Ovary Histology
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Direct answer
A single section through an ovary reads like a timeline: follicles at every stage crowd the cortex around a fibrocellular medulla, and the stage you see tells you the phase of the cycle. The surface is covered by a simple cuboidal layer called — misleadingly — the germinal epithelium, a modified peritoneum that is the origin of most ovarian carcinomas yet contributes no oocytes. Beneath it, primordial follicles (a primary oocyte arrested in prophase of meiosis I, ringed by a single flat cell layer) wait from foetal life; recruitment builds them through primary and secondary stages to the Graafian follicle with its antrum, cumulus oophorus and corona radiata. After ovulation, granulosa and theca cells luteinise into the progesterone-secreting corpus luteum, which regresses to the pale corpus albicans unless pregnancy rescues it.
What you must remember
- Surface epithelium: simple cuboidal "germinal" epithelium, a peritoneal derivative — source of most ovarian malignancies (serous, mucinous, endometrioid tumours).
- Primordial follicle: primary oocyte arrested at diplotene of meiotic prophase I in a single flat follicular layer; the lifetime stock falls from about 6-7 million at 20 weeks' gestation to 1-2 million at birth and 300,000-400,000 at puberty, of which only around 400 ever ovulate.
- Primary follicle: follicular cells turn cuboidal then multilayered granulosa, and the zona pellucida appears between oocyte and granulosa.
- Secondary (antral) follicle: Call-Exner bodies in the granulosa, antrum forming, theca interna (vascular, oestrogen-secreting) and theca externa (fibrous) differentiated.
- Graafian follicle: dominant antrum, oocyte on the cumulus oophorus with corona radiata; meiosis I completes just before ovulation, arresting at metaphase II.
- Corpus luteum: granulosa lutein (progesterone) and theca lutein (oestrogen) cells; regresses to corpus albicans in about 14 days without human chorionic gonadotrophin.
- Atresia: the fate of over 99 per cent of follicles — granulosa apoptosis with theca hypertrophy adding to the ovarian stroma of later life.
- Clinical anchors: polycystic ovary shows a thickened tunica with multiple atretic and cystic follicles; dermoids arise from germ cells.
Dating an ovary from one field of view
Pick the largest follicle and classify it. A single flat layer around a pale oocyte is primordial and could belong to any ovary from birth to menopause; multilayered granulosa with a visible zona pellucida means recruitment has begun. When an antrum appears, follicle-stimulating-hormone dependence is established, and when the antrum dominates with the oocyte on its cumulus hilllock, you are looking at a preovulatory Graafian follicle, up to about 2 centimetres across at mid-cycle.
If the field is a convoluted ribbon of large, eosinophilic, lipid-laden cells forming a yellow mass with a central clot, you have a corpus luteum, dating the ovary to the luteal phase or early pregnancy; rescue by human chorionic gonadotrophin converts it into the corpus luteum of pregnancy until the placenta takes over. A white, hyaline scroll of collagen is a corpus albicans, and many such scars plus scanty primordial follicles is the signature of the perimenopausal ovary.
Where students slip
Three confusions recur. First, the term germinal epithelium implies it generates oocytes; it does not — oogonia are germ-cell lineage set down in foetal life, while the surface is peritoneal mesothelium and the commonest tumour origin in this organ. Second, theca versus granulosa in steroid production: the avascular granulosa lacks the early enzymatic steps, so theca cells synthesise androgen substrate that granulosa aromatises to oestradiol — the two-cell theory, asked across anatomy, physiology and gynaecology. Third, meiotic timing: the oocyte hangs in prophase I until the luteinising-hormone surge, finishes meiosis I at ovulation releasing the first polar body, and completes meiosis II only if a sperm penetrates.
Frequently asked questions
Why is the term germinal epithelium a misnomer?
It is modified peritoneal mesothelium that generates no germ cells; oocytes derive from yolk-sac germ cells, while the surface layer is the source of most ovarian carcinomas.
Which follicle stage is FSH-dependent and develops an antrum?
The secondary (antral) follicle onwards — antrum formation marks the recruitment cohort from which one dominant follicle emerges each cycle.
Which cells form the corpus luteum and what do they secrete?
Luteinised granulosa cells chiefly secrete progesterone and luteinised theca cells oestrogen, sustaining the secretory endometrium.
When does the primary oocyte complete meiosis I?
Just before ovulation under the luteinising-hormone surge, releasing the first polar body and arresting at metaphase II until fertilisation.
What distinguishes polycystic ovary histology?
A thickened tunica albuginea with multiple subcapsular atretic and cystic follicles — the anatomy behind anovulation and hyperandrogenism.