The ovaries are small, paired, ovoid structures that exhibit a cortex and medulla when sectioned. On one side is a hilum for the transit of neu-rovascular structures; on this same side is a mesovarium that joins the ovary to the broad ligament. The functions of the ovary are the produc-tion of ova and the synthesis and secretion of estrogen and progesterone. In the cortex are numerous primordial follicles that are present at the time of birth and that remain unchanged until sexual maturation. The oogonia in these follicles are arrested in prophase of the first meiotic division. At puberty, under the influence of pituitary gonadotropins, the ovaries begin to undergo the cyclical changes designated the ovarian cycle. During each cycle, the ovaries normally produce a single oocyte that is ready for fertilization. At the beginning of the ovarian cycle, under the influence of pituitary follicle-stimulating hormone (FSH), some of the primordial follicles begin to undergo changes that lead to the development of a mature (Graafian) follicle. These changes include a proliferation of follicular cells and enlargement of the follicle. Although several primordial follicles begin these developmental changes, usually only one reaches maturity and yields an oocyte. Occasionally, two follicles will mature and ovulate, leading to the possibility of dizygotic twin development. The dis-charge of the oocyte and its adherent cells is called ovulation. At ovulation, the oocyte completes the first meiotic division. Only if fertilization occurs does the oocyte complete the second meiotic division. Whether or not fertilization occurs, the other follicles that began to proliferate in the same cycle degenerate, a process referred to as atresia.
Cortex, ovary, monkey, H&E ×120.
The cortex of an ovary from a sexually mature individual is shown here. On the surface, there is a single layer of epithelial cells designated the germinal epithelium (GEp). This epithelium is continuous with the serosa (peritoneum) of the mesovarium. Contrary to its name, the epithelium does not give rise to the germ cells. The germinal epithelium covers a dense fibrous connective
Early primary follicles, ovary, monkey, H&E ×450.
When a primordial follicle begins the changes leading to the formation of a mature follicle, the layer of squamous follicular cells becomes cuboidal, as in this figure. In addition, the follicular cells proliferate and become multilayered. A follicle
Primordial follicles, ovary, monkey, H&E ×450.
This figure shows several primordial follicles at higher magnification. Each follicle consists of an oocyte surrounded by a single layer of squamous follicular cells (F). The nucleus (N) of the oocyte is typically large, but the oocyte itself is so
Late primary follicle, ovary, monkey, H&E ×450.
The primary follicle in this figure shows a multilayered mass of follicular cells (FC) surrounding the oocyte. The innermost layer of follicular cells is adjacent to a thick eosinophilic layer of extracellular homogeneous material called the zona pellucida (ZP). At this stage of development, the oocyte has also enlarged slightly. The entire structure surrounded by the zona pellucida is actually the oocyte.
tissue layer, the tunica albuginea (TA); under the tunica albuginea are the primordial follicles (PF ). It is not unusual to see follicles at various stages of development or atresia in the ovary. In this figure, along with the large number of primordial follicles, there are four growing follicles (SF ), an atretic follicle (AF ), and part of a large follicle on the right. The region of the large follicle shown in the figure includes the theca interna (TI ), granulosa cells (GC ), and part of the antrum (A).