Spermatogenesis begins with spermatogonia that occupy the basal tissue of seminiferous tubules. These sperm precursors contain 46 chromosomes and replicate by mitosis, maintaining a constant supply of spermatogonia. Some of these cells are stimulated to enlarge to become primary spermatocytes, where they undergo meiosis. The first stage of meiosis splits chromosomes into identical chromatids. The second stage then further splits the chromatids into variant sets of 23 chromosomes that allow diversity among the genetic material contained in sperm cells. After undergoing meiosis, spermatocytes first differentiate into spermatids, then develop into mature sperm cells.
In the male, follicle-stimulating hormone (FSH) first stimulates production of spermatogonia in the testes. Luteinizing hormone (LH) later triggers the release of testosterone, which produces sex characteristics and begins the process of spermatogenesis by causing primary spermatocytes to undergo meiosis.
Oogenesis begins before birth with the division of cells in the ovaries by mitosis and subsequent growth to produce primordial follicles. The primary oocytes in these follicles then begin to degenerate so that of the several million that are formed before birth, only about 400,000 are present at puberty when the ovaries begin to undergo continual meiosis. Meiosis begins with unequal division of primary oocytes to produce a large secondary oocyte and a tiny polar body. The secondary oocyte then contains 23 chromosomes and the polar body degenerates to relieve the egg cell of excess cell material that would otherwise hamper the development of a zygote. Second meiosis occurs only upon entrance of a sperm, where the egg forms a second polar body to remove further cell material and chromatids diversify.
The epididymis receives sperm cells for maturation. Sperm cells gradually travel the length of the epididymis, which contains cells that release nourishing glycogen and other substances to promote maturation. Once matured, sperm are stored in the epididymis as well. Upon ejaculation, sperm cells leave the epididymis and are forced up through the vas deferens to the ejaculatory duct, where seminal vesicles release a slightly alkaline substance containing fructose, which provides energy for the sperm, and prostaglandins, which trigger muscular contractions in the female reproductive tract that aid sperm movement. From there, the mixture of sperm and seminal fluid travels to the prostate gland, which adds a milky, alkaline fluid to neutralize the slightly acidic sperm-containing fluid and enhance the motility of the recently non-motile sperm.
Spermatogenesis and oogenesis are quite differentfirstly in their time of occurrence. Oogenesis begins before birth with the growth of ovarian cells into follicles. Some meiosis takes place just after birth. Spermatogenesis, on the other hand, does not begin until puberty, although the precursors to sperm cells are present in the testes before that time. Once begun, however, spermatogenesis produces millions of mature sperm cells, while oogenesis produces only a few potential egg cells at a time. These cells must undergo further maturation and ovulation before they are able to be fertilized. Finally, products of spermatogenesis are stored and released in large quantities upon ejaculation, while products of oogenesis mature only a few at a time, and typically only one mature egg cell is released for fertilization.
Upon arriving at the egg, sperm cells attach themselves to the outermost layer, the zona pellucida, and release an enzyme from the acrosome on their heads to begin dissolving this tough outer layer. Gradually, the layer thins and a sperm cell is able to penetrate to the underlying egg cell. Once in contact with the sperm, the egg cell triggers cortical granules to release enzymes to harden the zona pellucida again to prevent more sperm from entering. The chromosome-containing nucleus of the fertilizing sperm cell then swells and unites with the nucleus of the egg cell to form a complete set of 46 chromosomes and complete the process of fertilization.
Human chorionic gonadotropin is a hormone secreted by the trophoblast layer of an embryo that keeps the corpus luteum from regressing while inhibiting secretion of LH and FSH. The combined effect is that the corpus luteum continues excreting estrogen and progesterone to maintain the lining of the uterine wall so the embryo is not aborted, and the lack of LH and FSH stops the menstrual cycle and allows the uterus to grow beyond normal cycle levels to support a fetus.
The placenta is a disk-shaped conglomeration of blood vessels where fetal and maternal blood mix and exchange nutrients and wastes are carried away from the fetus. The placenta is largely able to protect an infant from disease and toxins by acting as a filter as well. Finally, the placenta replaces the function of hCG hormone by secreting large amounts of estrogen and progesterone after the first trimester of pregnancy to keep the uterus growing to accommodate the fetus.