Male Reproduction
Sex determination & differentiation, SRY/Sox9, gonadal and phenotypic sex, PGC migration, disorders of sex development, male anatomy, spermatogenesis, testicular cooling, male endocrinology, and spermatogenesis mechanics (ANSC 224, Lectures 23–26)
A · Primary Germ Layers, Pituitary Development & Sex Determination Overview
- Digestive system, lungs, endocrine glands
- Muscle, skeleton, cardiovascular
- Reproductive system: gonads, uterus, cervix, part of vagina, epididymis, vas deferens, male accessory sex glands (MASG)
- Nervous system, skin, hair
- Mammary glands, hypothalamus, pituitary
- Part of vagina, penis, clitoris
Two parts derived from different tissues with different functions
- Origin: floor of the brain — infundibulum (neural ectoderm diverticulum)
- Releases ADH (vasopressin) and oxytocin
- Origin: roof of the mouth — Rathke's pouch (stomodeal ectoderm)
- Releases FSH, LH, GH, TSH, ACTH, prolactin
Reproductive tract development requires timing and coordination — errors affect ~0.5–1% of humans
Set at fertilization by the sex chromosomes: XY = male, XX = female. Y chromosome carries the SRY gene — the master switch.
SRY → testes develop (XY); no SRY → ovaries develop (XX, default). The bipotential gonad is directed by the hormonal environment.
Gonads secrete hormones (AMH, testosterone, DHT, estradiol) that drive development of internal ducts and external genitalia into male or female anatomy.
Testes → Sertoli cells (AMH) + Leydig cells (Testosterone) → male internal + external anatomy
Ovaries → no AMH → Müllerian ducts persist → uterus, oviducts, vagina. No positive hormone signal required.
B · The SRY Gene: Identifying the Master Sex Switch
In most mammals, presence of the Y chromosome → male; absence → female.
Quest timeline: researchers narrowed Y chromosome candidates from ~40,000 genes (1959) down to a single gene by 1990 — Sry.
In the presumptive testis, Sry gene is expressed in pre-Sertoli cells (somatic non-germ cells), producing Sry protein.
Sry protein → increases abundance of Sox9, another transcription factor.
Sox9 then:
- Alters transcription of many genes driving testis differentiation
- Drives production of FGF9 (testis-specific growth factor)
- Drives production of AMH (Anti-Müllerian Hormone)
- Turns off the Sry gene (Sry active only briefly, ~2 days in mice)
C · Hormones Drive Phenotypic Sex — Indifferent Gonad to Male or Female Tract
All embryos start with an indifferent gonad that has the potential to become either testis or ovary. At this stage, two duct systems co-exist:
→ In males: becomes epididymis, vas deferens, seminal vesicle, ampulla
→ In females: regresses (no testosterone)
→ In females: becomes oviduct, uterus, cervix, cranial vagina
→ In males: regresses under AMH
Also present: Urogenital sinus → becomes bladder (cranial) and vestibule/prostate (caudal)
→ Causes Müllerian duct inhibition/regression
→ Wolffian duct development: epididymis, vas deferens, seminal vesicle
(via 5α-reductase → DHT)
→ Male external genitalia: penis, scrotum, urethra
→ Brain sexual differentiation
Mesonephric tubules (future efferent ducts), mesonephric duct, paramesonephric (Müllerian) duct, undifferentiated sex cords, tunica albuginea forms
Epithelial cords → future seminiferous tubules; mesonephric tubules connect to rete testis; Müllerian duct still present but will regress
Rete testis → efferent ducts → epididymis; mesonephric duct → ductus deferens; seminiferous tubules; tunica albuginea
- PGC migration from yolk sac
- Sex cords develop in gonad; paramesonephric ducts develop
- Sex evident from gonadal structures
- Development of male ducts and testes OR female ducts and ovaries
- Formation of broad ligament (females)
- Testicular descent (species order: Bull & Ram → Boar & Human → Colt/Stallion latest)
D · Testis Descent, Hormone-Dependent Structures & Phenotype-Genotype Mismatches
The testes develop near the kidney and must migrate to the scrotum for spermatogenesis (requires cooler temperature).
- Gubernaculum — ligament from testis to scrotum; shortens and guides testis downward
- Testis passes through the inguinal ring → inguinal canal → scrotum
- Process driven by androgens (testosterone/INSL3) and differential growth
- Species timing varies (see 3rd trimester timing above)
The inguinal ring created for testis passage remains a weak point. Intestinal loops can herniate through it.
- Swine: 1/200 incidence
- Human children: 5/100 (5%)
- Can block intestinal blood flow; corrected by surgery
Epididymis, vas deferens (ductus deferens), seminal vesicle, ampulla — the Wolffian duct derivatives
Penis, scrotum, prostate, urethra (male external genitalia)
Requires: Testosterone → 5α-reductase → 5α-Dihydrotestosterone
Brain sexual differentiation: Testosterone → aromatase → Estradiol → masculinizes hypothalamic circuits
Genotype: 46;XY
Cause: 5α-reductase deficiency → little DHT in fetal life → female-appearing external genitalia at birth, undescended testes, normal internal male tract (testosterone-dependent)
At puberty: large testosterone surge overwhelms the deficiency → penis grows, testes descend → masculinization
Demonstrates distinction between T-dependent (internal) vs DHT-dependent (external) structures
SRY → Sox9 cascade establishes gonadal sex, but phenotypic sex depends on:
- Hormone production (Leydig/Sertoli cells functioning)
- Hormone receptors (androgen receptor intact)
- Enzyme activity (5α-reductase, aromatase)
Failure at any step = phenotype that doesn't match genetics
E · Primordial Germ Cell Biology, DSD & Freemartinism in Cattle
- Somatic cells — brain, liver, bone, muscle, skin, blood, etc.
- Germ cells — gametes and gamete precursors (sperm + oocytes); unique for undergoing meiosis
- Preserve genetic integrity through generations
- Generate genetic diversity (genetic recombination)
- Transmit genetic information to the next generation
- AIS (Androgen Insensitivity Syndrome) — Male pseudohermaphroditism: 46;XY, normal testes producing testosterone, but androgen receptor mutation → no response to androgens → female external phenotype, no uterus (AMH still works); 1:20,000
- Guevodoces — Male pseudohermaphroditism: 46;XY, 5α-reductase deficiency → female at birth; male at puberty
- CAH (Congenital Adrenal Hyperplasia) — Female pseudohermaphroditism: 46;XX, low glucocorticoids → adrenal overproduces androgens → masculinization of XX female
- Freemartin = female of mixed-sex (bull-heifer) twins
- Cause: shared placental vascular anastomosis (fusion of placentas) → blood exchange between twins
- AMH from male twin diffuses into female fetus → Müllerian (paramesonephric) duct inhibition → oviducts and uterus do not develop normally
- Variable reproductive tract abnormalities in the female twin
- Affects >90% of female twins co-gestated with a male
- Diagnosis: genetic test detecting Y chromosome in white blood cells (chimerism from fused blood supplies)
- Note: male co-twin is typically unaffected (testosterone dominates)
A · Male Gonads & The 5-Step Sperm Production System
Testis (singular) / Testes (plural)
Three functions:
- Gametogenesis — specifically spermatogenesis (sperm production)
- Produce male steroids — androgens (testosterone) and peptide hormones (inhibin)
- Produce fluid to move sperm through the ducts
1–25 × 10⁹ sperm/day (35,000–290,000 per second). "Plant must be air conditioned" — requires temp 2–10°C below body
Fluid absorption; membrane changes; nuclear & flagellar stabilization; motility acquisition; cytoplasmic droplet translocation (toward tail)
Stores 10–50 × 10⁹ spermatozoa; enough for 5–10 ejaculations; smooth muscle contractions move sperm at ejaculation
Add metabolic substrates (fructose, citrate), surface coatings, transport media. Glands: seminal vesicles, prostate, bulbourethral glands
Erection → Protrusion → Emission → Ejaculation
Most mammals have scrotal (extracorporeal) testes for cooling. Some species retain testes internally throughout life:
Why external? The reason is uncertain — but these internal-testis species all evolved alternate cooling mechanisms or heat-tolerant sperm.
B · Spermatic Cord Anatomy & Scrotal Cooling Adaptations
Extends from the body through the inguinal canal to each testis. Contains:
- Nerves, blood vessels, lymphatics
- Testicular artery (warm arterial supply from aorta)
- Pampiniform venous plexus (network of veins returning cool blood from testis) — surrounds the testicular artery
- Ductus deferens (vas deferens)
- Cremaster muscle (voluntary; elevates/lowers testis)
Spermatogenesis requires 2–10°C below core body temperature.
Mechanism: cool venous blood returning from the testis (≈33°C) surrounds and cools the warm testicular artery (entering at body temp ≈39°C) before it reaches the testis.
Warm incoming blood is cooled by outgoing blood — classic countercurrent exchange
A 2-lobed sac providing additional cooling adaptations beyond the pampiniform plexus
- Many sweat glands — evaporative cooling
- Very little hair — low insulation
- Very little subcutaneous fat — minimal insulation
- All features maximize heat dissipation
- Smooth muscle (involuntary)
- Has temperature sensors → trigger increased respiration rate when warm
- Cold: contracts → wrinkles scrotum → pulls testes toward body → warms
- Warm: relaxes → increases surface area → enhances cooling
- A pocket of peritoneum that descended with the testis
- Forms the vaginal cavity (fluid-filled space allowing testis to rotate freely)
- Visceral tunica vaginalis covers the testis surface directly
C · Testicular Temperature Problems & Cooling in Marine Mammals
- Bilateral: both testes retained → infertile (overheating → spermatogenesis fails) but normal testosterone and male appearance (Leydig cells still function)
- Unilateral: one testis descended → usually fertile
Many non-seasonal breeders show decreased fertility in warm months — testicular temperature rises as ambient temp increases
Testes too close to body wall → inadequate cooling → impaired spermatogenesis → subfertility
Abnormal dilation of pampiniform plexus veins → impaired countercurrent cooling → elevated testicular temperature → reduced sperm quality/fertility
Physicians suggest men with fertility problems wear boxers rather than briefs — reduces scrotal temperature by keeping testes away from body heat
Dolphins, whales, and seals have internal testes surrounded by warm body tissues and thick blubber. Yet they produce sperm. How?
Venous blood flowing through the dorsal fin and flukes is cooled by cold seawater via a periarterial venous rete (artery surrounded by ring of veins)
Cool venous blood from fins/flukes returns to juxtaposed arterial/venous plexuses near the aorta → cools the arterial blood heading to the testis
Same countercurrent heat exchange principle as the pampiniform plexus — just using fin/fluke venous return as the cooling source
The same blood plexus that cools the dolphin testis also cools the dolphin uterus — both are internal reproductive organs facing the same heat problem
A · Male Endocrinology
Releases GnRH (gonadotropin-releasing hormone) in pulses into the hypophyseal portal system. Pulse frequency and amplitude regulate FSH vs LH balance.
GnRH stimulates release of FSH and LH (gonadotropins) into systemic circulation.
FSH arm: FSH → Sertoli cells → inhibin (↓FSH feedback) + spermatogenesis support.
LH arm: LH → Leydig cells → testosterone → secondary sex characteristics + negative feedback.
Lipid-soluble; derived from cholesterol
- Examples: testosterone, estradiol, progesterone, cortisol
- Cross cell membranes freely
- Bind intracellular/nuclear receptors
- Receptor-hormone complex acts as transcription factor → gene expression
- Slow onset (hours–days) but prolonged effect
- Transported in blood bound to SSBG (sex steroid binding globulin)
Water-soluble; cannot cross membranes
- Examples: GnRH, FSH, LH, inhibin, activin, prolactin
- Bind membrane receptors (GPCRs)
- Signal via second messengers: cAMP → protein kinase A
- Fast onset (minutes) but shorter duration
- Circulate freely in blood (water-soluble)
- Produced by the liver
- Carries testosterone (and estradiol) in blood
- ~98% of testosterone is bound (inactive)
- Only free (~2%) testosterone is biologically active
- SSBG levels regulate testosterone bioavailability
All steroid hormones share this cholesterol precursor; pathway occurs mainly in Leydig cells
Testosterone is converted to more active forms in peripheral tissues
- Enzyme: 5α-reductase
- Sites: prostate, skin, hair follicles, genital skin
- DHT has greater androgen receptor affinity than testosterone
- Actions: prostate growth, male pattern baldness, external genital masculinization
- Enzyme: aromatase (CYP19)
- Sites: brain, adipose tissue, Sertoli cells
- Essential for: bone density, brain function/libido, spermatogenesis (in Sertoli cells)
- Males need some estrogen — deficiency causes osteoporosis and impaired spermatogenesis
Andropause
- Gradual decline in testosterone after age ~40
- Unlike menopause — not a complete cessation
- Symptoms: fatigue, decreased muscle mass, decreased libido, depression, decreased bone density
Testosterone Replacement Therapy (TRT)
- Forms: patches, gels, injections
- Improves symptoms of andropause
- Caution: exogenous T → negative feedback → ↓ GnRH/LH → ↓ endogenous testicular testosterone → ↓ spermatogenesis → infertility
A · Spermatogenesis & Testis Anatomy
- Seminiferous tubules — where spermatogenesis occurs
- Lined with Sertoli cells and developing germ cells
- Surrounded by contractile myoid cells
- Tubule lumen carries sperm toward rete testis → efferent ducts → epididymis
- Leydig cells — produce testosterone in response to LH
- Blood vessels (testicular artery, pampiniform veins)
- Lymphatics and nerves
- Macrophages and immune cells (regulated by BTB)
Characteristics
- Somatic cells — not germ cells
- Stop dividing at puberty → their number sets the maximum sperm production capacity
- Have FSH receptors and testosterone receptors
- Form tight junctions that create the blood-testis barrier
Functions
- Produce inhibin (↓FSH feedback) and androgen-binding protein (ABP)
- Provide nutrients, growth factors, and structural support to germ cells
- Phagocytose residual bodies after spermiogenesis
- Aromatize testosterone → estradiol (needed locally for spermatogenesis)
- Create immunologically privileged adluminal environment
- Formed by tight junctions between adjacent Sertoli cells
- Divides the tubule into two compartments:
Spermatogonia (stem cells) and early primary spermatocytes; connected to blood supply; mitosis occurs here
Meiotic cells (secondary spermatocytes) and spermatids; immunologically privileged; isolated from blood
Spermatocytogenesis (Mitosis)
- Spermatogonial stem cells (Type A) in basal compartment undergo mitosis
- Type A → Type B spermatogonia → primary spermatocytes (2n, diploid)
- Amplifies germ cell numbers; maintains stem cell pool
Meiosis (Reduction Division)
- Primary spermatocyte (2n) → Meiosis I → 2 secondary spermatocytes (n)
- Each secondary spermatocyte → Meiosis II → 2 spermatids (n)
- Net result: 1 primary spermatocyte → 4 haploid spermatids
- Occurs in adluminal compartment (above BTB)
Spermiogenesis (Differentiation — No Cell Division)
- Round spermatids → morphologically mature spermatozoa
- Acrosome formation from Golgi apparatus (enzyme cap for egg penetration)
- Flagellum develops from centrioles migrating to basal pole
- Nuclear condensation: histones replaced by protamines; DNA tightly packaged
- Mitochondria arrange into midpiece (energy for flagellar movement)
- Most cytoplasm shed as residual body → Sertoli cells phagocytose it