General Histological Plan
Externally, the organ is covered by a specialized mesothelium whose cells feature microvilli on their apical surface, facilitating egg release. Beneath the epithelium lies a dense fibrous connective tissue layer — the tunica albuginea.
On cross-section, the organ is clearly divided into two zones:
- Medulla (medulla). Located in the exact center, this narrow band consists of loose fibrous connective tissue containing major blood vessels, sympathetic and parasympathetic nerve fibers, and the rete ovarii — cords and tubules representing remnants of the mesonephros.
- Cortex (cortex). Occupies the wide peripheral zone. Its stroma is formed by thin layers of connective tissue with blood vessels. The primary component of the cortex is follicles at various stages of maturation or degeneration.
Stages of Follicular Development
Any follicle consists of at least two components: an oocyte (germ cell) and surrounding follicular cells resting on a basement membrane.
- Primordial (resting). The smallest and most numerous follicles, located beneath the capsule. The oocyte is surrounded by a single layer of squamous epithelial cells.
- Primary. The follicle begins to grow. Follicular cells become cuboidal and increase in number (1–2 layers). The zona pellucida composed of glycoproteins forms around the oocyte, and the connective tissue sheath (theca folliculi) begins to develop externally.
- Secondary (antral). The epithelium becomes stratified, and cells secrete fluid that accumulates in small cavities (antra). The theca differentiates into internal and external layers. The internal theca synthesizes androgens, which follicular cells convert into estrogens.
- Tertiary (Graafian follicles). Mature, very large follicles dominated by a single large fluid-filled cavity. The oocyte is displaced to the periphery and attached to the wall via the cumulus oophorus.
Ovulation and Modern Concepts of Follicular Growth
It was previously believed that a follicle grows from a primordial to a mature stage within two weeks of a single cycle. Modern histology disproves this: complete development takes from 2 to 5 menstrual cycles. Early growth stages occur without strict hormonal control. Only in the final stage (during a specific cycle) does FSH activate a pool of pre-existing secondary follicles.
Ovulation is the rupture of the follicular and ovarian walls. The trigger is the luteinizing hormone (LH) surge from the adenohypophysis.
As a result of hyperemia and edema, intrafollicular pressure rises, the wall is thinned by enzymes, and myofibroblasts in the theca externa contract. The follicle ruptures, and the oocyte, surrounded by the zona pellucida and a layer of follicular cells (corona radiata), is released into the peritoneal cavity to enter the uterine tube.
Follicular Atresia and Corpus Luteum Formation
The vast majority of follicles that begin growth do not reach ovulation; instead, they degenerate. This process is called atresia.
During atresia, the oocyte and follicular cells degenerate and undergo phagocytosis. The zona pellucida, however, shrinks and persists for a long time in the center of the resulting atretic body. The thecal cells do not die during atresia; instead, they proliferate, transforming into interstitial cells that continue active androgen secretion.
The fate of the single ovulated follicle is different. Influenced by LH, its remaining follicular cells proliferate and transform into lutein cells, forming the corpus luteum, a temporary endocrine gland that produces progesterone. If pregnancy does not occur, the corpus luteum regresses, leaving behind a connective tissue scar — the corpus albicans, which eventually resorbs completely.