Unit 17 — Female Reproductive · Question Bank

The secondary (antral) follicle is the stage at which a developing ovarian follicle becomes recognisable by the appearance of a single fluid–filled cavity, the antrum. It sits between the multilaminar primary follicle and the mature (Graafian) follicle, and it is the histological form most frequently tested because every named component of follicular anatomy is now present. Examiners expect you to walk through the structure from the oocyte at the centre outward to the theca externa, naming each shell in turn and giving its function.

The oocyte and its glycoprotein coat

At the core lies the primary oocyte, still arrested in prophase of meiosis I as it has been since fetal life. It is a very large cell (around 100–120 µm) with abundant cytoplasm and a large vesicular nucleus (germinal vesicle) containing a prominent nucleolus. Hugging the oocyte's plasma membrane is the zona pellucida, an eosinophilic glycoprotein shell composed mainly of ZP1, ZP2 and ZP3. Beyond its mechanical role, the zona pellucida is the species–specific sperm receptor and the substrate for the cortical reaction that blocks polyspermy.

Cumulus oophorus, corona radiata and the antrum

The oocyte does not float in the antrum — it is anchored to one side of the follicular wall by a hill of granulosa cells called the cumulus oophorus. The innermost ring of cumulus cells, sitting directly on the zona pellucida and sending gap–junctional processes through it, is the corona radiata. The antrum itself is a single, large, crescentic cavity filled with liquor folliculi, an oestrogen–rich, hyaluronan–containing transudate produced by the granulosa cells. The presence of this single antrum is by definition what makes the follicle “secondary.”

Stratum granulosum and basement membrane

The wall of the antrum is built from the stratum granulosum (membrana granulosa), several layers of granulosa cells resting on a distinct basement membrane. These cells express FSH receptors and aromatase: they receive androgens diffusing in from the theca interna and convert them to oestradiol, which is then released both into the antrum and into the bloodstream. The granulosa layer is famously avascular — no capillaries cross the basement membrane until after ovulation.

Theca interna and theca externa

Outside the basement membrane, the surrounding stroma has differentiated into a double sheath. The inner theca interna is vascular and steroidogenic; its cells are large, polygonal and full of smooth ER and lipid droplets, and under LH they secrete androgens (mainly androstenedione) that diffuse to the granulosa for aromatisation — the classic two–cell, two–gonadotropin model of ovarian oestrogen production. The outer theca externa is fibromuscular: spindle–shaped fibroblasts, collagen, and smooth muscle that will help squeeze the follicle at ovulation. The two thecae together provide structural, vascular and endocrine support to the follicle.

Clinical link

In polycystic ovary syndrome (PCOS), many antral follicles arrest at this stage just under the tunica albuginea, producing the “string of pearls” on ultrasound, with chronic anovulation and theca–driven hyperandrogenism. Granulosa cell tumour recapitulates the granulosa population and over–secretes oestrogen, producing endometrial hyperplasia or postmenopausal bleeding. The named layers of the secondary follicle are therefore not just a memorisation exercise — each is a target of pathology.

Folliculogenesis is the slow conversion of a dormant primordial follicle, set aside in fetal life, into a Graafian follicle ready to ovulate. The path runs through four recognisable morphological stages, each identified at the microscope by a small checklist: the number and shape of granulosa cells, the presence or absence of a zona pellucida, the presence or absence of an antrum, and whether the theca has formed. Only a few hundred follicles complete this journey in a lifetime; the rest die by atresia.

Primordial follicle — the resting reserve

The primordial follicle is the form in which the ovarian reserve is stored throughout life. It sits in the outer cortex, just beneath the tunica albuginea. It contains a primary oocyte arrested in prophase of meiosis I since fetal life, surrounded by a single layer of flat (squamous) follicular cells on a thin basal lamina. There is no zona pellucida, no theca and no antrum — recognising it relies on the flat single–cell envelope.

Primary follicle — unilaminar and multilaminar

When a primordial follicle is recruited, the flat cells thicken into cuboidal cells — the unilaminar primary follicle. As these granulosa cells proliferate to form several concentric layers, the follicle becomes a multilaminar primary follicle. Two further structures appear at this stage: an eosinophilic glycoprotein shell, the zona pellucida, deposited between the oocyte and the innermost granulosa cells, and a condensation of stromal cells outside the basal lamina that becomes the theca folliculi. The oocyte itself enlarges and the granulosa develops gap–junctional processes through the zona to support it.

Secondary (antral) follicle

Small lakes of liquor folliculi accumulate among the granulosa cells and coalesce into a single follicular antrum. The moment a single antrum is present the follicle is by definition secondary. The oocyte, still arrested in meiosis I and still wrapped in its zona pellucida and an inner ring called the corona radiata, is anchored to the wall by the cumulus oophorus. The granulosa is now a thick stratum granulosum on a basement membrane, and the theca has separated into a vascular androgen–secreting theca interna (LH–driven) and a fibromuscular theca externa. The two–cell, two–gonadotropin model is now operational: theca androgen + granulosa aromatase → oestradiol.

Mature (Graafian) follicle and ovulation

One dominant antral follicle outcompetes its cohort and balloons to 15–25 mm, bulging visibly on the ovarian surface — the mature (Graafian) follicle. Hours before ovulation, the mid–cycle LH surge finally releases the meiotic arrest: the oocyte completes meiosis I, extrudes the first polar body and becomes a secondary oocyte arrested in metaphase II. The same surge weakens the wall via proteases and triggers ovulation, during which the secondary oocyte, surrounded by its zona pellucida and corona radiata, is expelled into the oviduct.

Clinical link

Most recruited follicles never reach ovulation — they undergo atresia, identifiable on histology as collapsed follicles with a glassy “ghost” zona pellucida persisting after the oocyte has gone. In PCOS, dozens of antral follicles arrest at the secondary stage; in premature ovarian insufficiency the primordial pool is exhausted early.

The corpus luteum is a transient endocrine gland formed from the ovulated follicle. It is a striking demonstration that endocrine tissue can be improvised in days from cells whose previous job was completely different: yesterday's follicular wall is today's progesterone factory. Its life depends on continued LH–like signalling, and its fate — brief or sustained — depends entirely on whether implantation occurs.

Formation: from corpus haemorrhagicum to luteum

Immediately after ovulation the collapsed Graafian follicle bleeds into its central cavity, producing the short–lived corpus haemorrhagicum. Under continuing pituitary LH, the basement membrane breaks down, capillaries from the theca interna for the first time invade the formerly avascular granulosa layer, and both follicular cell populations hypertrophy and accumulate smooth ER and lipid — a process called luteinisation. Within days the structure is a folded, sausage–shaped, characteristically yellow gland (carotenoid pigment in lipid droplets) embedded in the ovarian cortex.

Two cell populations

Two histologically distinct cell types make up the corpus luteum. The granulosa lutein cells, derived from the granulosa, are large (30–50 µm), polygonal, pale and central; they form the bulk of the gland and secrete principally progesterone, with some oestrogen and inhibin. The theca lutein cells, derived from the theca interna, are smaller, darker, and arranged peripherally and in the folds between granulosa lutein cells, often hugging the new capillaries; they contribute oestrogen (and some progesterone). Both populations now release steroids directly into the bloodstream.

Corpus luteum of menstruation

If implantation does not occur, the corpus luteum is supported only by ovarian and pituitary LH, which fades after about 10–14 days. The luteal cells then undergo apoptosis, the parenchyma is replaced by hyalinised connective tissue, and the result is the corpus luteum of menstruation. The progesterone drop releases the brake on the endometrium and menstruation begins.

Corpus luteum of pregnancy

If the blastocyst implants on day 6–7, the syncytiotrophoblast secretes human chorionic gonadotropin (hCG), which binds the same LH receptor on lutein cells. This rescues the gland: it enlarges to the corpus luteum of pregnancy, reaching ~5 cm in early gestation, and sustains progesterone output until the placenta takes over steroidogenesis at around weeks 8–12. After the handover, this larger corpus luteum also involutes.

Fate and clinical link

Either pathway ends in a corpus albicans, the pale fibrous “white body” that persists in the cortex as a histological record of past ovulations. Clinically, β–hCG measured in urine or serum is essentially a readout of corpus–luteum rescue. A persistent fluid–filled corpus luteum cyst can bleed and mimic an ectopic pregnancy; luteal phase deficiency with inadequate progesterone underlies some early pregnancy losses.

The menstrual cycle is the cyclic structural and functional remodelling of the functional layer of the endometrium, driven by the hypothalamic–pituitary–ovarian axis. Day 1 is conventionally the first day of bleeding; the idealised cycle lasts 28 days. The endometrium is partitioned into two zones — a deep stratum basale, supplied by short straight basal arteries and never shed, and a superficial stratum functionale, supplied by progesterone–sensitive spiral arteries and remodelled every cycle.

Menstrual phase (days 1–4)

The cycle conventionally begins with bleeding. The corpus luteum of the previous cycle has died, and the consequent fall of progesterone (and oestrogen) removes vasodilator support from the spiral arteries. They go through a final intense vasoconstriction, ischaemia kills the functional layer, and then transient vasodilation lets the necrotic tissue desquamate with arteriolar blood — menstruation. The basal layer is supplied by separate basal arteries that are insensitive to progesterone withdrawal, so it is spared and immediately starts regenerating the surface epithelium even before bleeding has fully stopped.

Proliferative (follicular) phase (days 5–14)

As bleeding ends, growing antral follicles in the ovary secrete oestrogen, which drives the rebuilding of the functional layer from the basal stumps. The endometrium thickens from ~1 mm to 2–3 mm. Histologically the uterine glands are straight, narrow tubular structures lined by tall pseudostratified columnar cells with crowded basal nuclei and abundant mitoses. The stroma is compact, and the spiral arteries lengthen and begin to coil. This phase ends with ovulation around day 14.

Secretory (luteal) phase (days 15–28)

After ovulation the corpus luteum secretes progesterone, which converts the proliferative endometrium into a glycogen–rich, oedematous, fluffy bed ready for implantation. On days 17–18 the gland epithelium accumulates striking sub–nuclear glycogen vacuoles — the classical histological marker that ovulation has occurred. Over the next week the glands become extravagantly coiled and sawtoothed, dump glycogen and mucin into the lumen as luminal secretion, the stroma becomes oedematous and pre–decidual, and the spiral arteries grow even more tortuous. The endometrium peaks at 5–7 mm. If no blastocyst implants, the corpus luteum involutes, and the cycle restarts with menstruation.

Hormonal control

The whole sequence is driven by the hypothalamic–pituitary–ovarian axis. Pulsatile GnRH from the hypothalamus releases pituitary FSH and LH. Early in the cycle FSH grows the follicles, which secrete oestrogen (proliferative phase). A mid–cycle LH surge triggers ovulation. The resulting corpus luteum makes progesterone (secretory phase). Progesterone and oestrogen feed back negatively on the hypothalamus and pituitary; when the corpus luteum dies they fall, the brake comes off, FSH rises again and a new cohort of follicles begins to grow.

Clinical link

An endometrial biopsy can be “dated” to within a few days by gland and stromal morphology — useful in infertility workup. Unopposed oestrogen (anovulation, obesity, oestrogen–only HRT, tamoxifen) drives endometrial hyperplasia → adenocarcinoma, presenting as postmenopausal bleeding. Endometriosis is ectopic functional–layer–like tissue that still cycles, causing dysmenorrhoea and ovarian “chocolate cysts.”

The oviduct and the uterus are the two organs of the female tract that handle the ovulated oocyte and the resulting conceptus. The oviduct catches the egg, hosts fertilisation, and conveys the early embryo; the uterus then receives, implants and nurtures it. Each organ has a wall built on the same general plan of mucosa, muscular layer and outer coat, but tuned to its own job.

Oviduct: gross divisions

The oviduct is a 10–12 cm muscular tube with four segments. The funnel–shaped infundibulum bears finger–like fimbriae that sweep the ovarian surface at ovulation. The wide, thin–walled ampulla is the usual site of fertilisation. The narrow, thick–walled isthmus connects to the uterus, and the short intramural (uterine) segment pierces the uterine wall.

Oviduct mucosa and epithelium

The mucosa is famously thrown into elaborate branching folds, most exuberant in the ampulla (a cross–section there looks like a labyrinth of papillae) and almost flat in the isthmus. The epithelium is simple columnar with two cell types whose proportions vary across the cycle: ciliated cells dominate around ovulation, their cilia beating toward the uterus, and non–ciliated peg (secretory) cells produce a nutritive fluid that supports the oocyte and capacitates sperm.

Oviduct muscularis and function

Outside the lamina propria lies a two–layered smooth–muscle muscularis (inner circular, outer longitudinal). Coordinated cilia plus muscular peristalsis transport the oocyte from fimbriae to ampulla to uterus over ~3–4 days, by which time the conceptus is a morula or early blastocyst. The outer surface is covered by serosa.

Uterine wall: three layers

The uterine wall has three layers. The endometrium (mucosa) carries a simple columnar epithelium — ciliated and secretory cells — dipping into a cellular lamina propria as simple tubular uterine glands. The myometrium is a thick mass of smooth muscle in three to four ill–defined interwoven layers, the middle of which (stratum vasculare) is rich in vessels; during pregnancy it undergoes spectacular hyperplasia and hypertrophy. The outermost perimetrium is serosa where it faces the peritoneum and adventitia where the uterus is fused to neighbouring structures.

Functional and basal endometrium

The endometrium is divided functionally into a superficial stratum functionale, supplied by progesterone–sensitive spiral arteries — this is the layer that cycles, thickens, secretes and sheds — and a deep stratum basale, supplied by progesterone–insensitive short straight basal arteries that retain the gland bases. The basalis is never shed and regenerates the surface after every menstruation.

Clinical link

Tubal scarring from pelvic inflammatory disease (Chlamydia, gonorrhoea) is the main cause of tubal infertility and of tubal ectopic pregnancy, which most commonly implants in the ampulla and ruptures around 6–8 weeks. In the uterus, leiomyoma (fibroid) is the most common pelvic tumour (benign myometrial smooth muscle), adenomyosis is endometrium trapped within the myometrium, and endometrial adenocarcinoma arises from chronically unopposed oestrogen acting on the functional layer.