The anatomy of the ovaries was discussed in Chapter 8. Because components of the normal ovary may be observed in cytologic preparations, a brief summary of the histology is provided (Fig. 15-1).

The central portion of the ovaries is formed by the hilum, the site of entry of the vascular supply and lymphatic drainage. The hilum also contains clusters of large endocrine cells with eosinophilic, granular cytoplasm, similar to Leydig cells of the testis, that may contain rod-like Reinke’s crystalloids. The ovary is surrounded by a surface or germinative epithelium. The bulk of the ovary is formed by ovarian stroma.

The surface epithelium, which is closely related to the mesothelium, is composed of a single layer of cuboidal cells with scanty basophilic cytoplasm and spherical nuclei. The surface epithelium often forms invaginations into the cortex of the ovary or small cysts. It should be noted that cortical cysts may be mistaken for ovarian follicles on ultrasound examination and may be incidentally aspirated during the harvest of ova for in vitro fertilization.

The ovarian stroma is composed of small spindly cells, some of which are capable of endocrine function. The superficial part of the ovarian stroma, the cortex, contains ova in various stages of maturation. The ova, numerous at birth, are reduced in number in the mature ovary and reside in the cortical stroma, where each ovum is surrounded by a single layer of epithelial cells, forming a primitive follicle. The maturation of the ova begins at puberty. Under the impact of pituitary follicle-stimulating hormone (FSH), a few select follicles begin to enlarge. It is not known how and why the selection is taking place. The epithelial cells surrounding the ovum begin to multiply, become larger and multilayered, and are named granulosa cells. The ovum is separated from the granulosa cells by a homogeneous membrane, known as the zona pellucida. As the maturation of the ovum progresses, the number of cell layers of the granulosa increase. At the same time stromal cells surrounding the ovum become larger and, named theca cells, form a multilayered envelope around the follicle. The granulosa and theca cells secrete estrogens that induce the proliferative phase in the endometrium (see Chap. 13). As the follicle matures and enlarges, the granulosa cells form a cavity filled with a hormone-rich fluid. The ovum, still surrounded by granulosa cells, now protrudes into the follicular cavity;
the protrusion is named cumulus oophorus (Fig. 15-2A). At this point, the follicle is named after the Dutch anatomist who first described it in the 17th century, a follicle of De Graaf or Graafian follicle. The graafian follicles are now visible on the surface of the ovary as small protrusions, but normally only one of them will spontaneously rupture and discharge the mature ovum together with the follicular fluid into the peritoneal cavity, followed by bleeding into the cavity of the follicle. Again, it is not known how and why the single follicle is selected. The ovulation takes place under the impact of pituitary luteinizing hormone (LH), that also causes enlargement of the granulosa cells that converts the collapsed follicle into a large, grossly visible yellow structure, the corpus luteum, that secretes progesterone, thus inducing the secretory phase of the endometrium (Fig. 15-2B). The yellow color of the corpus luteum is due to a high lipid content of the hormone-producing component cells. The discharged ovum is captured by the fimbria of the fallopian tube, pending fertilization by a spermatozoon in the lumen of the tube. Unless pregnancy intervenes, the corpus luteum undergoes atrophy and fibrosis, resulting in a small white scar [corpus albicans or (plural) corpora albicantia] within the cortex of the ovary. If pregnancy occurs, the corpus luteum persists, becomes larger, and is known as corpus luteum of pregnancy.