17. Physiology of Reproduction: Male Reproductive System, Spermatogenesis, Hormonal Activity of the Testes, Androgens – Types, Physiological Effects, and Control of Secretion. Erection and Ejaculation

1. Male Reproductive System: Structures

• Testes: Primary male reproductive organs producing sperm and hormones.

• Sperm production ducts: Seminiferous tubules inside testes where spermatogenesis occurs.

• Reproductive glands:

  • Seminal vesicles

  • Prostate gland

  • Bulbourethral glands
    Produce fluids contributing to semen.

• Transporting ducts:

  • Epididymis (sperm maturation and storage)

  • Vas deferens (conveys sperm during ejaculation)

  • Urethra (ejaculates semen outside the body)

2. Spermatogenesis

• Process: Formation of spermatozoa from primordial germ cells in seminiferous tubules.

• Stages:

  • Spermatogonium (diploid) → primary spermatocyte (mitosis)

  • Primary spermatocyte → 2 secondary spermatocytes (meiosis I)

  • Secondary spermatocyte → 2 spermatids each (meiosis II)

  • Spermatid → mature spermatozoa (haploid)

• Outcome: 1 spermatogonium produces 4 spermatozoa.

• Role of Sertoli cells:

  • Maintain supportive environment for sperm development.

  • Secrete meiosis-initiating substances.

  • Produce testicular fluid aiding sperm transport.

  • Secrete androgen-binding protein (concentrates testosterone near sperm).

  • Protect spermatids from immune system attack.

3. Hormonal Activity of the Testes

• Androgens:

  • Testosterone:

    • Primary male androgen.

    • Stimulates sperm production.

    • Develops male secondary sexual characteristics (deep voice, muscle mass, genital growth).

  • Androstenedione:

    • Testosterone precursor.

    • Involved in early pubic and underarm hair growth.

  • Dihydrotestosterone (DHT):

    • Responsible for hair distribution.

    • Differentiates scrotum, penis, prostate.

  • Dehydroepiandrosterone (DHEA):

    • Weak androgen from adrenal glands, precursor for other androgens.

• Control of secretion:

  • GnRH (hypothalamus) → stimulates pituitary secretion of FSH and LH.

  • FSH:

    • Stimulates Sertoli cells → promotes spermatogenesis.

    • Sertoli cells produce inhibin B, inhibiting FSH (negative feedback).

  • LH:

    • Stimulates Leydig cells → testosterone production.

    • Testosterone inhibits LH and GnRH secretion via negative feedback.

  • CRH (hypothalamus) → stimulates ACTH → adrenal androgen production; regulated by testosterone feedback.

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4. Erection

• Triggered by mechanical stimulation of penis mechanoreceptors and CNS input (thoughts, emotions, sight).

• Parasympathetic nervous system activation causes:

  • Release of nitric oxide (NO) → arteriolar dilation → increased blood flow.

  • Blood fills corpora cavernosa → erection.

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5. Ejaculation

• Reflex mediated by the sympathetic nervous system.

• Process:

  1. Sperm move from epididymis to vas deferens.

  2. Seminal vesicles add seminal fluid in ampulla.

  3. Semen passes through ejaculatory ducts.

  4. Prostate gland adds prostatic fluid.

  5. Bulbourethral glands add lubricating fluid.

  6. Semen (sperm + seminal fluid + prostatic fluid + lubricant) is expelled via urethra during orgasm.

• Sperm count:

  • Normal: > 60 million/mL

  • Oligospermia: < 20 million/mL (low sperm count)i w