Spermatogenesis | Anatomy2Medicine
spermatogenesis

Spermatogenesis

    • Male Reproductive Hormones
      • Fetal Life
        • In normal males, an SRY antigen released from the Y chromosome stimulates the medullary portion of the indifferent gonad to develop into a testis. (MCQ)
        • Wolffian and Müllerian ducts are initially present in both male and female fetuses.
        • In the absence of hormonal input (ie, in the normal female fetus), female internal and external structures develop (ie, Müllerian ducts develop). (MCQ)
        • Normal male development requires the presence of three hormones:

 

  • Testosterone
  • dihydrotestosterone (DHT
  • Müllerian-inhibiting factor (MIH).

 

        • Sertoli cells secrete MIH, which inhibits Müllerian duct development. (MCQ)
        • Hormone chorionic gonadotropin (hCG) and LH stimulate Leydig cells to secrete testosterone, which stimulates Wolffian duct development. (MCQ)
        • In certain tissues (eg, prostate)
          • 5-alpha-reductase converts testosterone to DHT, which stimulates the development of organs from the urogenital sinus and genital tubercle. (MCQ)

 

  • In the absence of MIH, the Müllerian ducts develop, which differentiate into female internal structures.

 

        • Wolffian ducts differentiate into the majority of male internal structures, namely, epididymis, vasa deferentia, and seminal vesicles.

 

  • The urogenital sinus and genital tubercle differentiate into the scrotum, penis, and prostate gland.

 

      • Puberty
        • Hypothalamic control of gonadotropin secretion is by GnRH.
        • GnRH is secreted in episodic bursts, which causes pulsatile release of FSH and LH from the anterior pituitary. (MCQ)
        • At puberty, the amplitude of the LH pulses becomes greater, particularly during sleep, driving the mean level of LH higher.
        • This increased LH stimulates the Leydig cells to again secrete testosterone. (MCQ)
      • Major Cell Types of the Testis
        • LH receptors are located on the cell membranes of the interstitial cells of Leydig. (MCQ)
        • LH stimulation results in increased synthesis and secretion of testosterone.
        • Blood testosterone provides a negative feedback signal to both the hypothalamus and the anterior pituitary to regulate LH secretion.
        • Much of the testosterone synthesized by the Leydig cells diffuses into adjacent Sertoli cells. (MCQ)
        • An androgen-binding protein synthesized by the Sertoli cells and secreted into the lumen of the seminiferous tubules helps maintain a high local concentration of testosterone. (MCQ)
        • FSH increases the synthesis of androgen-binding protein
        • Testosterone acts locally to facilitate spermatogenesis via testosterone receptors on the nuclear chromatin of the Sertoli cell.
        • Membranes of the Sertoli cells surround the germ cells, and nutrients destined for the germ cells must pass through these membranes.
        • FSH is essential for the initiation of spermatogenesis. (MCQ)
        • FSH occupies receptors located on the plasma membrane of Sertoli cells to increase the production of proteins.
        • Both FSH and testosterone are required for normal spermatogenesis.
        • Sertoli cells are the source of MIH. (MCQ)
        • Sertoli cells also secrete aromatase, which converts androgens to estrogens, and Sertoli cell tumors are associated with feminization (MCQ)
      • Anabolic Actions of Androgens
        • Androgens increase GH secretion, which drives IGF-1 to increase long bone growth, stimulating a growth spurt.
        • IGF-1 is the major stimulus for cell division of the cartilage-synthesizing cells located in the epiphyseal plates of long bones. (MCQ)
        • Thus, androgens stimulate the growth of long bones and are responsible for the greater average height of men compared to women.
        • Near the end of puberty, androgens promote the mineralization (fusion or closure) of the epiphyseal plates of long bones.
        • Protein synthesis is stimulated in muscle, causing the larger muscle mass in men as compared with women.
        • Erythropoietin secretion is stimulated by the kidneys and increases red blood cell production.
      • Androgenic Effects of Androgens
        • Androgens induce development of male accessory reproductive organs.
        • They increase development of male secondary sex characteristics.
        • They are required for libido and potency.
        • Androgens inhibit GnRH via the hypothalamus and LH secretion via the anterior pituitary. (MCQ)

 

  • They are required for maintenance of spermatogenesis.

 

      • Regulation of Testicular Function
        • GnRH from the hypothalamus stimulates FSH and LH release from the anterior pituitary. (MCQ)
        • A pulsatile release of GnRH is required to up-regulate its own pituitary re- ceptors and to prevent down-regulation of its receptors.
        • FSH acts on Sertoli cells to regulate spermatogenesis(MCQ)
        • Sertoli cells secrete inhibin, a protein that regulates FSH secretion from the anterior pituitary by negative feedback. (MCQ)
        • LH stimulates Leydig cell testosterone secretion. (MCQ)
        • Testosterone inhibits LH by inhibiting release of GnRH from the hypothalamus and LH release from the anterior pituitary. (MCQ)
      • Regulation of Spermatogenesis
      • The scrotum provides an environment 4C cooler than the abdominal cavity via a countercurrent heat exchanger located in the spermatic cord.
    • Male Sexual Response
      • Erection
        • is caused by dilation of the blood vessels in the erectile tissue of the penis.
        • The increased blood flow compresses veins, blocking outflow.
        • Efferent parasympathetic fibers and nonadrenergic noncholinergic fibers mediate erection. (MCQ)
        • Important neurocrine mediators are vasoactive intestinal peptide or nitric oxide. (MCQ)

 

  • Emission

 

        • is the movement of semen from the epididymis, vas deferens, sem nal vesicles, and prostate to the ejaculatory ducts
        • mediated by sympathetic adrenergic transmitters.
        • A sympathetic adrenergic-mediated contraction of the internal sphincter of the bladder prevents retrograde ejaculation of semen into the bladder. (MCQ)
        • Destruction of this sphincter by prostatectomy often results in retrograde ejaculation. (MCQ)

 

  • Ejaculation

 

      • caused by the rhythmic contraction of the bulbospongiosus and ischiocavernous muscles of the urogenital diaphragm.