Female Reproductive System and Cycles

Anatomical Landmarks & Pathway of the Gamete

• Relative positions
  • Bladder = most anterior & inferior; uterus = posterior/superior; vagina runs inferiorly.
• Sequential structures (lateral ➔ medial)
  1. Ovary (a) – gonad producing the oocyte.
  2. Fimbriae (b) – finger-like projections; act as a “catcher’s mitt.”
  3. Uterine (fallopian) tube (c) – ciliated, muscular conduit; usual site of fertilisation.
  4. Uterus (d) – muscular organ for implantation, gestation & parturition.
  5. Vagina (e) – elastic, stratified squamous canal; birth & copulation passage.
• No direct tubal connection between ovary & fimbriae – potential for ectopic release into peritoneal cavity.
• Ligaments (ovarian, suspensory, broad) stabilise ovary & uterus within abdominopelvic cavity.

Ovarian Histology & Oogenesis

• Follicle = oocyte + surrounding follicular (granulosa) cells + fluid (antrum).
• Developmental vocabulary
  • Primordial → primary → secondary → Graafian (mature) follicle.
  • Post-ovulation: corpus luteum (yellow body) ➔ corpus albicans (white scar-like remnant).
• Oogenesis short-form chronology
  \text{oogonium (2n)} \xrightarrow{mitosis \text{ in fetus}} \text{primary oocyte (2n)} \xrightarrow{\text{meiosis I @ puberty}} \text{secondary oocyte (n)} + \text{1st polar body} \xrightarrow{fertilisation} \text{ovum (n)} + \text{2nd/3rd polar bodies}
• Key checkpoints
  • Primary oocytes arrest in prophase I before birth.
  • Secondary oocyte arrests in metaphase II; completes meiosis II only after sperm-nuclear fusion.
• Unequal cytokinesis → 1 functional ovum + 2–3 polar bodies per initial oogonium (vs 4 sperm in males).
• "Blister" metaphor: antral fluid builds pressure; LH thins follicular wall allowing rupture.

Numerical Timeline of Oocyte Reserve

• Fetal ovaries: 6\text{–}7\times10^{6} oogonia.
• At birth: \approx 2\times10^{6} primary oocytes.
• Puberty: \approx 2\times10^{5} per ovary.
• Lifetime ovulated: \sim 500 (≈ 40 yr × 12 cycles).
• Near age 40–50: reserve essentially exhausted → menopause.

Comparative Meiosis: Male vs Female

• Spermatogenesis: 1 spermatogonium → 4 spermatozoa.
• Oogenesis: 1 oogonium → 1 ovum + polar bodies.
• Biological implication: functional eggs are rarer & temporally limited; sperm production is continuous and lifelong.

Transport, Fertilisation & Early Support

• Uterine tube
  • Ciliated epithelium + circular/longitudinal smooth muscle ➔ peristalsis.
  • Fertilisation window: 12\text{–}24\,\text{h} post-ovulation.
  • Total gamete travel: 3\text{–}5\,\text{d}.
  • Chemotactic signal allurin secreted by oocyte guides sperm to correct tube.
  • Tube secretes nutrient-rich fluid for gametes & zygote.
• Uterus – roles
  • Mechanical protection, nutrient/waste exchange, powerful myometrial contractions.
  • Astonishing distensibility, yet ability to involute postpartum.

Uterus: Structure, Cyclic Remodeling & Menstruation

• Layers (outside ➔ inside)
  • Perimetrium – serosa.
  • Myometrium – thick smooth muscle; contracts during labour; target of oxytocin.
  • Endometrium – functional layer rebuilt monthly; rich spiral arteries & endometrial glands.
• Menstrual mechanism
  • In absence of implantation, corpus luteum degenerates ➔ ↓progesterone/estrogen.
  • Spiral arteries constrict → functional layer becomes ischemic & dies.
  • Rebound vasodilation floods tissue → sloughing (menses).
• Endometrial glands secrete glycogen during secretory phase; potential nutrient source for embryo pre-placenta.

Hormonal Regulation: Ovarian vs Uterine Cycles

• Master trigger: GnRH (hypothalamus) → anterior pituitary → FSH & LH.

Ovarian Cycle (≈ 28 d reference)

• Early follicular estrogen provides negative feedback on FSH/LH; suddenly flips to positive feedback (unknown switch) causing LH surge.
• Corpus luteum sustains high progesterone; involutes ≈ day 26 if no hCG signal ➔ hormone drop ➔ menstruation & FSH rise for next cycle.

Uterine Cycle

• Normal cycle length range 21\text{–}35\,\text{d} (sometimes up to 40 d).

Cervical Mucus Dynamics

• Estrogen-dominant (proliferative) – clear, stretchy, sperm-friendly.
• Progesterone-dominant (secretory) – thick “mucous plug,” sperm-restrictive.
• Basis for the rhythm/cervical-mucus method of contraception (variable reliability!).

Vagina & External Genitalia

• Vagina
  • Stratified squamous epithelium (friction); muscular & highly elastic.
  • Slightly acidic pH – bacterial defence.
  • Passageway for menses, coitus, semen depot, childbirth.
• Vulva landmarks
  • Labia minora (a), labia majora (b), hymen (c), clitoris (d) – erectile tissue analog of penis.
  • Greater vestibular (Bartholin’s) glands → mucous lubrication.

Mammary Glands & Lactation

• Architecture
  • Yellow = adipose shaping breast; pink "raspberries" = lobes/lobules (milk synthesis).
  • Suspensory ligaments (Cooper’s) support tissue.
  • Duct system: lobule → lactiferous duct → lactiferous sinus → multiple nipple pores (contrast to single-orifice bottle).
• Lactation physiology
  • Prolactin (anterior pituitary) – milk production within lobules.
  • Infant suckling → nipple mechanoreceptors → hypothalamus → posterior pituitary → oxytocin ↑ → myoepithelial contraction → “milk let-down.”
  • First 2–3 d: colostrum – antibody-rich, low-fat fluid.
• Stress & limbic input can inhibit oxytocin release, impairing let-down.

Parturition & Oxytocin Positive Feedback

• Fetal signal initiates labour; maternal posterior pituitary secretes oxytocin.
• Oxytocin → myometrium contracts → fetal head presses cervix → cervical stretch receptors → more oxytocin (neuro-endocrine loop).
• Loop is self-limiting; ends with delivery – classic physiological positive feedback example.

Contraception Strategies & Physiological Basis

• Combined oral contraceptives – steady exogenous estrogen + progesterone mimic luteal phase → negative feedback on GnRH/FSH/LH → no follicular maturation/ovulation.
• Progestin-only (“mini-pill”) – thickens cervical mucus, reduces GnRH pulse frequency.
• Barrier: condoms, diaphragms, etc.
• Rhythm/body-temperature method
  • Basal body temp rises ≈ 0.3\,^{\circ}\text{C} post-ovulation due to progesterone.
  • Combined with mucus observation – high user-failure rate.
• IUDs, implants, emergency contraception (not detailed in transcript, but contextually related).

Clinical Correlates & Terminology

• Dysmenorrhea – painful menstruation (inflammatory etiology common).
• Ectopic pregnancy – possible because ovum can escape into peritoneal cavity.
• Menopause – cessation of ovarian cycling once follicular reserve depleted; contrasts with lifelong male spermatogenesis.

Key Hormones & Their Primary Actions (cheat sheet)

• \text{GnRH} – hypothalamus → triggers FSH & LH pulses.
• \text{FSH} – granulosa cell proliferation; follicle growth; estrogen production.
• \text{LH} – Theca cell androgen synthesis; LH-surge triggers ovulation & corpus luteum formation.
• \text{Estrogen / Estradiol} – endometrial proliferation; secondary sexual traits; feedback (− then +).
• \text{Progesterone} – secretory conversion of endometrium; cervical mucus thickening; pregnancy maintenance.
• \text{Oxytocin} – uterine & mammary smooth-muscle contraction.
• \text{Prolactin} – milk synthesis.

Connections to Earlier Content & Broader Relevance

• Follicular “fluid-filled sac” analogous to thyroid follicles (lecture tie-in).
• Oxytocin recurs in multiple systems: social bonding, uterine labour, milk let-down – exemplifies pleiotropy.
• Female cycle illustrates negative vs positive feedback principles (endocrine physiology cornerstone).

Ethical & Practical Implications Mentioned

• Limitations of rhythm method → underscores need for comprehensive contraceptive education.
• Recognition that finite egg supply imposes reproductive time window for women; societal & medical implications (fertility counselling, assisted reproduction).