The hormones of female reproduction
Normal reproduction in the female is controlled by numerous hormones secreted from specialized glands called endocrine glands. The hormones are chemical substances developed by specialized tissues often acting at a distance and at very low concentrations on specific tissues to produce specific effects. As mentioned above these hormones are produced by the pituitary, the ovary and its structures, and the uterus. The hormones pass into the blood and lymph systems for transport to specific parts of the body where they produce their function. Reproductive hormones may originate in hypothalamus, pituitary, ovary, uterus, or placenta.
The ovary by means of Graafian follicle produces estrogens, of which estradiol is the most important in the dairy cow. Estradiol has several effects:
The development and function of the secondary sex organs (mammary gland);
The onset of behavioral estrus, i.e., the period of sexual receptivity (commonly referred to as heat);
The rate and type of growth, especially deposition of fat;
The development of prepuberal heifers and the regulation of post-partum cow for onset of cyclic sexual activity.
After ovulation the remaining cells in the follicle form the corpus luteum, which produces the hormone progesterone. Progesterone suppresses the further development of follicles and the secretion of estrogens. High levels of progesterone and low levels of estrogens are responsible for stopping the sign of estrus. Progesterone is necessary for preparing the lining of the uterus to receive the fertilized ovum and then for the implantation of the fetus, maintaining the lining of the uterus during pregnancy.
Estradiol and progesterone are not completely separate in their functions since both must be present for certain processes to occur. For example, estradiol/progesterone concentration ratio dictates the onset and duration of behavioral estrus. Development of the uterus is initiated by estradiol and completed by progesterone. Estradiol causes contraction of the uterus near the time of estrus and ovulation, which help in sperm transport. Progesterone has a quieting effect on the uterus so that there are no contractions which might disturb pregnancy. If fertilization takes place, the corpus luteum and the production of progesterone persist throughout pregnancy; otherwise, the corpus luteum regresses and another follicle develops.
The production of ovarian hormones is under direct influence of gonadotropic hormones produced by the anterior portion of the pituitary. Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are secreted from the pituitary and travel through the blood to the ovary. The release of FSH and LH is mediated by gonadotropin-releasing hormone (GnRH) produced by the hypothalamus to signal FSH and LH release from the pituitary. The hypothalamus is part of the brain and is located just above the pituitary gland. The release of FSH stimulates the growth, development and function of the follicle, while LH cause the follicle to rupture during ovulation and causes the subsequent development of the corpus luteum, and initiates progesterone production.
Many body tissues, but especially the uterus, produce the prostaglandins. The prostaglandin primarily affecting the cow's estrous cycle is prostaglandin F2α (PGFα), which is produced by uterus. PGF2α causes the natural regression of the luteal cells of the corpus luteum at the end of the estrous cycle or pregnancy. This effect allows a new estrous cycle to be initiated in the nonpregnant cow. In a pregnant cow, a signal is sent from the developing embryo to the uterus to prevent PGF2α release, which allows the corpus luteum to persist throughout pregnancy. Maternal recognition of pregnancy is believed to occur between days 16 and 17 after fertilization. Injecting cows or heifers with PGF2α between days 6 and 16 of the estrous cycle will cause premature regression of the corpus luteum, with the best results achieved among females injected on days 10 to 16.
Injecting PGF2α during the first five (1-2) and last five (17-21) days of the estrous cycle will generally not cause luteal regression. The luteolytic response allows the use of PGF2α in estrus synchronization programs in cow herds and initiates abortion in feedlot heifers.
The posterior portion of the pituitary produces the hormone oxytocin, which causes the contraction of the uterine wall to expel the calf at parturition. The oxytocin plays a role in the contraction of the myoepithelial cells during milk ejection.
Near the end of gestation the ovary produces the hormone relaxin. The relaxin is responsible for expansion of the uterus and relaxation of the cervix in preparation for parturition.