The Ovarian Cycle and Ovulation

The Follicular Stage

During a girl’s fetal life, the ovaries contain hundreds of thousands of eggs, which are scattered around in the connective tissue of the ovary. Most of these eggs disappear prior to birth. As she grows older some of them are surrounded by specialized cells that eventually form a capsule called the follicle, much like the shell of a hen’s egg. As sexual maturity approaches, the pituitary gland, under the stimulation of GnRH from the hypothalamic part of the brain, begins to secrete follicle stimulating hormone, FSH.

Under the influence of FSH, some of the follicles now develop a fluid, and those that are near the ovarian surface may bulge through the surface. These are called graafian follicles. In an entirely random manner, one or, at most, a few grow far larger than the rest. This growth begins during menstruation, and is called the follicular phase of the ovarian cycle. Follicles that do not grow simply shrivel and disappear. The follicular stage of the ovarian cycle, which takes place simultaneously with the menstrual and proliferative phases of the endometrial cycle, is variable in length.

Ovulation

When a certain level of growth is reached, the graafian follicle falls under the influence of another hormone from the pituitary gland ­ luteinizing hormone or LH. At midcycle there is a surge of LH, which follows the peak of estrogen secretion by about 24 hours-see the graph on page 15. LH stimulates further development of the follicle until, within 12 to 36 hours of the LH surge, it bursts through the surface of the ovary and pops the egg with its surrounding mantle of follicle cells into the waiting folds of the fallopian tubes. This is called ovulation. From the estrogen peak to actual ovulation is considered the ovulatory phase of the ovarian cycle.

In humans, ovulation is totally independent of sexual intercourse, occurring with equal frequency in the sexually developed virginal woman and the sexually active woman. If fertilization does not occur obviously its occurrence is relatively infrequent the tiny unfertilized egg quickly dies and fragments into many pieces, which white blood cells then eat up.

The Luteal Phase

Following ovulation, the follicle walls fall inward and, under the influence of LH, the follicle changes into the corpus luteum. The corpus luteum secretes some estrogen, but more progesterone. This phase of the ovarian cycle is termed the luteal phase. The luteal or progesterone­dominant phase of the ovarian cycle corresponds in its timing to the secretory phase of the endometrial cycle. It is relatively constant in length, varying from 12 to 16 days in all women.

When a woman becomes pregnant, the corpus luteum will remain viable for several months, secreting progesterone necessary to maintain the pregnancy. The corpus luteum is maintained by human chorionic gonadotropin (hCG). (In medical literature, a lower case “h” is often used to distinguish naturally occurring human hormones from synthetic ones.) Human chorionic gonadotropin is a hormone secreted first by the fertilized egg, starting about 6 days after fertilization, and later by the placenta. The chorion is the layer of the embryo from which the placenta and the outer of the two fetal membranes develop.

When a woman does not become pregnant-which is true for most of her cycles-the corpus luteum will regress, progesterone levels will then drop, and the menstrual flow will begin.

The elegant interplay and miraculous coordination of multiple mechanisms to make reproduction work successfully is a source of cease­less amazement. Many more eggs are fertilized than babies born. Fertilization, implantation, and early development of the ovum are each so complex that something frequently goes wrong and further growth ceases. If this occurs within the first 10 to 12 days, the menstrual period is not even late. If development of the ovum is arrested, say, around the fifteenth day, the menses are delayed a week or so but no recognizable tissue is passed. However, if pregnancy progresses several weeks before it comes to a halt, a discernible miscarriage occurs. Some experts believe ,hat at least 40 to 50 percent of fertilized eggs never end up as a baby.