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Human Body Study

Seminiferous Tubules

The site of sperm production — tightly coiled tubes packed into each lobule of the testis. About 1–4 tubules per lobule, each 30–80 cm long when uncoiled, summing to roughly 250 m of tubule per testis.

Schematic of the testis A labelled diagram of the testis. The left figure is a cut-away sagittal view showing the fibrous capsule (tunica albuginea), the septa dividing the testis into lobules, the coiled seminiferous tubules within each lobule, the mediastinum testis with its rete testis, the efferent ductules leaving the testis, the epididymis draped along its posterior border (head, body, tail), the vas deferens, and the spermatic cord with its testicular artery and pampiniform venous plexus. The centre figure is a magnified cross-section of a single seminiferous tubule with adjacent interstitium, showing tall Sertoli cells spanning the tubule wall, the spermatogenic lineage maturing from basement membrane to lumen, peritubular myoid cells, and Leydig cells in the interstitial space. The testis — cut-away and tubule cross-section scrotum mediastinum head body tail testicular artery pampiniform plexus vas deferens seminiferous tubule (cross-section) lumen capillary Scrotum & spermatic cord the skin sac that holds both testes outside the body,and the cord that suspends each testis carrying itsvessels, nerves, and vas deferens. Vasculature & nerves the testicular artery and the pampiniform venousplexus that cools it; lymphatics; autonomic nerves. Tunics & lobules the tough fibrous capsule (tunica albuginea), theserous outer tunica vaginalis, and the septa carvingthe testis into ~250 wedge-shaped lobules. Seminiferous tubules the tightly coiled tubes within each lobule wheresperm are made — about 250 m of tubule per testis. Intratesticular ducts straight tubules feed the rete testis (a networkin the mediastinum), which drains via efferentductules to the head of the epididymis. Sertoli cells tall nurse cells spanning the tubule wall;their tight junctions form the blood-testis barrier. Spermatogenic cells the germ-cell lineage, base to lumen — spermatogonia→ spermatocytes → spermatids → spermatozoa. Leydig cells endocrine cells between the tubules — thesource of testosterone, driven by LH. Interstitial tissue the loose stroma between tubules — capillaries,lymphatics, macrophages, and connective tissue. Left: sagittal cut-away of one testis with its epididymis and spermatic cord. Centre: magnified seminiferous tubule and adjacent interstitium. Not to scale.

Wall layers (outside in)

Section titled “Wall layers (outside in)”
  • Peritubular myoid cells — a single ring of slim, contractile smooth-muscle-like cells just outside the basement membrane. Their slow, rhythmic squeeze pushes immotile sperm along the tubule toward the rete testis (sperm cannot swim until they finish maturing in the epididymis).
  • Basement membrane — the thin extracellular sheet on which the seminiferous epithelium sits.
  • Seminiferous epithelium — the working tissue of the wall, made of two cell families:
    • Sertoli cells — tall somatic “nurse” cells that span from basement membrane to lumen. Their tight junctions divide the wall into a basal compartment (open to blood) and an adluminal compartment (sealed off) — the blood-testis barrier.
    • Spermatogenic cells — the germ-cell lineage, layered between Sertoli cells from base (spermatogonia) to lumen (spermatids and spermatozoa).
  • Lumen — the central channel, filled with Sertoli-secreted tubular fluid that carries newly released sperm toward the rete testis.

What it does

Section titled “What it does”
  • Spermatogenesis — the entire ~74-day journey from a spermatogonial stem cell to a mature spermatozoon takes place between the Sertoli cells of this wall.
  • Conducts sperm — the peritubular myoid ring contracts in slow waves, and the tubular fluid carries sperm into the intratesticular ducts.
  • Hormonal output — Sertoli cells secrete inhibin B (which feeds back negatively on pituitary FSH) and, in fetal life, anti-Müllerian hormone (AMH).
  • Compartmentalises the germline — the blood-testis barrier walls off the developing germ cells from the immune system and from the body’s normal extracellular environment.

Spermatogenic wave

Section titled “Spermatogenic wave”

Adjacent stretches of tubule are not synchronised — each segment runs the cycle at a different phase, so at any one time some segments are releasing mature sperm into the lumen while others are still in mitosis or meiosis. The result, summed over the whole testis, is a steady output of ~100–200 million sperm per day rather than a synchronous burst.

Key idea

Section titled “Key idea”

The seminiferous tubule is a factory in tube form — Sertoli cells line the conveyor belt, germ cells progress along it from base to lumen as they mature, and the peritubular myoid ring keeps the product moving. The blood-testis barrier exists because the product (sperm) carries antigens that did not exist when the immune system learned what self meant — the wall must hide them.