General Plan of Histological Structure
Externally, the organ is covered by a specialized mesothelium, the cells of which bear microvilli on their apical surface — this facilitates the release of the oocyte. Beneath the epithelium lies dense fibrous connective tissue, the tunica albuginea (tunica albuginea).
In cross-section, the organ is clearly divided into two zones:
- Medulla (medulla). Located in the very center. It is a narrow band of loose fibrous connective tissue. Large main blood vessels and sympathetic and parasympathetic nerve fibers pass here. The medulla also contains the rete ovarii (rete ovarii), which are cords and tubules remaining as remnants of the primitive kidney.
- Cortex (cortex). Occupies a wide peripheral zone. Its foundation (stroma) is formed by thin layers of fibrous connective tissue with blood vessels. The main component of the cortex consists of 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. Located beneath the capsule. The oocyte is surrounded by a single layer of flattened epithelial cells.
- Primary. The follicle enters the growth phase. Follicular cells become cuboidal, and their number increases (1–2 layers). The zona pellucida (zona pellucida) composed of glycoproteins forms around the oocyte, and the connective tissue capsule, the theca (theca folliculi), begins to develop externally.
- Secondary (antral). The epithelium becomes stratified, and the cells secrete fluid that accumulates in small cavities (antra). The theca differentiates into the internal and external layers. The internal theca begins synthesizing androgens, while follicular cells convert them into estrogens.
- Tertiary (Graafian follicles). Mature, very large follicles. The main volume is occupied by a single huge fluid-filled cavity. The oocyte is pushed to the periphery and connected to the wall via the cumulus oophorus (cumulus oophorus).
Ovulation and Modern Perspectives on Growth
It was previously believed that a follicle grows from primordial to mature within two weeks of a single cycle. Modern histology refutes this: complete development takes from 2 to 5 menstrual cycles. Early stages of growth proceed without strict hormonal control. Only in the final stage (in a specific cycle) does FSH activate a pool of already prepared secondary follicles.
Ovulation is the process of rupture of the follicular and ovarian walls. The trigger is the surge of luteinizing hormone (LH) from the pituitary gland.
As a result of hyperemia and edema, intra follicular pressure rises, the wall is thinned by enzymes, and myofibroblasts of the external theca contract. The follicle ruptures, and the oocyte, surrounded by the zona pellucida and a layer of follicular cells (corona radiata), exits the organ to enter the uterine tube.
Follicular Atresia and Corpus Luteum Formation
The vast majority of follicles that begin growth do not reach ovulation, but instead degenerate. This process is called atresia.
During atresia, the oocyte and follicular cells break down and undergo phagocytosis. The zona pellucida, conversely, shrinks and persists for a long time in the center of the forming atretic body. The theca cells do not die during atresia; instead, they proliferate, transforming into interstitial cells that continue to actively secrete androgens.
The fate of the single follicle that undergoes ovulation is different. Under the influence of LH, its remaining follicular cells proliferate and transform into lutein cells, forming the corpus luteum. This is 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 completely resorbs.
Briefly: answers to topical questions
From which embryonic primordia does the ovary develop?
The ovary develops from a complex of embryonic primordia including epithelial, mesenchymal, and germ elements.
- Gonadal ridges — paired bulgings of the coelomic epithelium; in early stages, they show no signs of differentiation into testes or ovaries.
- Mesenchyme — participates in stroma formation; the cortical and medullary zones of the ovary differentiate from it.
- Primordial germ cells (gonocytes) — form outside the gonads, associate with the yolk sac, and then migrate into the gonadal ridges.
Class and exam questions
What causes the follicular wall to rupture during ovulation?
Rupture occurs due to a combination of factors: a sharp increase in fluid pressure (edema secondary to LH surge), destruction of intercellular contacts by hydrolytic enzymes, and active contraction of myofibroblasts in the external theca.
Why does the ovary produce high amounts of androgens during the second phase of the cycle?
During the atresia of numerous follicles, follicular cells (which used to convert androgens to estrogens) die, but the theca cells proliferate. As part of atretic bodies, they continue to synthesize androgens, which are left without any cells to metabolize them.
Do the cells forming the corpus luteum and atretic body differ?
Yes. The corpus luteum is formed predominantly from the follicular epithelium of the ovulated follicle. The atretic body consists of proliferated theca (interstitial) cells of the degenerated follicle surrounding the shrunken zona pellucida.
What else to study on this topic
- Morphological differences between the corpus luteum of menstruation and the corpus luteum of pregnancy
- Comparative characteristics of oogenesis and spermatogenesis
- Hormonal regulation of the ovarian-menstrual cycle (role of FSH, LH, and gonadocrinin)
- Histological structure of the uterine tubes and the mechanism of oocyte capture
- Homology of female and male reproductive organs (clitoris, vestibular bulbs)
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