At puberty, the female begins to undergo regular monthly cycles. These sexual cycles are controlled by the hypothalamus. Gonadotropin-releasing hormone (GnRH) produced by the hypothalamus acts on cells of the anterior pituitary gland, which in turn secrete gonadotropins. These hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), stimulate and control cyclic changes in the ovary. At the beginning of each ovarian cycle, 15 to 20 primary (preantral) stage follicles are stimulated to grow under the influence of FSH. (The hormone is not necessary to promote development of primordial follicles to the primary follicle stage, but without it, these primary follicles die and become atretic.) Thus, FSH rescues 15 to 20 of these cells from a pool of continuously forming primary follicles (Fig. 2.1). Under normal conditions, only one of these follicles reaches full maturity, and only one oocyte is discharged; the others degenerate and become atretic. In the next cycle, another group of primary follicles is recruited, and again, only one follicle reaches maturity. Consequently, most follicles degenerate without ever reaching full maturity. When a follicle becomes atretic, the oocyte and surrounding follicular cells degenerate and are replaced by connective tissue, forming a corpus atreticum. FSH also stimulates maturation of follicular (granulosa) cells surrounding the oocyte. In turn, proliferation of these cells is mediated by growth differentiation
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