Q1 LONG ANSWER 10M ★ High-yield

Describe the ovarian and uterine changes of the menstrual cycle. Explain its hormonal control.

Physiology · 1st Professional — Pre-Clinical

Definition

Menstrual cycle = the cyclical changes occurring in the ovary and endometrium of a woman of reproductive age, under the control of the hypothalamo–pituitary–ovarian axis.

Parameter	Normal
Duration	28 days (range 21–35)
Menstrual flow	3–5 days (range 2–7)
Blood loss	30–80 mL
Menarche	11–15 years
Menopause	45–50 years

The luteal phase is constant at 14 days; it is the follicular phase that varies and accounts for differing cycle lengths

Ovarian Cycle

Phase	Days	Events	Dominant hormone
Follicular	1–13	Primordial → primary → secondary → Graafian follicle; one becomes dominant, the rest undergo atresia	FSH → oestrogen
Ovulation	14	Rupture of the Graafian follicle; release of the secondary oocyte	LH surge
Luteal	15–28	Corpus luteum forms; regresses to corpus albicans by day 26 if no pregnancy	Progesterone

Uterine (endometrial) Cycle

Phase	Days	Endometrium
Menstrual	1–5	Shedding of the functional layer; the basal layer is retained and regenerates it
Proliferative	6–13	Oestrogen → thickens from 1 to 3–4 mm; straight glands; spiral arteries grow
Secretory	15–28	Progesterone → 5–6 mm; glands become tortuous and secrete glycogen; stroma oedematous — the endometrium is now receptive to a blastocyst
Ischaemic	27–28	Corpus luteum regresses → ↓ progesterone → spiral artery spasm → ischaemic necrosis → menstruation

Hormonal Control

Hypothalamus → GnRH (pulsatile) → Anterior pituitary → FSH and LH → Ovary → oestrogen, progesterone, inhibin → Feedback on hypothalamus and pituitary

Feedback — the key point

Low and moderate oestrogen → negative feedback → suppresses FSH and LH
High, sustained oestrogen (> 200 pg/mL for > 36 h) → positive feedback → triggers the LH surge → ovulation
Progesterone → always negative feedback
Inhibin from granulosa cells selectively suppresses FSH, which is how the dominant follicle starves the others

Hormone Profile Through the Cycle

Hormone	Peak	Note
FSH	Early follicular, and a small mid-cycle peak	Recruits follicles
LH	Day 13–14 — the surge	Ovulation follows 24–36 h after the surge begins
Oestrogen	Day 12–13 (pre-ovulatory) and a second luteal rise	Proliferative changes
Progesterone	Day 21–22	A level > 5 ng/mL on day 21 confirms ovulation

Actions of Ovarian Hormones

	Oestrogen	Progesterone
Endometrium	Proliferation	Secretory change
Cervical mucus	Thin, watery, abundant; ferning positive; spinnbarkeit high	Thick, scanty, tacky; ferning absent
Myometrium	↑ Excitability and oxytocin receptors	↓ Excitability — "quietens" the uterus
Basal body temperature	No change	↑ 0.3–0.5 °C — thermogenic
Breast	Duct growth	Alveolar growth
Bone	Closes epiphyses; prevents osteoporosis	—
Secondary sex characters	Develops and maintains them	—

Oogenesis — in Brief

Oogonium (46) — multiplies only in fetal life → Primary oocyte — arrested in prophase of meiosis I from birth → At ovulation: meiosis I completed → Secondary oocyte + first polar body → Arrested in metaphase of meiosis II → Completed only if fertilised → Ovum + second polar body

7 million oogonia at 5 months of fetal life → 2 million at birth → 400,000 at puberty → only 400 ovulate in a lifetime
The long arrest in prophase I explains the rising risk of non-disjunction and Down syndrome with maternal age

Applied Aspects

Amenorrhoea — primary (no menarche by 16) or secondary (cessation for 6 months). Pregnancy must be excluded first
Polycystic ovary syndrome — anovulation, hyperandrogenism, polycystic ovaries; a raised LH:FSH ratio; insulin resistance
Dysfunctional uterine bleeding — usually anovulatory cycles with unopposed oestrogen
Oral contraceptive pills act chiefly by suppressing the LH surge through negative feedback, and by thickening cervical mucus
Unopposed oestrogen (as in PCOS or oestrogen-only HRT) predisposes to endometrial hyperplasia and carcinoma
Dysmenorrhoea — due to prostaglandin F2α causing myometrial ischaemia; relieved by NSAIDs, which block its synthesis
Premenstrual syndrome — symptoms in the late luteal phase, relieved by menstruation
Endometriosis — endometrial tissue outside the uterus bleeds cyclically → dysmenorrhoea, dyspareunia, infertility

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