Male Reproductive System

Introduction to the Male Reproductive System

  • The Gonads as Endocrine Glands: The primary function of gonads (both male and female) is to support the development and maturation of germ cells.

  • Male Gonads (Testes): Responsible for:

    • The development and maturation of sperm.

    • The synthesis and secretion of testosterone, the primary male sex steroid hormone.

  • Female Gonads (Ovaries): Responsible for:

    • The development and maturation of ova.

    • The synthesis and secretion of estrogen and progesterone.

Sexual Differentiation

  • Components of Differentiation: Includes the development of the gonads, the internal genital tract, and the external genitalia.

  • Characterization of Sex:

    • Genetic Sex: Defined by sex chromosomes (XY for males, XX for females).

    • Gonadal Sex: Defined by the presence of testes or ovaries.

    • Phenotypic (Genital) Sex: Defined by the physical appearance of the person as male or female.

Timeline of Differentiation

  • First 5 Weeks: Gonads are considered indifferent or bipotential (neither male nor female).

  • Week 7 (Males): The sex-determining region of the Y chromosome (SRYSRY gene) produces a gene product that triggers testes development.

  • Week 9 (Females): In the absence of the SRYSRY gene, ovaries begin to develop.

  • Key Principle: Genetic sex determines gonadal sex, and male gonads appear slightly earlier than female gonads.

Cell Types of the Testes

  • Germ Cells: Produce spermatogonia.

  • Sertoli Cells: Synthesize a glycoprotein hormone known as antimüllerian hormone (also called müllerian inhibiting substance or MIS).

  • Leydig Cells: Synthesize and secrete testosterone.

Determination of Phenotypic Sex

  • The "Default" Pathway: In the absence of testes (and thus no antimüllerian hormone or testosterone), the fetus will develop as a phenotypic female by default.

  • The Male Pathway: Two decisive factors from the testes ensure male phenotyping:

    • Antimüllerian Hormone (AMH): Produced by Sertoli cells; causes the atrophy of the müllerian ducts (which would otherwise become the female genital tract).

    • Testosterone: Produced by Leydig cells; stimulates the growth and differentiation of the wolffian ducts.

  • Wolffian Duct Derivatives: Give rise to the epididymis, vas deferens, seminal vesicles, and ejaculatory ducts.

  • External Genitalia (Male): Includes the scrotum and the penis.

  • Internal Genital Tract (Male): Includes the prostate, seminal vesicles, vas deferens, and epididymis.

Anatomy of the Male Reproductive System

Classification of Organs

  • Primary Sex Organs (Gonads): Testes.

  • Accessory Sex Organs:

    • Seminal vesicles.

    • Prostate gland.

    • Urethra.

    • Penis.

Internal vs. External Structures

  • Internal Structures: Vas deferens, prostate, seminal vesicle, bulbourethral gland.

  • External Structures: Scrotum, penis, testes, epididymis.

Specific Anatomical Landmarks

  • Urinary Bladder and Symphysis Pubis.

  • Ductus Deferens (and the Ampulla of the ductus deferens).

  • Penis Components: Corpora Cavernosa, Corpora Spongiosum, Glans penis, and Prepuce.

  • Ejaculatory Duct and Bulbourethral Gland.

  • Epididymis (located atop the testis).

Physiology of Puberty

The Hypothalamic-Pituitary Axis

  • GnRH (Gonadotropin-Releasing Hormone): Activity begins as early as gestational week 4, but levels remain very low until the onset of puberty.

  • FSH and LH: Secretion of Follicle-Stimulating Hormone and Luteinizing Hormone begins between gestational weeks 10 and 12, remaining low until puberty.

Lifespan Variations in FSH and LH

  • Childhood: FSH levels are relatively higher than LH levels.

  • Puberty and Reproductive Years:

    • GnRH, FSH, and LH levels increase and exhibit pulsatile secretion.

    • The ratio reverses: LH levels become higher than FSH levels.

    • Pulsatility stimulates the secretion of testosterone and estradiol, leading to secondary sex characteristics.

  • Senescence (Old Age): FSH levels once again become higher than LH levels, similar to childhood.

Physical Changes in Male Puberty

  • Hormonal Activation: Growth of Leydig cells and increased testosterone synthesis.

  • Testicular Growth: Driven by an increase in the number of seminiferous tubules.

  • Accessory Organs: Growth of the prostate and other sex accessory organs.

  • Secondary Characteristics:

    • Pronounced linear growth spurt (followed by the closure of epiphyses).

    • Growth of facial, pubic, and axillary hair.

    • Growth of the penis.

    • Lowering of the voice due to increased larynx and vocal cord size.

    • Spermarche: The initiation of spermatogenesis.

Growth Determinants

  • Infantile (15% of adult height): Nutrition, thyroid hormones, health/happiness.

  • Childhood (40% of adult height): Growth hormone, thyroid hormones, genetics, health/happiness.

  • Pubertal (15% of adult height): Testosterone/estrogen and growth hormone.

  • Fetal (30% of adult height): Uterine environment.

The Testes and Spermatogenesis

Environment

  • Temperature Regulation: The testes are located in the scrotum outside the body cavity to maintain a temperature of 35∘C−36∘C35^{\circ}\text{C}-36^{\circ}\text{C}.

  • Importance: This is 1∘C−2∘C1^{\circ}\text{C}-2^{\circ}\text{C} below normal body temperature, which is essential for normal spermatogenesis.

Seminiferous Tubules

  • Comprise 80% of the adult testes.

  • Structure: Convoluted loops (120−300 μm120-300\,\mu\text{m} in diameter) arranged in lobules.

  • Epithelial Cell Types:

    1. Spermatogonia: Stem cells.

    2. Spermatocytes: Cells in the process of becoming sperm.

    3. Sertoli Cells: Support cells for developing sperm.

Specialized Functions of Sertoli Cells

  • Support and nourish spermatogenic cells until release.

  • Aromatase Secretion: Converts androgens into estrogen.

  • Androgen-Binding Protein (ABP) Secretion: Essential for maintaining high testosterone levels during spermatogenesis.

  • Inhibin Secretion: Inhibits FSH release from the anterior pituitary.

  • Activin Secretion: Increases FSH release (opposite of inhibin).

  • MRF (Müllerian Regression Factor): Responsible for the regression of müllerian ducts in the fetus.

Leydig Cells

  • Comprise 20% of the adult testes.

  • Found in the connective tissue between tubules.

  • Function: Synthesize and secrete testosterone.

  • Effects of Testosterone:

    • Paracrine: Locally supports spermatogenesis in Sertoli cells.

    • Endocrine: Systemic effects on organs like skeletal muscle and the prostate.

The Process of Spermatogenesis

  • Duration: One full cycle takes approximately 64 days.

  • Production Rate: 2 million spermatogonia begin the process daily; since each produces 64 spermatozoa, the daily yield is approximately 128 million sperm.

  • Stages (Periphery to Lumen):

    1. Spermatogonium (4646 chromosomes): Undergoes mitosis.

    2. Primary Spermatocyte (4646 chromosomes): Undergoes meiosis I.

    3. Secondary Spermatocyte (2323 chromosomes): Undergoes meiosis II.

    4. Spermatid (2323 chromosomes): Undergoes transformation (Spermiogenesis) to reduce cytoplasm.

    5. Spermatozoa (2323 chromosomes): Released into the lumen (initially immotile).

Maturation in the Epididymis

  • Structure: Head, body, and tail.

  • Timeframe: Sperm take about 12 days to mature and develop motility in the epididymis.

  • Storage: Mature sperm are stored in the tail of the epididymis until ejaculation.

Anatomy of a Sperm Cell

  • Head: Contains the nucleus and very little cytoplasm.

    • Acrosome (Cap): Covers the head; filled with lysosomes/enzymes for fertilization.

  • Midpiece: Contains abundant mitochondria to generate energy for movement.

  • Tail (Flagellum): Provides motility (Principal piece and end piece).

Copulation and Ejaculation

Physiological Response

  • Arousal: Vasodilation increases blood flow to the corpora cavernosa and corpus spongiosum, leading to penile enlargement and erection.

  • Ejaculation Mechanism: Smooth muscle contractions in the epididymis push sperm into the vas deferens. The vas deferens joins the seminal vesicle duct near the prostate to form the ejaculatory duct.

Glandular Secretions (Semen Composition)

  • Seminal Vesicles:

    • Fructose: Energy for sperm motility.

    • Citrate & Fibrinogen.

    • Prostaglandins: React with cervical mucus to make it penetrable and induce peristaltic contractions in the female tract to propel sperm.

  • Prostate Gland: Secretes an alkaline fluid to help thicken the semen and neutralize acidic environments.

  • Bulbourethral (Cowper’s) Glands: Release a thick fluid to lubricate the urethral opening and clear urine residue from the urethra.

Capacitation and Fertilization

  • Sperm cannot fertilize an egg immediately after ejaculation.

  • Capacitation: Occurs after 4–6 hours in the female reproductive tract.

    • Inhibitory factors are washed away.

    • Cholesterol is withdrawn from the sperm membrane.

    • Surface proteins are redistributed.

    • Ca2+Ca^{2+} Influx: Increases motility to a whiplike motion.

  • Acrosomal Reaction: The acrosomal membrane fuses with the outer sperm membrane, creating pores for hydrolytic and proteolytic enzymes to escape and penetrate the ovum's protective layers.

Male Sex Hormones (Androgens)

Types of Androgens

  1. Testosterone: Secreted in the largest quantities.

  2. Dihydrotestosterone (DHTDHT): More potent/active than testosterone.

  3. Androstenedione.

Transport and Metabolism

  • 90% of circulating testosterone is bound to plasma proteins (Sex Hormone-Binding Globulin and Albumin).

  • Testosterone Synthesis Pathway (Leydig Cell):

    • Cholesterol →\rightarrow Pregnenolone (via CSCCE enzyme).

    • Two pathways exist: Delta-4 and Delta-5.

    • Intermediates include Progesterone, 17-alpha-hydroxyprogesterone, Dehydroepiandrosterone (DHEA), and Androstenedione.

    • Enzymes involved: 3-β-HSD3\text{-}\beta\text{-HSD}, 17-α-hydroxylase17\text{-}\alpha\text{-hydroxylase}, 17-20 Lyase17\text{-}20\text{ Lyase}, and 17-ketosteroid reductase17\text{-ketosteroid reductase}.

  • Conversion: Testosterone is converted to the more active DHTDHT by the enzyme 5-α-reductase5\text{-}\alpha\text{-reductase} in target tissues.

Mode of Action

  • Testosterone and DHTDHT bind to intracellular receptors (RR), which then influence DNA transcription in the nucleus.

  • Primary Effects:

    • Spermatogenesis.

    • Sexual differentiation (Wolffian stimulation and virilization).

    • Sexual maturation.

    • Control of gonadotropin secretion.