Reproductive System

The ovaries are oval-shaped and about 1-1.5 in (2.5-3.8 cm) long. They are connected to the body of the uterus by an ovarian ligament that tethers the ovaries in place. The ovaries parallel the testes in that they release sex hormones and develop gametes (ova or sperm). However, the job of the ovaries differs from that of the testes: while sperm are created daily through a man's life after puberty, all of a female fetus's eggs have been created by the sixth gestational month. Several million primordial follicles capable of forming ova are formed. About 1 million primordial follicles mature into primary follicles that still exist at birth. (The rest have degenerated.) When puberty begins, about 400,000 follicles remain. Mature eggs leave alternating ovaries monthly beginning in puberty in a process called ovulation. Unfertilized eggs are lost through menstruation, when the uterine lining is shed. Women typically menstruate for 30-40 years losing 360-480 eggs in a lifetime. Ovulation is hormonally suppressed during pregnancy and shortly after childbirth.

The formation of mature ova in the ovaries is called oogenesis. The anterior pituitary (AP) hormones LH and FSH, which regulate spermatogenesis, also orchestrate oogenesis. However, unlike spermatogenesis which occurs daily, oogenesis is on an average 28 day (or monthly) cycle. During embryonic development, primordial follicles are formed, each of which contains an oocyte surrounded by a layer of spindle-shaped cells. These spindle cells multiple during the mid-fetal stage of development and become granulosa cells which surround the egg. Granulosa cells function much like the Sertoli cells in men: they prevent destructive drugs from getting to the egg while also providing essential nutrients for its development. Granulosa cells also secrete a rich substance that forms a follicular coating called the zona pellucida. Before birth, the cellular layers surrounding the follicle differentiate into a layer of cells called the theca interna. At birth, a baby girl's ova are suspended at the first meiotic division inside the primary follicles. After the onset of puberty, a new follicle enters the next phase of follicular growth monthly.

The first two weeks of the menstrual cycle are called the follicular phase because of the follicular development that occurs during that time. High FSH levels trigger this development. Although more than one follicle begins to mature each month, one follicle outgrows the others, and slow-growing follicles stay in the ovary to degenerate by a process called atresia. The granulosa cells of the dominant follicle secrete estrogens into the fluid bathing the oocyte inside the follicle. The highly vascular theca interna layer, which is outside the granulosa cells, releases estrogens which enter the female circulation. A build up of circulating estrogen will signal release of additional FSH and LH that initiate the second half of the menstrual cycle. Around day 14 of the cycle, LH and FSH surge to initiate ovulation. Ovulation entails the release of the mature oocyte from the ovarian follicle as it ruptures from the surface of the ovary into the abdominal cavity. Once released, the ovum is caught by the fimbria, which are finger-like projections off the ends of the fallopian tubes. The follicle that housed the growing egg remains in the ovary and is transformed into the corpus luteum. The corpus luteum secretes high levels of progesterone and some estrogen. The corpus luteum secures a position near the ovarian blood vessels to supply these hormones which prevent another follicle from beginning maturation. If the ovum is fertilized, then these hormone levels continue into pregnancy to prevent another cycle from beginning. However, if fertilization does not occur, then the corpus luteum degenerates allowing the next cycle to start. The second 14 days of the menstrual cycle are called the luteal phase because of the corpus luteum's hormonal control over this half of the cycle.