Male Reproductive System & Gametogenesis

Basic Genetic Concepts

• Humans are diploid most of life

  • Denoted 2n; each chromosome pair has one copy from the mother & one from the father

  • Instructor’s metaphor: genetic variants are like two blueberry-pie recipes (one with, one without a top crust) – both make pie but differ in details

• Meiosis in gonads

  • Specialized “double division” producing haploid n gametes (egg or sperm)

  • Halves chromosome number so fertilization restores diploid state 2n = 46

• Mitosis

  • Normal body-cell division; makes identical daughter cells for growth & healing

• Chromosome counts

  • Somatic cells: 46 total (i.e.
    23 pairs)

  • Gametes: 23 single chromosomes

• Karyotype basics

  • Pairs 1!\rightarrow!22 = autosomes

  • Pair 23 = sex chromosomes (XX or XY)

Key Terminology

• Gamete – haploid sex cell (egg or sperm)

• Gonads – organs that make gametes (testes / ovaries)

• Genitalia – external structures specialized for gamete transfer

• Female-specific

  • Early cell stage: oocyte

  • Mature gamete: ovum (egg)

• Male-specific

  • Early cell stage: spermatocyte (primary / secondary)

  • Mature gamete: spermatozoon (commonly “sperm”)

• Semen – fluid medium containing sperm; rich in sugars & other factors that power motility

Male Reproductive Anatomy (gross overview)

• Letter key from diagram cited in lecture

  • A Testes – sperm production & testosterone secretion

  • B Epididymis – coiled tube for sperm storage & maturation

  • C Ductus (vas) deferens – thick-walled transport & storage duct

  • D Ampulla of ductus deferens – local widening near bladder

  • E Prostate – donut-shaped gland encircling prostatic urethra

  • F Seminal vesicles – posterior bladder; contribute ~60 % of semen volume

  • G Bulbourethral (Cowper’s) glands – neutralize & lubricate urethra

  • H Urethra – common channel for urine & semen (prostatic ➜ membranous ➜ spongy)

• Support structures

  • Scrotum separated by midline raphe

  • Cremaster muscle – elevates or lowers testes to regulate temperature

  • Spermatic cord – connective-tissue sheath housing vasculature, nerves & ductus deferens

• Temperature control critical: testes stay ~2 °C below core for optimal spermatogenesis

Developmental Sex Differentiation

• All human embryos start with “indifferent” gonads plus two duct systems

  • Müllerian (paramesonephric) ducts

  • Wolffian (mesonephric) ducts

• Presence of Y-chromosome SRY gene ➜ Testes-determining factor (TDF)

  • Promotes formation of testes & secretion of testosterone + Müllerian-inhibiting factor (MIF)

  • MIF causes Müllerian ducts to regress; Wolffian ducts develop into epididymis, ductus deferens, ejaculatory ducts

  • External genitalia masculinize into penis & scrotum

• Absence of TDF ➜ default female pathway

  • No MIF → Müllerian ducts form uterine tubes & uterus; Wolffian ducts degenerate

  • External genitalia become clitoris, labia, vagina (separate from urethra because ducts never merge)

• Clinical note: Cryptorchidism – undescended testes; may self-correct or require surgery

Seminiferous Tubule Micro-Anatomy & Cell Types

• Spermatogonia (2n stem cells) along basal lamina

• Primary spermatocytes ➜ meiosis I ➜ secondary spermatocytes ➜ meiosis II ➜ spermatids (haploid)

• Spermiogenesis – morphological maturation of spermatids to spermatozoa

  • Lose excess cytoplasm

  • Form acrosomal cap (enzyme packet for egg penetration)

  • Pack mitochondria in mid-piece (ATP for flagellum)

  • Develop flagellum (tail)

  • Process takes ≈ 9 weeks

• Sustentacular (Sertoli) cells

  • Nourish developing gametes, form blood–testis barrier, secrete inhibin

• Interstitial (Leydig) cells in connective tissue between tubules

  • Produce testosterone & other androgens

Epididymis & Ductus Deferens

• Epididymis ≈ 7\,\text{m} (23 ft) coiled tube; functions

  • Sperm storage ~14 days further maturation

  • Remove damaged sperm

  • Propel immotile sperm via fluid currents & peristalsis

• Ductus deferens

  • Thick smooth-muscle wall; stores sperm for months

  • Strong peristaltic contractions during ejaculation move sperm to ejaculatory duct (formed where ductus meets seminal-vesicle duct behind bladder)

Accessory Glands & Seminal Fluid Composition

• Seminal vesicles (~60 % fluid)

  • Fructose → energy for mitochondria

  • Prostaglandins → smooth-muscle stimulant

  • “Fibrinogen-blocker” element – evolutionary remnant of mating plug mechanism

  • Provide chemical signal that activates flagellar motility

• Prostate (~30 % fluid)

  • Acidic prostatic fluid containing seminalplasmin (antibiotic) & zinc (supports testosterone balance)

• Bulbourethral glands (~5 % fluid)

  • Thick alkaline mucus → neutralizes residual urinary acidity & lubricates glans penis

• Semen statistics

  • Ejaculate volume ≈ 5\,\text{mL}

  • Sperm count 20$ – $100\times10^6 sperm/mL → \approx (1!\text{–}!5)\times10^8 to 5\times10^8 sperm per ejaculation

  • Relative contributions: Seminal vesicles 60 %, prostate 30 %, bulbourethral 5 %, sperm 5 %

Endocrine Regulation

• Hypothalamus releases Gonadotropin-releasing hormone (GnRH) in steady pulses after puberty

• Anterior pituitary secretes:

  • Follicle-stimulating hormone (FSH)

  • Targets sustentacular cells → promotes spermatogenesis & secretion of androgen-binding protein

  • Luteinizing hormone (LH)

  • Targets interstitial (Leydig) cells → stimulates testosterone production

• Testosterone effects

  • Sustains spermatogenesis

  • Stimulates protein synthesis & muscle growth (anabolic)

  • Drives secondary male characteristics (facial hair, voice deepening, body mass)

  • Enhances libido & CNS function

  • Exerts negative feedback on hypothalamus & pituitary to regulate GnRH, FSH, LH

• Inhibin from sustentacular cells provides additional negative feedback on FSH release

• Clinical note: Castration eliminates testicular testosterone → no negative feedback → markedly elevated FSH & LH levels

Key Numbers & Equations (quick reference)

• Diploid chromosome number: 2n = 46

• Haploid gamete number: n = 23

• Spermatogenesis timeline: \approx 9\,\text{weeks} for a cohort to mature

• Epididymal storage: \sim 14\,\text{days} before sperm fully motile

• Ejaculate: 20$ – $100\times10^6 sperm per mL × 5\,\text{mL} = (1!\text{–}!5)\times10^8 to 5\times10^8 total sperm

Real-World & Clinical Connections

• Temperature regulation (cremaster & scrotum) connects to fertility; fevers or tight clothing can impair spermatogenesis.

• Prostate enlargement (benign prostatic hyperplasia) can constrict prostatic urethra, affecting urination & ejaculation.

• Understanding TDF & sex differentiation explains intersex conditions and underlies genetic counseling.

• Antibiotic property of prostatic fluid shows evolutionary protection against urinary tract pathogens.

Conceptual Flowchart (verbal)

1. GnRH from hypothalamus ➜ 2 hormones from anterior pituitary (FSH & LH)

2. FSH → sustentacular cells → nourish & coordinate gamete development

3. LH → interstitial cells → testosterone

4. Testosterone + ABP maintain high local androgen concentration → complete spermatogenesis

5. Negative feedback: testosterone & inhibin dampen GnRH, FSH, LH

Study Tips & Memory Aids

• Blueberry-pie recipe analogy for allele variants.

• “Wolffian sounds masculine” → Wolffian ducts persist in males; Müllerian in females.

• Remember SRY → “S ex-determining R egion of Y .”

• Seminal vesicle starts the Swimming (Sugar, Stimulants).

• Bulbourethral = “buffer urethra” (alkaline mucus).