Anatomy & Physiology II – Reproductive System (Chapter 27)

Learning Objectives

• Describe anatomy & histology of male and female reproductive systems, their accessory glands, and composition/function of semen.
• Explain hypothalamic-pituitary regulation of reproductive function in both sexes.
• Trace gamete pathways from formation to fertilization.
• Detail meiotic events of spermatogenesis and oogenesis that convert diploid germ cells to haploid gametes.
• Outline hormonal changes of ovarian & menstrual cycles and pre-ovulatory events.
• Identify importance of testosterone and emergence of secondary sex characteristics at puberty.

Core Vocabulary & Functional Overview

• Gamete = sex cell with 23 chromosomes.
  • Sperm = male gamete.
  • Oocyte = female gamete.
• Functional summary:
  • Male – produce & deliver gametes + secrete androgens.
  • Female – produce gametes & hormones + support gestation and parturition.

Male Reproductive System – Gross Anatomy

• Principle organs: testes (within scrotum), epididymis, ductus deferens, ejaculatory duct, accessory glands (seminal vesicles, prostate, bulbourethral), urethra, penis.
• Accessory ducts join urinary tract at prostatic urethra → common outlet for urine & semen (dual function necessitates pre-ejaculatory cleaning via bulbourethral glands).

Scrotum & Temperature Regulation

• Skin-covered, pigmented sac posterior to penis housing both testes.
• Musculature:
  • Dartos (smooth) wrinkles skin, minimizing surface area heat loss.
  • Cremaster (skeletal) elevates/lowers testes; critical for maintaining ideal spermatogenic temperature ≈ 2{-}3\,^{\circ}!\text{C} below core.

Testes Structure

• Paired ovals \approx 4{-}5\,\text{cm} long.
• Connective-tissue layers:
  • Tunica vaginalis – parietal & visceral layers derived from peritoneum.
  • Tunica albuginea – fibrous capsule; septa partition parenchyma into \sim 250 lobules.
• Each lobule contains 1{-}4 coiled seminiferous tubules → straighten → rete testis → efferent ductules → epididymis.

Seminiferous Tubule Histology & Cell Types

• Sustentocytes (Sertoli cells)
  • Columnar, cradle developing sperm; secrete growth factors, form blood-testis barrier via tight junctions, produce inhibin & androgen-binding protein (ABP).
• Spermatogenic cells – germ lineage from spermatogonia → sperm.
• Myoid cells – peritubular smooth-muscle-like, contract rhythmically to propel luminal contents.
• Leydig (interstitial) cells – between tubules; synthesize androgens (testosterone + small estrogen).

Spermatogenesis

• Location: seminiferous tubules.
• Sequence (diploid 2n → haploid n):
  1. Spermatogonium (stem cell) on basement membrane.
  2. Primary spermatocyte – enters meiosis I.
  3. Secondary spermatocytes – complete meiosis II.
  4. Spermatids – haploid, mature via spermiogenesis → spermatozoa.
• Time course ≈ 64 days; output ≈ 100{-}300\,\text{million} sperm/day.

Mature Sperm Structure & Function

• Head – condensed haploid nucleus + acrosome (hydrolytic enzymes facilitating zona pellucida penetration).
• Mid-piece – helical mitochondria supply ATP for motility.
• Flagellum – microtubule axoneme; whip-like propulsion.

Sperm Transport Pathway

1. Seminiferous tubule lumen (immotile) → 2. Straight tubule / rete testis → 3. Efferent ductules → 4. Epididymis (≈ 6\,\text{m} coiled) – sperm gain motility, stored several weeks.
2. Ductus deferens in spermatic cord ascends inguinal canal; vasectomy severs this tube (highly effective contraception, sperm production persists but can’t exit).
3. Meets duct from seminal vesicle → ejaculatory duct → prostatic urethra → penile urethra.

Semen Composition & Accessory Glands

• Semen = sperm (\approx 5\%) + glandular secretions.

1. Seminal vesicles (paired; \approx 70\% of fluid)
   • Fructose (energy), prostaglandins, clotting proteins; alkaline.
2. Prostate (walnut-sized, inferior to bladder)
   • Milky fluid with citrate & PSA; activates sperm and modulates coagulation/decoagulation.
3. Bulbourethral (Cowper’s) glands
   • Thick salty mucus; lubricates urethra & neutralizes residual urine (pre-ejaculate may carry sperm already present).

Penis & Erectile Physiology

• Erectile bodies:
  • Corpus cavernosum (paired dorsolateral) – houses cavernosal arteries.
  • Corpus spongiosum (ventral, surrounds urethra) – forms glans.
• Erection triggered by parasympathetic release of \text{NO} → smooth-muscle relaxation → arterial dilation → venous compression; occurs during sexual arousal & REM sleep.
• Detumescence via sympathetic vasoconstriction = venous drainage > inflow.

Testosterone – Production, Actions, Regulation

• Synthesized by Leydig cells; daily output 6{-}7\,\text{mg}, intratesticular concentration ≈100× systemic.
• Embryonic role (7th week gestation) – wolffian duct differentiation, external genitalia masculinization.
• Pubertal/adult systemic effects:
  • Spermatogenesis.
  • Anabolic – muscle, bone growth.
  • Secondary sex traits – larynx enlargement, facial/body hair, deepened voice, libido maintenance.
• Minor production: female ovaries (mostly aromatized to estradiol) & adrenal cortex in both sexes.
• Endocrine feedback loop:
  • Hypothalamus releases GnRH (pulsatile) → anterior pituitary secretes LH & FSH.
  • LH → Leydig → \uparrow testosterone.
  • FSH → Sertoli → ABP (keeps intratubular T high) + inhibin.
  • Negative feedback: testosterone inhibits GnRH/LH/FSH; inhibin blocks FSH specifically.

Female Reproductive System – Gross Anatomy

• Primary organs: ovaries; accessory: uterine tubes, uterus, cervix, vagina, external genitalia (vulva), mammary glands.
• Purpose: gamete production, cyclic endocrine activity, gestation, lactation.

External Genitalia (Vulva)

• Mons pubis – fatty pad over pubic symphysis.
• Labia majora – hair-covered skin folds; homologous to scrotum.
• Labia minora – thin pigmented folds; protect vestibule.
• Clitoris – erectile; same embryologic origin as glans penis, densely innervated.
• Hymen – partial mucous membrane across vaginal orifice; variable integrity.
• Bartholin’s glands – flank vaginal opening, secrete mucus for lubrication.

Vagina

• Elastic, fibromuscular canal; functions as reproductive tract entrance, birth canal, menstrual exit.
• Wall layers

1. Fibrous adventitia.
2. Smooth-muscle muscularis.
3. Mucosa with rugae (stratified squamous epithelium).

• Microflora – lactobacilli dominate; produce lactic acid maintaining pH<4.5 (protective). Over-cleansing alters pH → infection susceptibility.

Ovaries – Structure & Histology

• Paired ovals 2{-}3\,\text{cm} long; stabilized by mesovarium + ovarian/broad/suspensory ligaments.
• Surface epithelium (simple cuboidal) over tunica albuginea.
• Cortex – houses follicles within stromal CT.
• Medulla – vasculature, lymphatics, nerves.

Oogenesis (Gamete Formation)

• Oogonia form during fetal life via mitosis.
• Enter meiosis I → primary oocytes, arrest in prophase I until puberty.
  • Numbers: \approx2\times10^{6} prenatally → 4\times10^{5} at birth → depleted at menopause.
• Each cycle, one oocyte completes meiosis I → secondary oocyte + first polar body; ovulated cell arrests at metaphase II.
• Meiosis II only finishes upon sperm penetration → mature ovum + second polar body.

Folliculogenesis – Stages & Fates

• Functional unit = follicle = oocyte + granulosa cells; theca cells develop later.

1. Primordial follicle – squamous pre-granulosa layer.
2. Primary follicle – single cuboidal granulosa layer + zona pellucida.
3. Secondary follicle – multiple granulosa layers; theca forms, secretes androgens.
4. Vesicular (tertiary/antral) follicle – fluid-filled antrum; corona radiata surrounds oocyte.

• Outcomes:
  • Atresia (≈99.9\%) via apoptosis.
  • Ovulation – once per cycle, dominant follicle ruptures.

Ovarian Cycle (≈28 days)

• Follicular phase (days 1{-}14): growth of vesicular follicles.
• Ovulation (day 14): LH surge triggers oocyte release.
• Luteal phase (days 14{-}28): corpus luteum forms, secretes progesterone ± estrogen.

Hormonal Control of Ovarian Cycle

• GnRH → FSH & LH on granulosa/theca.
• Rising estradiol exerts negative feedback reducing FSH (selection of dominant follicle) while inhibin assists.
• Dominant follicle’s high estradiol flips to positive feedback → LH surge:
  • Completion of meiosis I.
  • Ovulation via proteolytic weakening of ovarian wall.
  • Luteinization of remaining follicle → corpus luteum (CL).
• CL secretes progesterone (and some estrogen):
  • Preps uterus, inhibits GnRH/FSH/LH (prevents new follicle).
  • Without implantation, CL regresses after 10{-}12 days → corpus albicans; progesterone fall lifts inhibition → next cycle.

Uterine Tubes (Oviducts)

• Sections: fimbriae → infundibulum → ampulla (fertilization site) → isthmus → uterine entry.
• Histology: serosa, smooth-muscle layer, inner mucosa with ciliated & secretory epithelial cells.
• Post-ovulation, estrogen increases tubal peristalsis + cilia beating guiding oocyte; open communication with peritoneum poses infection risk (e.g., pelvic inflammatory disease).

Uterus & Cervix

• Regions: fundus (superior), body, cervix (projects into vagina; mucus thins under estrogen to facilitate sperm entry).
• Wall layers:

1. Perimetrium – serosa.
2. Myometrium – thick smooth muscle, generates labor contractions, menstruation expulsion & sperm guidance.
3. Endometrium – mucosal; subdivided into:
   • Stratum basalis – permanent regenerative layer.
   • Stratum functionalis – proliferates under estrogen/progesterone, vascularized by spiral arteries, shed as menses.

Menstrual Cycle (Uterine Cycle)

• Day 1 = first day of bleeding; length 21{-}32 days (average 28).

1. Menstrual phase (days 1{-}5)
   • Low progesterone & gonadotropins → spiral artery spasm → ischemia & shedding of stratum functionalis.
2. Proliferative phase (days 6{-}14)
   • Rising estradiol (from maturing follicle) rebuilds endometrium.
   • Cervical mucus thins; negative feedback suppresses FSH allowing dominant follicle selection; LH surge ends phase via ovulation.
3. Secretory phase (days 15{-}28)
   • Corpus luteum progesterone stabilizes thickened endometrium; glands secrete glycogen-rich fluid, spiral arteries elaborate.
   • If no implantation, CL regression ↓ hormones → prostaglandin-mediated vasoconstriction & menstruation restart.

Mammary Glands (Breasts)

• Modified sweat glands; function = lactation.
• Components: lobes → lobules housing alveoli (milk-producing cells) → lactiferous ducts & sinuses → nipple; areola contains sebaceous areolar glands lubricating during nursing.
• Responsive to estrogen (growth) & progesterone (alveolar development); prolactin & oxytocin regulate milk synthesis/ejection postpartum.

Puberty & Secondary Sex Characteristics

• At ≈8 yrs hypothalamic sensitivity to steroid negative feedback diminishes → ↑GnRH pulses → ↑LH/FSH → gonadal steroid surge.
• Male traits: penile & scrotal enlargement, facial/axillary/pubic hair, voice deepening, broad shoulders, musculature.
• Female traits: breast development, hip widening, pubic/axillary hair, onset of menarche, cyclic folliculogenesis.

Selected Clinical & Practical Notes

• Vasectomy: ligation of ductus deferens; hormones & ejaculation volume largely unchanged; reversal possible but not guaranteed.
• Self-cleaning vagina: commercial douches disrupt pH & microbiota increasing infection risk.
• Pre-ejaculate caution: bulbourethral secretion can carry residual sperm → potential pregnancy.
• Open oviduct-peritoneum connection: explains risk of peritonitis from STIs and ectopic implantation.
• Corpus luteum support in pregnancy: if implantation occurs, embryonic hCG rescues CL, maintaining progesterone until placental takeover.

Numerical & Statistical References (concise)

• Testis length 4{-}5\,\text{cm}; intratesticular T \approx 100× plasma.
• Daily spermatogenesis 100{-}300\,\text{million} cells.
• Testosterone output 6{-}7\,\text{mg}/day.
• Ovarian reserve: 2\times10^{6} (fetal) → 4\times10^{5} (birth).
• Cycle lengths: ovarian/menstrual \approx 28 days; CL functional 10{-}12 days if no pregnancy.
• pH vaginal microflora <4.5.

Inter-Lecture & Real-World Connections

• Embryology link: genital homology (clitoris ↔ penis; labia majora ↔ scrotum) underscores shared developmental pathways.
• Endocrine principles: negative vs positive feedback exemplified by low estradiol suppression vs high estradiol LH-surge induction.
• Ethical/clinical: contraceptive strategies (hormonal pills mimic luteal hormones), vasectomy permanence counseling, societal misconceptions about hymen integrity.