During an ovarian cycle, a pregnancy can occur if the ovum becomes fertilized. The events that occur during pregnancy are also controlled by hormones.
In mammals, fertilization normally occurs in the oviduct. When the fertilized embryo is implanted in the uterus, it is nourished as a result of the glandular and vascular buildup of the uterine wall. This buildup is maintained by estrogen and progesterone secreted by the corpus luteum. As the embryo grows, it begins to produce a portion of the placenta; the other portion is produced by the uterus. Cells in the embryonic chorion of the placenta produce cho-rionic gonadotropin, which "rescues" the corpus luteum and prevents its becoming deactivated by a prostaglandin produced by the ovary as a signal to deactivate the corpus luteum if fertilization has not occurred. During the latter stages of a pregnancy, the placenta takes over the production of estrogen and progesterone in some species. During pregnancy, chorionic gonadotropin will be present in the urine. During the last days of pregnancy, the ovary begins to produce a new hormone called relaxin. Relaxin is a very ancient hormone; it has been found in sharks and birds as well as mammals. Relaxin softens the ligaments connecting the bones in the birth canal. It also stimulates uterine contractions. The uterine contractions begin the process of labor, resulting in birth. The signal for the onset of labor is a fetal hormone, cortisol. Cortisol crosses the placenta to the maternal blood, where it causes the synthesis of another prostaglandin. The strongest support for this conclusion comes from experiments in which the adrenal glands were removed from fetal goats; the result of such in utero surgery is significantly delayed parturition (birth). This uterine prostaglandin intensifies the uterine contractions to initiate labor and thus parturition.
Following birth, the suckling of the infant at the mother's mammary glands stimulates the release of the posterior pituitary hormone oxytocin. Oxytocin stimulates contraction of muscles in the mammary glands to eject milk. Oxytocin also stimulates uterine contraction to pass the placenta (afterbirth) through the vagina. Prolactin, which stimulates mammary development during pregnancy, has nothing to do with milk ejection during lactation. Prolactin is also present in lower vertebrates that lack mammary glands. In fish, prolaction stimulates sodium conservation. In amphibians, prolactin decreases skin water permeability and increases sodium uptake across the skin. In some salamanders, prolactin initiates "water drive," which is the migration of adults back to water for reproduction. In pigeons and similar birds, prolactin stimulates the production of crop-sac milk, which the mother regurgitates to feed her young; a process remarkably similar, in effect, to lactation.
directly to blood calcium concentrations. When calcium levels are low, parathyroid hormone (PTH) is secreted into the blood to stimulate three centers. In bone, PTH mobilizes calcium to elevate blood levels of this ion. Because mobilization of bone also elevates phosphate, which can be toxic at high concentrations, the kidneys become important. PTH stimulates the kidneys to increase calcium conservation and potassium excretion. PTH also stimulates uptake of calcium in the small intestine. Vitamin D enhances the action of PTH. Working antagonistically to PTH, calcitonin, produced in the thyroid gland, responds directly to high blood calcium to move this ion into bone.
The digestion and assimilation of food are also controlled by hormones. In meat-eating animals, beginning in the stomach, stretch and the presence of protein stimulate the secretion of gastrin into blood vessels in the wall of the stomach. This gastrin stimulates the secretion of hydrochloric acid into the lumen of the stomach to digest protein. When the partially digested food enters the small intestine for the completion of digestion and assimilation, a slightly alkaline pH is required. The walls of the small intestine detect the acidity and secrete another pair of hormones into blood vessels. Secretin travels to the pancreas and stimulates sodium bicarbonate secretion. The sodium bicarbonate travels through the common bile duct to the small intestine, where it neutralizes the acid. Gastric inhibitory polypeptide travels to the stomach to inhibit acid secretion and stomach contractions. Another peptide, cholecystokinin-pancreozymin (CCKPZ), responds to fats and proteins in the small intestine and is thus secreted into the blood. This hormone travels to the gallbladder, causing it to contract and release its bile through the common bile duct to aid digestion of fats in the small intestine. CCKPZ also stimulates secretion of a whole host of enzymes by the pancreas. These enzymes also move through the common bile duct to the small intestine to aid in the digestion of carbohydrates, fats, and protein. At least two other digestive hormones have been dis covered that are not well understood at present. Motilin is secreted by the small intestine and stimulates stomach muscle contractions. Vasoactive intestinal polypeptide also is secreted by the small intestine and it, in turn, stimulates sodium bicarbonate secretion by the walls of the small intestine. Both hormones are of obvious benefit, but key details of their function, such as what triggers their secretion, are not clearly understood. It is important to realize that all of the hormones of the stomach and small intestine are secreted into the blood vessels in the walls of the organs, not into their lumens.
The two pituitary hormones that are involved in reproduction are called the gonadotropins, FSH and LH. These hormones are identical in males and females. The gonadal hormones differ between the two sexes. Females produce estrogens and progesterone in their ovaries. Males produce androgens (primarily testosterone) in the testes.
The mammalian menstrual cycle has two components. Both the ovarian cycle and the uterine cycle proceed simultaneously and last approximately four days in rats, sixteen days in sheep, and twenty-eight days in humans. The length and pattern of the cycle vary with species. For the sake of comparison, the human cycle is described here. The first five days of each cycle is called the menstrual period, and during this period the built-up walls of the uterus (resulting from the previous cycle) are shed and discharged through the vagina. At this time the concentrations of FSH and LH in the blood are about the same. From the close of the menstrual period until ovulation is the follicular cycle. FSH stimulates the ovaries to begin the growth and maturation of an egg-containing follicle. This follicle produces estrogen. Estrogen feeds back negatively on FSH, causing its levels in the blood to drop. At the same time, estrogen is feeding back positively on LH, causing its levels to rise.
At the midpoint of the ovarian cycle, LH peaks and causes the now mature follicle to burst and eject an egg (ovum) into the oviduct. The ruptured follicle now becomes a corpus luteum and continues to secrete estrogen, but also begins to secrete progesterone. The estrogen, and now the progesterone, stimulate the walls of the uterus to thicken and produce glandular tubes and blood vessels. This goes on for the final half of the cycle, which is called the follicular phase in the ovaries and the proliferative phase in the uterus. If fertilization of the ovum in the oviduct fails to occur during this period, a hormone, probably a prostaglandin, builds up in the corpus luteum, causing it to stop producing estrogen and progesterone. With the loss of these two steroids, the thickened wall of the uterus is shed and the menses flows during the first five days of the next cycle.
Male reproductive endocrinology is much different. The first striking difference is that, although the pituitary hormones FSH and LH are the same, the patterns of secretion are different. Instead of the cyclic peaks found in females, males secrete constant levels of gonadotrophins. FSH stimulates sperm production and maturation in the seminiferous tubules of the testes. LH stimu lates testosterone secretion by the interstitial cells of the testes. Testosterone helps FSH to stimulate sperm maturation. This androgen also stimulates such primary sex characteristics as penis and epi-didymal growth during puberty. The epididymis is a tubular structure that stores sperm in preparation for ejaculation. Testosterone also stimulates secondary sex characters, such as the deepening of the voice and development of muscle mass that manifest during puberty in humans.
—Daniel F. Stiffer See also: Circulatory systems of vertebrates; Courtship; Digestion; Endocrine systems of invertebrates; Fertilization; Gametogenesis; Growth; Hormones and behavior; Hormones in mammals; Kidneys and other excretory structures; Lactation; Mating; Metabolic rates; Muscles in vertebrates; Nervous systems of vertebrates; Osmoregulation; pH maintenance; Pheromones; Reproduction; Reproductive strategies; Reproductive system of female mammals; Reproductive system of male mammals; Sexual development; Thermoregulation; Vertebrates; Water balance in vertebrates.
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