Module 3, Lecture 1: Female Reproduction and Development

Anatomy and Organisation of the Female Reproductive System

  • Female reproductive organs and cells responsible for production of sex hormones and gametes

  • Ovaries contain: stromal matrix (connective tissue, nerves, lymphatics, blood vessels), follicles, tunica albuginea, surface epithelium

  • Supporting structures: suspensory (infundibular) ligaments, broad ligament with mesosalpinx/mesovarium/mesometrium, ovarian and uterine ligaments, uterine tubes (fallopian tubes) with fimbriae, uterus (fundus, body, cervix), cervix, vagina, round ligament

  • Uterine wall composed of endometrium (functional layer + basal layer), myometrium, perimetrium

  • Ureter and uterine/ovarian vasculature run in proximity to reproductive organs

  • Follicle-containing ovary includes: primordial follicles, primary follicles, secondary follicles, early antral follicles, antral follicles, preovulatory (Graafian) follicle, corpus luteum

Learning objectives – Female Repro (Key points)

  • Identify and describe the female reproductive organs and cells responsible for production of sex hormones and gametes

  • Identify and describe the two major functions of meiosis

  • Identify and describe the production, regulation and function of reproductive hormones

  • Describe how the Hypothalamic-Pituitary-Gonadal (HPG) axis regulates female hormone production

  • Identify and explain the hormones and phases of the menstrual cycle

Development and Fate of Ovarian Follicles

  • Each follicle nurtures 1 egg/oocyte

  • At birth: millions of primordial follicles

  • Only ~450 ovulate in a lifetime; the rest undergo atresia

  • Menopause: <1000 primordial follicles left; gametes are not replenished (unlike males)

  • Follicular development sequence: Primordial → Primary → Secondary → Early Antral → Antral → Preovulatory

  • Corpus luteum forms from ruptured follicle after ovulation

  • Ovarian reserve and follicle dynamics are summarized in Fig. 27.23

Ovarian reserve perspective

  • Primordial follicles are non-growing; growth activation leads to follicle recruitment

  • Follicle growth and ovulation are regulated by intraovarian (paracrine) factors and systemic gonadotropins

  • Menopause marks depletion of the active follicle pool; <1000 primordial follicles remain

Follicle Growth, Folliculogenesis, and the Theca/Granulosa Cells

  • Folliculogenesis involves growth and maturation of the follicle that houses the oocyte

  • Two somatic cell types in the follicle:

    • Theca cells: respond to LH; produce androgens from cholesterol

    • Granulosa cells: respond to FSH; express aromatase to convert androgens to estrogens

  • Regulatory transitions:

    • Primordial and preantral follicles are largely gonadotropin-independent for growth

    • Primary/secondary/early antral follicles become FSH/LH dependent for growth

  • Key relationships include regulatory receptors:

    • Follicle cells acquire FSHR and LHR as maturation progresses

  • Intraovarian factors balance recruitment and quiescence (activation vs apoptosis)

2-cell, 2-gonadotropin hypothesis (thecal/granulosa interaction)

  • Thecal cells produce androgens under LH influence:

    • Cholesterol → androgens (androgen synthesis in the theca interna)

  • Granulosa cells convert androgens to estrogens via aromatase under FSH influence:

    • Androgens + aromatase → estrogens (primarily estradiol)

  • Regulation is modulated by inhibins (from granulosa cells) and feedback on FSH/LH and GnRH

  • Summary reaction chain (conceptual):

    • Theca cell LH → androgen production

    • Granulosa cell FSH → aromatase activity → estrogen production

    • Inhibin from granulosa cells → negative feedback on FSH

  • These interactions underpin the estrogen-dominated follicular phase and the LH surge leading to ovulation

Meiosis and Gamete Formation (Overview)

  • Making Gametes – Meiosis (Fig. 27.2)

    • Prophase I, Metaphase I, Anaphase I, Telophase I; followed by Prophase II, Metaphase II, Anaphase II, Telophase II

    • 2n (diploid) chromosome number in germ cells becomes n (haploid) after meiosis (reduction division) 2n<br>ightarrown2n <br>ightarrow n

    • Purpose: produce haploid gametes and promote genetic diversity (via crossing over/recombination)

    • Females: oogenesis; Males: spermatogenesis

  • Key points about oocyte development in humans (folliculogenesis + oogenesis):

    • Before birth: oogonium → primary oocytes arrested in prophase I

    • Throughout life: primordial follicles grow; most undergo atresia

    • After puberty: follicle growth resumes; a single dominant follicle emerges each cycle

    • Meiosis I completes just before ovulation to form a secondary oocyte + first polar body

    • Meiosis II completes only if fertilization occurs, forming the ovum + second polar body

  • Terminology recap (useful for exams):

    • Diploid: 2n2n

    • Haploid: nn

    • Oogonium → Primary oocyte (arrest in prophase I at birth)

    • Primary oocyte → Secondary oocyte (Meiosis I completed before ovulation)

    • Secondary oocyte → Ovum (Meiosis II completed upon fertilization)

    • Polar bodies: byproducts of meiosis, often degenerate

Regulation of the Ovarian Cycle (HPG Axis and Folliculogenesis)

  • HPG axis components (Fig. 24, 25):

    • Hypothalamus releases GnRH into the hypophyseal portal system

    • Anterior pituitary secretes FSH and LH in response

    • Gonads (ovaries) produce estrogens, progesterone, inhibin

    • Target cells respond to sex steroids; negative/positive feedback modulates GnRH/LH/FSH release

  • Portal system overview (NB: not required for exam):

    • Hypothalamic hormones travel via portal veins to anterior pituitary, releasing hormones into a secondary capillary plexus that feeds general circulation

  • GnRH pulse frequency and gonadotropin secretion (Fig. 26):

    • Fast GnRH pulse rate → ↑ LH secretion

    • Slow GnRH pulse rate → ↑ FSH secretion

    • Desensitization can reduce LH/FSH output and downstream steroid production

Regulation of ovarian cycle phases (Fig. 27.24)

  • Early and mid-follicular phase:

    • GnRH stimulates FSH and LH secretion

    • Thecal cells (androgens) + Granulosa cells (estrogens) operate in the two-cell model

    • The dominant follicle with FSH receptor/LH receptor becomes steroidogenically active

  • The 2-cell, 2-gonadotropin pathway (detailed):

    • Theca cell: LH → androgens

    • Granulosa cell: FSH → aromatase → estrogens

    • Estrogen production increases, providing negative feedback on FSH and GnRH; inhibin from granulosa cells also inhibits FSH

  • Midcycle: estrogen positive feedback leads to LH surge

  • Late follicular phase: estrogen rising to a threshold triggers positive feedback and an LH surge

  • Post-ovulation: LH surge transforms ruptured follicle into corpus luteum; corpus luteum secretes progesterone (and some estrogen)

  • End of luteal phase: decline in progesterone and estrogen if no fertilization leads to corpus luteum degeneration and cycle restart

  • Key phrases: negative feedback dominate luteal and early follicular phases; positive feedback dominates mid-late follicular phase

Hormones and targets (Estrogen and Progesterone) – Table 27.2 summary

  • Estrogens (primarily estradiol):

    • Major source: developing follicles and corpus luteum (ovaries)

    • Primary stimuli: FSH and LH

    • Feedback: both negative and positive on GnRH and gonadotropins

    • Effects: growth and maturation of reproductive organs and breasts; promote proliferative phase of uterine cycle; stimulate watery cervical mucus; promote oogenesis and ovulation; during pregnancy support uterine growth and mammary development

  • Progesterone:

    • Major source: corpus luteum (ovaries)

    • Primary stimulus: LH

    • Feedback: negative on GnRH and gonadotropins

    • Effects: supports secretory phase of uterine cycle; promotes viscous cervical mucus; promotes endometrial secretions and prepares uterus for pregnancy; during pregnancy helps quiet the myometrium and supports mammary gland maturation

The Menstrual Cycle and the Uterine Cycle

  • The menstrual cycle comprises ovarian (follicular, ovulatory, luteal phases) and uterine (endometrial) cycles; coordinated by ovarian hormones

  • Ovarian phases and hormones (simplified):

    • Follicular phase: rising estrogen from growing follicles; FSH stimulates follicle growth; LH stimulates theca cells to provide androgens

    • Ovulation (around day 14): LH surge triggers ovulation and meiosis I completion; dominant follicle releases secondary oocyte; corpus luteum forms

    • Luteal phase: progesterone dominates; corpus luteum maintains endometrium

  • Endometrial (uterine) phases: days 1–5 menstrual, days 6–14 proliferative, days 15–28 secretory

    • Menstrual phase: shedding of functional layer; occurs early in cycle; estrogen rising begins endometrial regeneration

    • Proliferative phase: estrogen-driven regrowth of functional layer; endometrium thickens

    • Secretory phase: progesterone-driven glandular secretions; endometrium becomes highly vascular and edematous to support potential implantation

  • Endometrium structure: functional layer (shed during menses) and basal layer (regenerates functional layer)

  • Interplay between cycles: Follicular/ovarian phases align with proliferative/secretory phases of uterus; estrogen is dominant in the follicular/proliferative phases, progesterone in luteal/secretory phases

Endometrium and Uterine Changes Across the Cycle

  • Endometrial glands develop and secrete nutrients to support embryo if implantation occurs

  • Blood vessels proliferate; thickness of the functional layer increases during proliferative phase and secretory phase

  • Cervical mucus changes with cycle hormones: watery mucus in estrogen-dominant phases; viscous mucus under progesterone dominance

Exam Preparation and Typical Questions

  • Typical exam question: status of the oocyte

    • Consider: diploid or haploid? primary or secondary oocyte? phase of meiosis arrested (Prophase I or Metaphase II)

  • Other exam focus areas:

    • Labeling hormone concentration curves (GnRH, FSH, LH, estrogen, progesterone) across the cycle

    • Identifying stages of follicle development (Primordial → Primary → Secondary → Early Antral → Antral → Preovulatory)

    • Describing the 2-cell/2-gonadotropin hypothesis and its implications for estrogen production

  • Terminology and concepts to recall (from Essential terminology slide):

    • LH, FSH, GnRH, HPG axis, Theca cells, Granulosa cells, Androgens, Progesterone, Estrogen, Inhibin, Haploid, Diploid

  • Practical connections: hormonal regulation has clinical relevance for fertility treatments, contraception, and reproductive health management

Key Numerical and Conceptual References (quantitative/structural)

  • Ovulation count: ~450 oocytes ovulated in a lifetime; millions at birth; menopause when primordial follicles drop to <1000

  • Follicle development timelines and follicle diameters (Table 8.2 concepts):

    • Preantral follicles: <0.5 mm

    • Very early antral: <2 mm

    • Early antral: 1–6 days of cycle; 2–7 mm; FSH/LH receptor presence rises with development

    • Dominant follicle emerges during the follicular phase; estrogen concentration rises locally within the follicular fluid

  • Hormone measurements across cycles: estrogen levels rise during the follicular phase; progesterone rises during the luteal phase; LH surge triggers ovulation

  • Phases of the menstrual cycle (endometrial):

    • Menstrual phase (days 1–5)

    • Proliferative phase (days 6–14)

    • Secretory phase (days 15–28)

  • Hormonal feedback dynamics: both negative and positive estrogen feedback; inhibitory effects of inhibin and progesterone on FSH; positive estrogen feedback triggers LH surge

Definitions and Key Concepts (Glossary-style recap)

  • Oogenesis: development of the oocyte inside the ovary

  • Folliculogenesis: growth and development of the ovarian follicle

  • Primordial follicle: earliest stage; contains a primary oocyte arrested in prophase I

  • Primary follicle: granulosa cell layers begin to proliferate

  • Secondary follicle: multiple granulosa cell layers present; theca cells become more defined

  • Preantral/Antral follicle: antrum forms; increased follicular fluid accumulation

  • Corpus luteum: temporary endocrine gland formed from ruptured follicle after ovulation; secretes progesterone and estrogen

  • Gonadotropins: FSH and LH regulate follicle growth and steroid production

  • Theca and Granulosa cells: key somatic cell types in follicle; work in concert for estrogen synthesis

  • Androgens, Estrogens, Progesterone: steroid hormones driving reproductive physiology and menstrual cycle dynamics

  • Inhibin: A granulosa cell product that inhibits FSH

  • HPG axis: Linked hypothalamic GnRH, pituitary FSH/LH, and gonadal hormone production; forms a feedback loop that coordinates reproduction

Quick Visual References (from slides)

  • Figure 27.1: Hypothalamic-Pituitary-Gonadal axis schematic with stimulating/inhibiting relationships

  • Figure 27.16: Photomicrograph of the mammalian ovary showing follicles, stroma, tunica albuginea, surface epithelium

  • Figure 27.17a: Uterus and ovary anatomy (posterior view)

  • Figure 27.22: Folliculogenesis and Oogenesis flow with primordial to corpus luteum progression

  • Figure 27.24: Regulation of the ovarian cycle (two-cell/two-gonadotropin mechanism and feedback loops)

  • Figure 27.25: Uterine cycle alignment with ovarian cycle (ovarian hormones vs gonadotropic hormones vs uterine changes)

Note: This summary consolidates the material presented in the lecture slides and is intended for exam preparation. It preserves major and minor points, key mechanisms, and the relationships between ovarian and uterine cycles, hormonal regulation, and cellular processes in gametogenesis and follicle development.