Endocrine Signalling & Reproductive Biology 4

  • Introduction to Reproduction in Mammals

    • Reproductive control in mammals without strong seasonal breeding is simpler in males than females.

  • Anatomy of Male Reproductive System

    • Key Structures:
    • Testis: Where sperm production occurs.
    • Epididymis: Sperm storage site, holds sperm for up to 15 days.
    • Ductus Deferens: Tubing that conducts ejaculate out of the body.

  • Sperm Production

    • Occurs in seminiferous tubules:
    • Diameter: about 150 microns, length: around 80 cm.
    • Adult males have 300 – 900 meters of tubules, producing waves of spermatogenesis.
    • Join at rete testis, drain into epididymis via efferent ducts.

  • Ejaculation Process

    • Sperm follows a circuitous route to pass through accessory glands.
    • Accessory Glands: Seminal vesicles, prostate gland, bulbourethral gland.
    • Ejaculate composition: 5% sperm, 95% fluids from glands.
    • Functions of accessory glands:
    • Protect, support, feed, and buffer sperm.
    • Lubricate sperm during muscular contractions through the ductus deferens.

  • Sperm Production Statistics

    • Average semen volume: 1.5 ml.
    • Sperm per ejaculate: approximately 39 million.
    • Sperm vitality: about 58%.
    • Progressive motility: 32% of sperm.
    • Morphological normal forms: only about 4%.

  • Testis Structure

    • Compartmentalization in Adult Testis:
    • Coiled seminiferous tubules lead to epididymis.
    • Important to have normal structure for fertility.
    • Cell Types:
    • Leydig Cells: Produce androgens (testosterone).
    • Sertoli Cells: Form tight junctions, support germ cells, secrete inhibin B.

  • Germ Cell Development

    • Spermatogonia: Divide by mitosis to form primary spermatocytes.
    • Meiosis Process:
    • First meiotic division: Two haploid secondary spermatocytes.
    • Second meiotic division: Results in groups of four spermatids connected by cytoplasmic bridges.
    • Spermiogenesis:
    • Development into spermatozoa with acrosome, tail, and condensed nucleus.
    • Takes about 75 days from spermatogonia to mature spermatozoa.

  • Capacitation

    • Process occurring during ejaculation, enabling sperm to swim effectively.
    • Sperm must be injected directly into the egg site if retrieved from seminiferous tubules (intracytoplasmic sperm injection).

  • Feedback Regulation of Male Reproduction

    • Feedback mechanisms are simpler in males:
    • Pulses of gonadotropin-releasing hormone (GnRH) control LH and FSH secretion from the pituitary gland.
    • LH stimulates testosterone secretion from Leydig cells.
    • FSH supports Sertoli cell function and inhibin B secretion.

  • Testosterone Functions

    • Maintains spermatogenesis and supports accessory glands.
    • Balancing act between local (testicular) and systemic (circulation) effects.
    • Other effects include development of secondary sexual characteristics, maintenance of muscle mass, and sexual behavior.

  • Hypogonadism

    • Decreased testosterone production leads to lower sex drive, muscle mass, and sperm count, impacting fertility.
    • Two types: Primary (testicular damage) and Secondary (pituitary gland issues).

  • Inhibin B and Fertility Markers

    • Indicative of Sertoli cell function; normal levels correlate with good fertility.
    • Each Sertoli cell supports 30-50 developing sperm.
    • Klinefelter's syndrome: Results in low inhibin B and infertility due to abnormal chromosomes.

  • Conclusion

    • The complex interplay of hormones, cell types, and structural relationships in male reproduction are crucial in ensuring fertility and overall reproductive health.