The Female Reproductive System

Oogenesis and the sexual cycle.

    At the beginning of puberty, around the age of 11 to 15 years old, the woman becomes fertile. From that moment, the woman's body goes into a cycle of about 28 days (the sexual or menstrual cycle). At each of these cycles (around the middle of the cycle), one of her ovaries release an egg that is ready to be fertilized. If the egg is fertilized, the woman will become pregnant, and the cycle will stop momentarily until delivery of the baby, and for few more months in woman who lactate, and will resume thereafter. This cycle will repeat until menopause (usually in the fifth decade), and the woman will then cease to be fertile.

    The eggs develop in the ovaries of women from primordial follicle cells (primary oocytes). At birth, the baby girl already has 750.000 of these cells distributed in both ovaries. But this number decreases rapidly, reaching 400.000 at puberty. Of these, only 450 will develop into egg and will be expelled into the fallopian tubes, and the other will degenerate. Each month, only one egg is normally released, and each month, the uterus prepares to receive the fertilized egg. If the egg is not fertilized, the uterus peels (which produces menstruation), and the cycle will repeat.

    The development of the egg is due to complex hormonal processes. When the woman reaches the age of 8 years old, the anterior pituitary gland (located at the base of the brain) gradually begins to secrete follicle stimulating (FSH) and luteinizing (LH) hormones. Around the age of 11 to 15 years old, the blood levels of these hormones are sufficient to initiate the sexual cycle. The figure below shows the changes in hormone levels and their relationship with the sexual cycle.

Hormonal cycle in woman.

     Each month, the follicle stimulating hormone stimulates the development of about 20 follicles. First, the eggs grow and become surrounded by several layers of cells that form a theca around the egg (primary follicle). Then, it is mainly these cells of the theca that will secrete the estrogen and progesterone. Shortly before ovulation, one of these follicles will start to grow, at the expense of others that will degenerate. Meanwhile, stimulated by estrogen and progesterone, the endometrium of the uterus grows and prepares to receive the fertilized egg.

    About 14 days after the onset of menstruation, an increased release of luteinizing hormone stimulates the follicle to fully developed and grow further. Then, the growing pressure within the follicle makes it explode and expel the egg that was inside. The role of the luteinizing hormone is to reduce the production of estrogen and increase the progesterone secretion by the empty follicle, which then becomes the corpus luteum.

    The corpus luteum survives about ten days if the egg is not fertilized. If the egg is fertilized however, then the corpus luteum will survive all the pregnancy time through chorionic gonadotropin secretion by the chorion. If the egg is not fertilized, the corpus luteum will degenerate into a white body and will stop producing progesterone. Without progesterone the uterus, which was stimulated, starts to degenerate, resulting in menstruation.

Sexual Act.

    As the man's penis, the clitoris of the woman is also an erectile organ and, as the man's penis, it is a very sensitive organ. But, unlike the penis, the clitoris is not the main reproductive organ of the female. Rather, it is the vagina that would be the main female reproductive organ, it is through the vagina that a woman can receive the seed from the man. Another important difference between men and women is that women do not really need to have an orgasm to be fertilized. Nonetheless, like the man, the woman is able to reach orgasm and experience sexual pleasure.

    As for man, the perineal region of women is sensitive to stimulation. These regions include the pubis, the clitoris, the labia majora and minora, the anal and urinary tracts, and the vaginal mucosa. The impulses produced by these stimuli travel through the pudendal nerve and other nerves of the sacral plexus to the spinal cord. Although we do not know very well what region of the brain translates sexual pleasure, we know that these feelings produce a reflex, at the sacred level of the spinal cord, and this produces the erection of the clitoris. It is the impulses from the parasympathetic nervous system which cause dilation of arteries and constriction of veins of the erectile bodies of the clitoris. This causes an increase in blood supply, increase in pressure and erection of the clitoris.

    These impulses are also involved in muscle constrictions which tighten the vaginal opening around the man's penis and stimulate him further. At the same time, the vestibular glands are stimulated to secrete a lubricant mucus to facilitate penetration.

    When stimulation is at its climax, the woman reaches an orgasm which is somewhat similar to that of man, in that it is accompanied by contractions of the genitals, and, at times, abundant secretions. While the orgasm is not essential for the fertilization of the ovum, it is believed that it can help. Indeed, it is possible that the vaginal and uterine contractions facilitate the movement of sperm. Likewise, uterine and vaginal secretions may offer a favorable environment for the movement of sperm.