventually leads to ovulation and the formation of an ovum.

            The primary oogonia cells migrate from the yolk sac of the embryo to the genital ridge at 5 to 6 weeks of gestation. There, in the developing ovary, they produce oogonia by mitotic division until 20 to 24 weeks of gestation, where the total number of oogonia has reached a maximum of 7 million. Beginning at 8 to 9 weeks of gestation, some oogonia start to enter the prophase of meiosis and become primary oocytes. This process continues until 6 months after birth, when all oogonia have been converted to oocytes. From the start of oogenesis, however, a process of oocyte attrition also occurs. By birth, only 1 to 2 million primary oocytes remain, and by the onset of puberty the number falls to 400,000.

            In most immature follicles, called primordial follicles, a single oocyte is surrounded by a single layer of flattened follicular epithelial cells (granulosa cells). As follicles continue to develop, under the influence of FSH, the follicular cells become active and grow into a single cuboidal layer, forming the primary follicles. A hyaline, amorphous extracellular material called zona pellucida then becomes interposed between the oocyte and the follicular epithelium. The follicular cells at this stage have become proliferative and are contributing to a continuously growing layer of follicular cells, and the follicular epithelium becomes multilayered. With the maturation of the follicles, the granulosa cells begin to secrete glycoprotein into the spaces surrounding them. These glycoproteins coalesce into small lakes called Call-Exner bodies, and these in turn fuse to form a single, eccentrically placed cavity called the antrum. The follicle is now known as a secondary follicle.

            The growth of the antrum pushes the oocyte to one side of the follicle where the oocyte remains surrounded by the cumulus oophorus, a mound composed of granulosa cells. At the same time, the follicular epithelial cells are still increasing in number, causing a striking increase in the overall volume of the follicle. The follicle eventually becomes so large that it forms a blister on the surface of the ovary and occupies much of the entire width of the ovary. This type of follicle is called a Graafian follicle.

            Just before ovulation, the oocyte completes its first meiotic division. One of the daughter cells, the secondary oocyte, receives most of the cytoplasm, while the other, the first polar body, fragments and disappears. The secondary oocyte immediately begins the second meiotic division, but his division stops at metaphase and is completed only when a sperm penetrates the oocyte. The cumulus oophorus separates from the granulosa, and the secondary oocyte now floats freely inside the antrum, surrounded by a layer several cells thick, known as the corona radiata.

            In a short period immediately prior to ovulation, the follicle swells rapidly due to a sudden burst of follicular proliferation and an accumulation of follicular fluids. The follicle then ruptures through the wall of the ovary, expelling the oocyte with the attached corona radiata. The oocyte is then swept rapidly into the oviducts by the beating of cilia on the fimbriae. Following ovulation, the remains of the follicles undergoes a process called luteinization, forming a hormone-producing body, the corpus luteum.