Ovary

The ovary has two distinct functions: germ cell production and steroid hormone biosynthesis. Germ cell support occurs in microscopic structures known as ovarian follicles. Resting follicles each contain a primitive or primordial oocyte surrounded by a single layer of cells, the granulosa cells. Surrounding the granulosa cells are a collar of cells known as theca cells. Theca cells produce androgens that are then converted to estrogens by the granulosa cells (Chapter 2). Steroid hormones produced by the ovary act within the follicle to support the developing oocyte and outside the ovary on target tissues.

The human ovary contains about 2 million oocytes at birth but only 100 000 at puberty. The number of oocytes continues to decrease throughout a woman’s reproductive lifespan. This decrease occurs because mitosis of the primitive oogonia stops midway through fetal life and does not resume. At the time mitosis stops, the newly formed oocytes enter into the prophase of the first meiotic division. They will remain in meiotic prophase until either they are stimulated to mature for ovulation or they degenerate in a process called atresia.

The primordial follicles are scattered just beneath the connective tissue capsule covering the ovary (Fig. 10.1). This superficial position permits ovulation into the abdominal cavity. The earliest signs of follicular growth are: (i) an increase in size of the oocyte; (ii) a change in the shape of the surrounding granulosa cells from flat to cuboidal; (iii) an increase in granulosa cell number; and (iv) the appearance of a zona pellucida around the oocyte. The zona pellucida is a sphere of gelatinous protein matrix immediately surrounding the oocyte. Once growth of the granulosa cells has produced three to four layers of cells, fluid begins to accumulate between the cells. This fluid resembles blood plasma and contains high concentrations of several protein and steroid hormones. When this follicular fluid accumulates around the oocyte, the follicle is known as a Graafian follicle and is approaching ovulation. Although as many as 20 follicles begin to mature in each wave of recruitment, typically only one successfully ovulates.

Ovulation involves expulsion of the egg through a thinned-out area known as the stigma. Stigmata can be seen with the naked eye as “blisters” on the surface of the ovary. Once the oocyte is released, the follicle collapses and the granulosa cells proliferate to fill the space left by the oocyte and its associated follicular fluid. They undergo transformation into plump, endocrinologically active cells known as lutein cells. These lutein cells produce a yellow pigment and the structure containing these cells is appropriately called the corpus luteum, or yellow body. During corpus luteum formation, blood vessels penetrate the follicular basement membrane.