Male Reproductive System

Read the seminiferous epithelium like a vertical timeline: the youngest cells sit on the basement membrane, the most mature cells dangle into the lumen. Spermatogenesis takes roughly 74 days in the human, and the ladder runs in a fixed order.

Spermatogonia are the diploid stem cells on the basement membrane. Type A-dark cells are the reserve stem pool, type A-pale cells are the renewing pool, and type B cells are the ones committed to enter meiosis. Once a type B divides, its daughters become primary spermatocytes — these are the largest germ cells you see, and because they spend ~22 days in meiotic prophase (pachytene), their thick condensed chromatin makes them unmistakable.

Secondary spermatocytes are produced fleetingly — they finish meiosis II within hours, so you rarely catch them on a slide. Their daughters, the spermatids, are small haploid cells near the lumen. Spermatids do not divide; instead they undergo spermiogenesis — a remodelling in which the Golgi assembles the acrosome, the centriole grows the flagellum, the manchette of microtubules shapes the head, mitochondria spiral into the midpiece, and excess cytoplasm is shed as a residual body (phagocytosed by Sertoli cells). The product is the spermatozoon, released into the lumen carrying either 23+X or 23+Y.

Sertoli cells are the unsung directors of the whole tubule. Each one is a tall, irregular, columnar cell that stretches all the way from the basement membrane to the lumen, with deep lateral indentations cradling the developing germ cells at every stage of meiosis. The nucleus is pale and ovoid with a prominent nucleolus — once you recognise that pale nucleus near the basement membrane, you have found a Sertoli cell.

Their jobs are several. They nourish the germ cells (germ cells have no direct blood supply once they leave the basal compartment). They phagocytose residual bodies shed during spermiogenesis. They carry FSH receptors and respond by secreting androgen-binding protein (ABP), which traps testosterone inside the tubule at concentrations 100× serum; they also secrete inhibin (feedback to the pituitary) and, in the foetus, anti-Müllerian hormone that regresses the Müllerian duct system.

Most importantly, neighbouring Sertoli cells weld themselves together with occluding tight junctions, creating the blood-testis barrier. This barrier splits the epithelium into a basal compartment (containing spermatogonia and early primary spermatocytes) and an adluminal compartment (containing later spermatocytes, spermatids and sperm). The point of the barrier is immunological: meiotic and post-meiotic cells express novel surface antigens the immune system has never seen, so they would be attacked as foreign. Sertoli tight junctions hide them.

• Tall columnar cells spanning basement membrane → lumen, with an irregular outline embracing the germ cells; pale ovoid nucleus + prominent nucleolus.
• Functions: support & nourish germ cells; form the blood-testis barrier (Sertoli–Sertoli tight junctions) dividing basal & adluminal compartments; phagocytose residual bodies; secrete androgen-binding protein, inhibin & anti-Müllerian hormone.

The mature spermatozoon is built like a miniature submarine: a sharp warhead, a power core, and a long whip. Total length about 60 µm, of which the head is only ~5 µm.

The head houses the haploid nucleus, whose DNA is so tightly packed (histones replaced by protamines) that it stains deeply and uniformly. The anterior two-thirds of the head is covered by the acrosomal cap, a Golgi-derived lysosome-like vesicle loaded with hyaluronidase, acrosin and other hydrolases that digest the corona radiata and zona pellucida at fertilisation.

Behind the head comes the short neck (connecting piece, proximal centriole), then the midpiece — this is where the action lives. The midpiece contains the central axoneme (the universal 9+2 microtubule arrangement) surrounded by outer dense fibres, all wrapped in a helical mitochondrial sheath that supplies ATP for tail-beating. The principal piece is the longest segment, where mitochondria are replaced by a tough fibrous sheath. The end piece is the bare axoneme tip.

Now you are following sperm out of the testis. As they leave the rete (cuboidal epithelium, irregular anastomosing lumina), they enter 10–15 efferent ductules. The signature of these ductules under the microscope is a scalloped, wavy lumen — because the lining alternates between groups of tall ciliated columnar cells and groups of short non-ciliated cuboidal cells. The ciliated tufts beat sperm toward the epididymis; the short cells absorb most of the testicular fluid.

The epididymis is a single, immensely coiled tube about 6 m long folded into a comma along the posterior testis (head, body, tail). Its lining is uniform pseudostratified columnar epithelium with very long, branched, non-motile stereocilia (which are actually giant microvilli, not true cilia). Two cell types — tall principal cells with stereocilia, and basal cells at the basement membrane. A smooth muscle coat surrounds it, thin in the head and progressively thicker toward the tail. Functionally, sperm mature here (acquire forward motility and the ability to fertilise) and are stored in the tail until ejaculation. Travel time is roughly 12 days.

The ductus (vas) deferens is the muscular tube that pushes sperm from the tail of the epididymis through the spermatic cord into the ejaculatory duct. Its epithelium is similar pseudostratified columnar with short stereocilia, but the wall is famous: three layers of smooth muscle — inner longitudinal, middle circular, outer longitudinal. That sympathetic-driven peristaltic burst at ejaculation comes from this thick muscle coat.

Only ~5–10% of ejaculate is actually sperm. The rest is the contribution of three pairs (or one) of accessory glands. Learn each by its histology and its secretion.

The seminal vesicle is a paired, coiled pouch whose mucosa is so elaborately folded that on cross-section it looks like a honeycomb. The lining is pseudostratified columnar epithelium studded with yellow lipochrome pigment granules. It contributes about 60–70% of semen volume: viscous, alkaline, fructose-rich fluid (the sperm energy source), prostaglandins, fibrinogen and seminal-vesicle-specific coagulation proteins that gel the ejaculate.

The prostate is a walnut-sized fibromuscular organ surrounding the proximal urethra. It contains 30–50 tubuloalveolar glands arranged in three concentric zones (McNeal zones): the peripheral zone (~70% of the volume — the seat of prostate adenocarcinoma), the central zone (~25%), and the small transition zone surrounding the urethra (~5% — the seat of benign prostatic hyperplasia). The glandular lumina often contain concentric eosinophilic lamellated concretions called corpora amylacea; they increase in number with age and have no clinical importance, but they are a giveaway on slides. The prostate adds ~30% of semen volume: thin, slightly acidic fluid containing PSA (a serine protease that liquefies the coagulum after a few minutes), citric acid, fibrinolysin and zinc.

The paired bulbourethral (Cowper) glands are small compound tubuloalveolar mucous glands lying within the urogenital diaphragm. Their clear pre-ejaculatory secretion lubricates the urethra and neutralises residual urinary acidity ahead of the sperm.