Reproductive Development and Male Reproductive Physiology

Introduction to Sex and Gender

• Definitions and Terminology:

  • Sex (Biological Sex): Determined by reproductive organs and their functions. It is anatomical and influenced by chromosomes, genes, and hormones.

  • Gender: Socially or culturally determined traits, characteristics, or expectations of being masculine or feminine. It is culturally specific and fluid.

  • Gender Identity: An individual's psychological sense of self, which may not align with the sex assigned at birth.

  • Gender Expression: The communication of gender through behavior or appearance, which can be constrained by social or cultural factors.

• Key Identity Terms:

  • Intersex: An umbrella term for individuals born with reproductive/sexual anatomy, chromosomes, or hormonal patterns that do not fit typical binary definitions of male or female. This applies to approximately $1.7\%$ of all births.

  • Genderqueer: An umbrella term for identities falling outside the traditional male/female binary.

  • Bigender: An individual whose identity is split over two genders, either simultaneously or at different times.

  • Androgynous: An individual who identifies or presents as neither distinguishably masculine nor feminine.

Sexual Differentiation

• Levels of Sexual Differentiation:

  • Chromosomal Sex: The set of chromosomes possessed at fertilization (typically $XX$ or $XY$).

  • Genetic Sex: Depends on the presence of the $SRY$ gene on the $Y$ chromosome. $SRY$ expression leads to testis development; its absence leads to ovary development.

  • Gonadal Sex: The development of ovaries or testes based on the $SRY$ gene.

  • Anatomical Sex: Internal and external genitalia, which depend on exposure to testicular hormones.

• Historical Context:

  • Alfred Jost (1940s): Conducted experiments removing fetal gonads to observe sexual differentiation. This provided the first scientific indication that biological sex involves multiple factors beyond a simple binary.

• Chromosomal Variations (Karyotypes):

  • Normal Female: $46,XX$

  • Normal Male: $46,XY$

  • Turner Syndrome: $45,XO$ (one $X$ chromosome is partial or missing).

  • Triple X Syndrome: $47,XXX$.

  • XYY Syndrome: $47,XYY$.

  • Klinefelter Syndrome: $47,XXY$.

Prenatal Development and Variations

• Gonadal Formation:

  • Development begins around the $5^{th}$ week of gestation.

  • Testosterone and Anti-Mullerian Hormone (AMH): Crucial for male internal genitalia development. In their absence, female genitalia develop.

  • Internal Duct Systems:

    • Paramesonephric Ducts (Mullerian): Regress in males due to $AMH$.

    • Mesonephric Ducts (Wolffian): Differentiate into male structures under hormonal influence.

  • External Genitalia: Dependent on the presence of $5\alpha$-dihydrotestosterone ($DHT$).

• Variations in Sexual Development (VSD):

  • Complete Androgen Insensitivity Syndrome (CAIS):

    • Frequency: Approximately $2$ to $5$ in every $1,000,000$ $XY$ births.

    • Genetics: $XY$ chromosomes, $SRY$ present (genetic male).

    • Phenotype: Appears female at birth; external genitalia are female, but the individual has undescended testes.

    • Mechanism: Cells cannot respond to testosterone due to a receptor mutation. $AMH$ still causes female internal ducts to regress, resulting in no internal male or female genitalia and a short vagina.

  • Androgen Deficiency ($5\alpha$-reductase deficiency):

    • Mutation in enzymes synthesizing testosterone or converting it to $5\alpha$-$DHT$.

    • Often results in males with external female characteristics at birth, with masculinization occurring at puberty due to a surge in androgens.

  • Androgen Excess (Congenital Adrenal Hyperplasia - CAH):

    • Frequency: $1$ in every $15,000$ live births.

    • Mechanism: Mutation in enzymes converting progestin to corticosteroids, leading to excess androgen production and masculinization of female infants.

Anatomy of the Male Reproductive System

• The Testes:

  • The male gonads responsible for spermatogenesis (sperm production) and androgen production.

  • Temperature Regulation: Testes must remain $2^{\circ}C$ to $4^{\circ}C$ below core body temperature.

  • Cremaster and Dartos Muscles: Contract in the cold to bring testes closer to the body; relax in heat to encourage airflow.

  • Structures:

    • Tunica Vaginalis: A serous membrane lining the scrotal cavity to reduce friction.

    • Tunica Albuginea: Dense connective tissue forming partitions (septate testis).

    • Seminiferous Tubules: Approximately $800$ tightly coiled tubules per testis where sperm is produced. Total length is about half a mile.

• Cellular Composition:

  • Sertoli Cells (Supporting Cells):

    • Maintain the blood-testis barrier via tight junctions.

    • Support mitosis and meiosis via stimulation by $FSH$ and testosterone.

    • Provide nutrients and chemical stimuli to spermatids.

    • Secrete Inhibin (negative feedback for $FSH$ and $GnRH$).

    • Secrete luminal fluid and androgen-binding protein.

    • Secrete Mullerian Inhibiting Factor in embryos.

  • Leydig Cells (Interstitial Cells): Located in connective tissue; produce $95\%$ of male testosterone.

Spermatogenesis and Sperm Structure

• The Process of Spermatogenesis:

  1. Spermatogonia: Stem cells divide by mitosis; one remains a stem cell, the other becomes a primary spermatocyte ($16$ days).

  2. Primary Spermatocytes: Undergo meiosis I to form secondary spermatocytes ($24$ days).

  3. Secondary Spermatocytes: Undergo meiosis II to produce four haploid spermatids (takes a few hours).

  4. Spermiogenesis: Physical maturation of spermatids into spermatozoa ($24$ days).

• Sperm Anatomy:

  • Head: Contains DNA and the Acrosome (a cap filled with lysosomal enzymes for fertilization).

  • Midpiece: Contains mitochondria for $ATP$ production.

  • Tail (Flagellum): Provides motility.

  • Sperm production rate: $100$ to $300,000,000$ per day.

The Male Duct System and Accessory Glands

• Path of Sperm: Seminiferous tubules $\rightarrow$ Straight tubules $\rightarrow$ Rete testis $\rightarrow$ Efferent ductules $\rightarrow$ Epididymis $\rightarrow$ Vas deferens $\rightarrow$ Ejaculatory duct $\rightarrow$ Urethra.

• Epididymis: A coiled tube where sperm mature for up to $3$ weeks, gaining motility and stability.

• Vas Deferens (Ductus Deferens): Approximately $45\,cm$ long; part of the spermatic cord. Thick smooth muscle layer for peristaltic transport during ejaculation.

• Accessory Glands:

  • Seminal Vesicles ($60\%$ of semen): Secrete alkaline fluid with fructose ($ATP$), prostaglandins (motility), and clotting proteins.

  • Prostate Gland ($25\%$ of semen): Secrete milky, slightly acidic fluid containing citrate, acid phosphatase, and proteolytic enzymes to liquefy coagulated semen.

  • Bulbourethral (Cowper's) Glands: Produce pre-ejaculate to neutralize urethral acidity.

Physiology of the Sexual Response

• Erection Mechanism:

  • Stimulated by erotic thoughts, sight, or tactile stimuli (mechanoreceptors).

  • Parasympathetic activation triggers the release of Nitric Oxide ($NO$).

  • $NO$ increases $cGMP$, which relaxes smooth muscle in penile arterioles.

  • Vasodilation occurs; blood traps in the corpora cavernosa and corpus spongiosum via passive vein compression.

  • Viagra (Sildenafil): Inhibits $PDE5$, the enzyme that breaks down $cGMP$, thereby maintaining the erection.

• Ejaculation Phases:

  1. Emission: Sympathetic nervous system causes sperm and fluid to move into the urethra.

  2. Ejaculation: Powerful rhythmic contractions discharge semen from the urethra.

  3. Resolution: Detumescence (loss of erection) occurs as blood leaves the erectile tissue.

Hormonal Regulation (HPG Axis)

• Hypothalamus: Pulsatile release of GnRH every $1$ to $3$ hours.

• Anterior Pituitary: Secretes FSH (acts on Sertoli cells for spermatogenesis) and LH (acts on Leydig cells for testosterone production).

• Negative Feedback Loops:

  • Testosterone: Inhibits $GnRH$ and $LH$ at high levels.

  • Inhibin: Secreted by Sertoli cells; inhibits $FSH$ release when sperm production is high.

Questions & Discussion

• Quiz 1: What is the biological basis of sex vs. gender?

  • Answer: Sex is anatomical/biological (organs, chromosomes); Gender is a social/cultural construct.

• Quiz 2: True or False - $1.7\%$ of births are intersex?

  • Answer: True.

• Quiz 3: Name the three accessory glands of the male reproductive system.

  • Answer: Seminal vesicles, Prostate gland, Bulbourethral glands.

• Quiz 4: What is the primary function of Sertoli cells?

  • Answer: Supporting developing sperm and maintaining the blood-testis barrier.

• Quiz 5: Which hormone is responsible for the development of male internal genitalia?

  • Answer: Testosterone and Anti-Mullerian Hormone ($AMH$).