Define spermatogenesis. Describe its stages, the cells of the testis and the factors controlling it.

Definition

• Begins at puberty and continues throughout life, unlike oogenesis
• Duration 64–74 days
• Output about 120 million sperm a day

Stages

Spermatogonium (46, XY — diploid) → Primary spermatocyte (46, XY) → First meiotic division → 2 secondary spermatocytes (23 — haploid) → Second meiotic division → 4 spermatids (23) → Spermiogenesis → 4 spermatozoa

• One primary spermatocyte therefore yields 4 functional sperm — in contrast to oogenesis, which yields one ovum and three polar bodies

Spermiogenesis

• Transformation of a round spermatid into a motile spermatozoon, without further division
• Golgi apparatus → acrosome, containing hyaluronidase and acrosin for penetrating the zona pellucida
• Nucleus condenses → forms the head
• Centriole → tail (flagellum)
• Mitochondria arrange spirally in the middle piece → supply ATP for motility
• Excess cytoplasm shed as the residual body

Cells of the Testis

Blood–testis barrier

• Formed by tight junctions between Sertoli cells
• Isolates the developing germ cells from blood → prevents an autoimmune reaction, since sperm antigens appear only after the immune system has learned self-tolerance
• Maintains a special fluid environment rich in androgen-binding protein

Hormonal Control

Hypothalamus → GnRH → Anterior pituitary → FSH and LH → FSH → Sertoli cell; LH → Leydig cell → testosterone → Both required for spermatogenesis

• Testosterone feeds back on LH; inhibin feeds back on FSH
• Intratesticular testosterone is 100 times the plasma level — hence exogenous testosterone suppresses spermatogenesis by shutting off LH

Factors Affecting Spermatogenesis

Spermatogenesis VS Oogenesis

Maturation and Capacitation

• Sperm leaving the testis are immotile and infertile
• They gain motility during 18–24 hours in the epididymis
• Capacitation occurs in the female genital tract over 1–10 hours — removal of surface glycoproteins allows the acrosome reaction and fertilisation
• This is why sperm cannot fertilise immediately after ejaculation, and why it matters in IVF

Applied Aspects

• Vasectomy — the vas is divided; sperm are still made but cannot pass. Testosterone and libido are unaffected. Sterility is confirmed only after semen analysis at 3 months
• Cryptorchidism must be corrected before 2 years of age; it also carries a raised risk of testicular malignancy
• Male infertility accounts for about 40% of infertile couples; semen analysis is the first investigation
• Varicocele — commonest correctable cause; raises scrotal temperature; commoner on the left because the left testicular vein drains at a right angle into the renal vein
• Mumps orchitis after puberty → may cause permanent damage to the seminiferous epithelium
• Anti-sperm antibodies may form if the blood–testis barrier is breached by trauma or vasectomy reversal
• Chemotherapy and radiotherapy damage the rapidly dividing spermatogonia → sperm banking should be offered before treatment