Updated About encyclopedia.com content Print Article Share Article
views updated

ovaries The ovaries lie one each side of the peritoneal cavity, just above the brim of the pelvis. Each is close to the open, fringed end of a Fallopian tube, leading to the uterus which lies between and below them. The ovaries are whitish and ovoidal (3 cm × 2 cm × 1 cm) and weigh 5–8 gm. At birth each ovary contains about 200 000 eggs, each enclosed in a single layer of cells: these are the primordial follicles. Unlike the testes, where there is continuous production of sperm after puberty and throughout life, the ovaries do not produce any more eggs and women are born with all the eggs they will ever have.

Each ovary is enclosed in a tough, fibrous capsule and consists of an outer cortex and an inner medulla. The cortex is essentially the working part of the ovary containing all the follicles and remains of ruptured follicles embedded in vascular fibrous tissue. The medulla is where the blood vessels, lymphatics, and nerves enter the ovary. The appearance of the ovaries varies with the age of the woman. Before puberty they are smooth and rather solid in consistency. Between puberty and the menopause their surface becomes more corrugated in appearance due to the activity during each monthly ovarian cycle. After the menopause the ovaries shrink and by then are covered with scar tissue where month after month a follicle has ruptured and released its mature egg.

Development of the follicles

Spontaneous growth of primordial follicles to primary and secondary follicles continues from birth to the menopause though many of these follicles never get past the first stage of development. The first stage is characterized by an increase in the size of the primary oocyte although meiosis is not reactivated. As the oocyte enlarges the cells surrounding it (granulosa cells) divide rapidly and produce a protein material that forms a coating round the oocyte known as the zona pellucida. Contact between the oocyte and the granulosa cells is maintained by cytoplasmic processes of the granulosa cells that penetrate this barrier. At the same time ovarian cells condense on the outside of the follicle wall and form a loose network of spindle shaped cells known as the thecal cells. They are separatd from the granulosa cells by a basement membrane. This first phase of follicular development is independent of any hormanal stimulation. Nothing further can happen without the increased secretions of luteinizing hormone (LH) and follicle stimulating hormone (FSH) from the pituitary gland that occur at puberty and normally initiate cyclical ovarian activity and menstruation. At a critical stage of development the secondary follicle becomes sensitive to FSH and LH, enabling follicles to continue their (hormone-dependent) development. This is thought to occur over a period of three menstrual cycles during which time many of these follicles will degenerate. Of those that enter their final development stage at the beginning of the last of these cycles only one will normally be selected and recruited for releasing its egg. This is known as the dominant follicle. The rest simply degenerate, the eggs die and the follicle is invaded by white blood cells and becomes scar tissue. At the beginning of this second phase of development the ‘thecal’ cells become richly supplied with blood vessels and the underlying layer of ‘granulosa’ cells, which surround the egg itself, rapidly divide. LH stimulates the thecal cells to produce progesterone and androgens; the androgens diffuse into the inner layer where the granulosa cells convert them to oestrogens under the influence of FSH. As the follicle grows it secretes increasing concentrations of oestrogen, and the inner part of the follicle becomes filled with fluid (follicular fluid); at this stage is called a Graafian follicle (after de Graaf, a Dutch anatomist and physician who described the maturing ovarian follicle in a major work on the female reproductive cycle published in 1672 — a year before he died of the plague aged 32).

The dominant follicle and ovulation

At the beginning of each menstrual cycle several follicles will have reached a certain size and maturity that would allow them to undergo their final stage of development so that a fully mature ovum could be released mid-cycle. For some unexplained reason normally only one follicle becomes more successful than the others and towards mid-cycle this dominant Graafian follicle rises to the surface of the ovay. At mid-cycle the granulosa cells become responsive to LH and a surge of LH secretion circulating in the blood from the pituitary gland reaches the ovary via the circulation. This LH surge stimulates the egg (the primary oocyte) to complete its first meiotic division into secondary oocytes. This means that the primary oocyte, containing 46 chromosomes, produces two daughter cells, each with 23 chromosomes, one of which is the X (female) sex chromosome. The second meiotic division is initiated but (unlike sperm) is not completed until and unless fertilization has taken place. Under the further influence of the LH surge, the mature follicle now ruptures and releases the egg, enclosed in its encircling cell layers (the cumulus oophorus), into the Fallopian tube.

The corpus luteum

The cells left behind in the lining of the ruptured follicle then proliferate rapidly, forming a corpus luteum (from Latin, ‘yellow body’). Under the influence of LH and FSH the corpus luteum grows and produces progesterone. Unless fertilization occurs, it begins to degenerate after about 10 days, along with the decline in pituitary gonadotrophin secretion towards the end of the cycle; it becomes a fibrotic ‘corpus albicans’, and finally all that is left of the follicle's life is scar tissue. If fertilization and pregnancy ensue, the corpus luteum is saved from decay by gonadotrophins secreted by cells of the implanted embryo itself, and its secretion of progesterone in turn maintains the requisite uterine conditions for early pregnancy. This ovarian secretion remains vital for the pregnancy in the first three months, until the fully developed placenta takes over the secretion of progesterone and other hormones which support gestation.

At the menopause the ovaries become completely redundant. The remaining egg-containing follicles, which have been inactive since birth, simply degenerate, and all that is left of past activity in the ovaries are the scars from dominant follicles which have reached full maturity, ruptured, and released an egg for possible fertilization.

Saffron Whitehead

See urogenital system.See also menstrual cycle; ova; sex hormones.