Gonadotropic Hormones and Their Effects on the Ovaries
The ovarian changes that occur during the sexual cycle depend completely on the gonadotropic hormones FSH and LH, secreted by the anterior pituitary gland. In the absence of these hormones, the ovaries remain inactive, which is the case throughout childhood, when almost no pituitary gonadotropic hormones are secreted. At age 9 to 12 years, the pituitary begins to secrete progressively more FSH and LH, which leads to onset of normal monthly sexual cycles beginning between the ages of 11 and 15 years. This period of change is called puberty, and the time of the first men-strual cycle is called menarche. Both FSH and LH are small glycoproteins having molecular weights of about 30,000.
During each month of the female sexual cycle, there is a cyclical increase and decrease of both FSH and LH, as shown in the bottom of Figure 81â€“3. These cyclical variations cause cyclical ovarian changes, which are explained in the following sections.
Both FSH and LH stimulate their ovarian target cells by combining with highly specific FSH and LH receptors in the ovarian target cell membranes. In turn, the activated receptors increase the cellsâ€™ rates of secretion and usually the growth and proliferation of the cells as well. Almost all these stimulatory effects result from activation of the cyclic adenosinemonophosphate second messenger system in the cellcytoplasm, which causes the formation of proteinkinase and multiple phosphorylations of key enzymes that stimulate sex hormone synthesis.