OVARIAN STRUCTURES AND DEVELOPMENT
The ovaries develop from a thickening of cells that form ridges medial to the müllerian and wolfﬁan bodies. These germinal ridges appear at the sixth week. Primary oocytes, arising in the umbilical vesicle (yolk sac) and migrating along the mesentery of the hindgut, arrive in the embryonic gonads and are thought to provide the countless thousands of ova that crowd the ovary at birth.
By the third month of gestation, the ovaries descend toward the pelvis. The pull of the gubernaculum—an abdominal fold that grows more slowly than the rest of the fetus—exerts a downward traction on the gonadal ridges. Later, these folds fuse in their midportion with that part of each müllerian duct that develops into the uterine fundus. The lateral half and the medial portion of the folds become the round ligaments and the suspensory ligaments of the ovary, respectively.
The infant ovary is a sausage-shaped structure, with a pale and smooth surface. A gradual thickening and shortening occur throughout the ﬁrst decade of life. The major gain in size and weight takes place after the menarche and during adolescence. Two layers, the germinal epithelium and the tunica albuginea, constitute the surface of the prepubertal ovary. They are crowded with primordial ova that are surrounded by dark-staining cells, the origin of the future granulosa cells. The granulosa cells are polygonal and rather uniform, round, with sharply outlined nuclei in a poorly stainable cytoplasm that contains, however, numerous granules, from which this layer derives its name.
As the primordial follicle develops, it sinks, with its single layer of epithelial cells, toward the center of the ovary. The attendant cells proliferate to form a manylayered coating of granulosa cells around the developing follicle. A crescentic cavity forms eccentrically, in which follicular ﬂuid accumulates. From the surrounding ovarian stroma, a capsule of theca cells differentiates. The theca interna is rich in capillaries, upon which the avascular theca granulosa must depend for nourishment. Before menarche, while still little or no follicle-stimulating hormone is present, these follicles develop no further but degenerate and become atretic.
The mature gonad is an approximately almond-shaped structure, pitted and scarred by the stigmata of ovulation. Spiral arteries enter at the hilum and are involved in sequential changes during the cyclic ebb and sway of follicle growth and development of corpora lutea. In the hilum are also found cells with morphologic and histochemical properties, similar to the interstitial cells of the testis—vestiges from the fetal period, before sex differentiation took place. Proliferation of these cells or tumor formation may result in virilization.
In the ripening follicle, the oocyte is a spherical body composed of clear protoplasm. It contains a round, dark-staining nucleus, with a deﬁnite surrounding membrane and an eccentric nucleolus. A transparent membrane, the zona pellucida, encloses a ﬂuid-ﬁlled perivitelline space in which the egg ﬂoats freely. A dense layer of granulosa cells, the cumulus oophorus, closely envelops the egg and attaches it to the follicular wall. The cumulus cells immediately next to the zona arrange themselves radially outward to form the corona radiata.
The two-layered theca envelope coats the follicle. The theca interna is composed of large epithelioid cells interspersed in connective tissue and rich in blood and lymph vessels. The theca externa is thick and dense, consisting of circularly arranged connective tissue ﬁbers.
In the follicles that do not mature but degenerate, the granulosa layer ﬁrst becomes disorganized. The corona loses its radial arrangement. Thereafter, the follicular cavity shrinks, and soon the egg itself loses its characteristic features. Hyaline is deposited in a wavy, concentric band. Up to this point, the theca interna has continued to be a prominent layer of large, vesicular, nucleated cells. Degenerative changes rapidly progress until nothing is left except an amorphous hyaline scar.