Usually by the tenth to fourteenth day after the first day of menstruation only one follicle has continued its growth to become a fully mature graafian follicle (fig. 20.30). Other secondary follicles during that cycle regress and become atretic—a term that means "without an opening" in reference to their failure to rupture. Follicle atresia, or degeneration, is a type of apoptosis that results from a complex interplay of hormones and paracrine regulators. The gonadotropins (FSH and LH), as well as various paracrine regulators and estrogen act to protect follicles from atresia. By contrast, paracrine regulators that include androgens and FAS ligand (chapter 15) promote atresia of the follicles.
The follicle that is protected from atresia and that develops into a graafian follicle becomes so large that it forms a bulge on the surface of the ovary. Under proper hormonal stimulation, this follicle will rupture—much like the popping of a blister— and extrude its oocyte into the uterine tube in the process of ovulation (fig. 20.31).
The released cell is a secondary oocyte, surrounded by the zona pellucida and corona radiata. If it is not fertilized, it will degenerate in a couple of days. If a sperm passes through the corona radiata and zona pellucida and enters the cytoplasm of the secondary oocyte, the oocyte will then complete the second meiotic division. In this process, the cytoplasm is again not divided equally; most remains in the zygote (fertilized egg), leaving another polar body which, like the first, degenerates (fig. 20.32).
Changes continue in the ovary following ovulation. The empty follicle, under the influence of luteinizing hormone from the anterior pituitary, undergoes structural and biochemical
(graafian) Primary Germinal follicle follicle epithelium
Growing Corpus primary albicans follicle
■ Figure 20.30 An ovary containing follicles at different stages of development. An atretic follicle is one that is dying by apoptosis. It will eventually become a corpus albicans.
Fimbria of uterine tube
Fimbria of uterine tube
■ Figure 20.31 Ovulation from a human ovary. Notice the cloud of fluid and granulosa cells surrounding the ovulated oocyte.
changes to become a corpus luteum (= yellow body). Unlike the ovarian follicles, which secrete only estrogen, the corpus luteum secretes two sex steroid hormones: estrogen and progesterone. Toward the end of a nonfertile cycle, the corpus luteum regresses to become a nonfunctional corpus albicans. These cyclic changes in the ovary are summarized in figure 20.33.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.