Reproduction
The primary reproductive organs (ovaries and testes) arise from the bipotential indifferent gonads in fetal development. Ovaries develop automatically unless a protein coded for on the Y chromosome is present. The female internal accessory organs (Fallopian tubes, uterus, and inner vagina) arise from the embryo.
Male Reproductive Function
Spermatogenesis The male gonads, the testes, produce sperm in the somniferous tubules and secrete testosterone from the Leydig cells. The meiotic divisions of spermatogenesis result in each sperm containing 23 chromosomes compared to the original 46 of the spermatogonia.
Transport of sperm From the somniferous tubules, the sperm pass through the epididymis, where they mature and are concentrated. The epididymis and vas deferens store the sperm, and the seminal vesicles and prostate secrete the bulk of the semen. Erection of the penis occurs because of vascular engorgement accomplished by relaxation of the arterioles and passive occlusion of the veins. Ejaculation includes emission emptying of semen into the urethra--followed by expulsion of the semen from the urethra.
Hormonal control of male reproductive function Hypothalamic GnRH stimulates the anterior pituitary to secrete FSH and LH, which then act on the testes: FSH on the Sertoli cells to stimulate spermatogenesis and inhibin secretion, and LH on the Leydig cells to stimulate testosterone secretion. Testosterone exerts a negative-feedback inhibition on both the hypothalamus and the anterior pituitary to reduce LH secretion. Inhibin exerts a negative-feedback inhibition on FSH secretion. Testosterone, acting locally on the Sertoli cells, is essential for maintaining spermatogenesis. Testosterone also maintains the accessory reproductive organs and male secondary sex characteristics and stimulates growth of muscle and bone. In many of its target cells it must first undergo transformation to dihydrotestosterone or (in the brain) to estradiol.
Female Reproductive Function
Ovarian functions The female gonads, the ovaries, produce ova and secrete estrogen, progesterone, inhibin, and relaxin. The two meiotic divisions of oogenesis result in each ovum having 23 chromosomes, in contrast to the 46 of the original oogonia. The follicles surrounding the ova consist of inner layers of granulosa cells and outer layers of theca cells. At the beginning of each menstrual cycle, several follicles begin to develop into antral follicles, but soon only the largest (dominant) follicle continues its development to full maturity and ovulation. Following ovulation the remaining cells of that follicle are transformed into a corpus luteum, which lasts about 10 to 14 days if pregnancy does not occur. The menstrual cycle can be divided, according to ovarian events, into a follicular phase and a luteal phase, which are approximately 14 days each and separated by ovulation.
Control of ovarian function During the early and middle follicular phases, FSH stimulates the granulosa cells to proliferate and secrete estrogen, and LH stimulates the theca cells to proliferate and produce the androgens that the granulosa cells use to make estrogen. During this period, estrogen exerts a negative feedback on the hypothalamus and anterior pituitary to inhibit the secretion of GnRH and the gonadotropins. During the late follicular phase, plasma estrogen becomes high enough to elicit a surge of LH, which then causes, via the granulosa cells, completion of the oocyte's first meiotic division and cytoplasmic maturation, ovulation, and formation of the corpus luteuin. During the luteal phase, under the influence of small amounts of LH, the corpus luteum secretes progesterone and estrogen. Regression of the corpus luteum results in a cessation of the secretion of these hormones.
Uterine changes in the menstrual cycle The ovarian follicular phase is equivalent to the uterine menstrual and proliferative phases, the first day of menstruation being the first day of the cycle. The ovarian luteal phase is equivalent to the uterine secretory phase. Menstruation occurs when the plasma progesterone and estrogen levels fall as a result of the regression of the corpus luteum. During the proliferative phase, estrogen stimulates growth of the endometrium and myometrium and causes the cervical mucus to be readily penetrable by sperm. During the secretory phase, progesterone converts the estrogen-primed endometrium to a secretory tissue and makes the cervical mucus relatively impenetrable to sperm.
Pregnancy After ovulation, the ovum is swept into a uterine tube, where a sperm, having undergone capacitation and activation in the female tract, fertilizes it. Following fertilization the ovum undergoes its last meiotic division, and the nuclei of the ovum and sperm fuse. Reactions in the ovum block penetration by other sperms and trigger cell division and embryogenesis. The zygote undergoes cleavage, becoming a blastocyst that implants in the endometrium on approximately day 7 after ovulation. The trophoblast gives rise to the fetal part of the placenta whereas the inner cells mass develops into the embryo proper. Although they do not mix fetal blood and maternal blood both flow through the placenta, exchanging gases, nutrients, and waste products. The fetus is surrounded by amniotic fluid in the amniotic sac. The progesterone and estrogen required maintaining the uterus during pregnancy come from the corpus luteum for the first two months, their secretion stimulated by chorionic gonadotropins produced by the trophoblast. During the last 7 months of pregnancy, the corpus luteum is not important because the placenta itself produces large amounts of progesterone and estrogen. The high levels of progesterone, in the presence of estrogen, inhibit the secretion of GnRH and, thereby, that of the gonadotropins, so that menstrual cycles are eliminated. Parturition occurs by rhythmical contractions of the uterus, which first dilate the cervix and then move the infant, followed by the placenta, through the birth canal. The contractions are stimulated by a decrease in the progesterone to estrogen ratio (progesterone inhibits and estrogen stimulates contractions), by oxytocin released from the posterior pituitary in a reflex triggered by uterine mechanoreceptors, and by uterine prostaglandins.
The breasts develop markedly during pregnancy as a result of the combined influences of estrogen, progesterone, prolactin, and placental lactogen. Prolactin secretion is stimulated during pregnancy by estrogen acting on the anterior pituitary, but milk is not synthesized because high concentrations of estrogen and progesterone inhibit the milk-producing action of prolactin on the breasts. As a result of the suckling reflex, large bursts of prolactin and oxytocin occur during nursing, the oxytocin causing milk ejection.
