Unit 16 — Male Reproductive · Question Bank

The seminiferous tubule is the functional unit of the testis — the place where diploid stem cells are converted into haploid spermatozoa. A working answer walks the examiner through the tubule wall from outside in and then names the cells in each layer in the order they sit.

Limiting (peritubular) membrane

The outermost coat of the tubule is the limiting membrane, a sleeve of collagen fibrils interleaved with 3–5 layers of flattened, smooth-muscle-like contractile myoid cells. Their slow rhythmic contractions create the gentle peristaltic waves that move the immotile spermatozoa — suspended in Sertoli-secreted fluid — along the tubule toward the rete testis.

Basement membrane

Immediately deep to the limiting layer is a continuous basement membrane on which the seminiferous epithelium rests. Its integrity is essential because every germ cell begins its journey here as a spermatogonium attached to it.

Seminiferous (germinal) epithelium — spermatogenic cells

The epithelium is thick and stratified-looking because it contains successive cohorts of germ cells in different stages of meiosis, arranged in a vertical timeline from basement membrane to lumen. The youngest cells — the spermatogonia — are diploid stem cells sitting on the basement membrane. Type A-dark spermatogonia are a reserve pool, type A-pale are the renewing pool, and type B cells are the ones committed to meiosis. Their daughters become primary spermatocytes, the largest cells of the line, which spend roughly 22 days in the prophase of meiosis I; their thick condensed chromatin (pachytene) makes them unmistakable on a slide. Meiosis I yields fleeting secondary spermatocytes that complete meiosis II within hours, so they are rarely caught on section. The resulting haploid spermatids are small cells near the lumen that undergo spermiogenesis — a remodelling, with no further division, in which the Golgi builds the acrosomal cap, the centriole grows the flagellum, mitochondria spiral into the midpiece, and excess cytoplasm is cast off as a residual body. The end product, the spermatozoon, is released into the lumen carrying either 23+X or 23+Y. The entire cycle takes about 74 days in the human male.

Seminiferous epithelium — Sertoli cells

Threaded between the germ cells are the tall columnar Sertoli (sustentacular) cells, each spanning basement membrane to lumen, with an irregular outline that cradles every developing germ cell in its lateral indentations. They scaffold and nourish spermatogenesis, phagocytose residual bodies, carry FSH receptors, and secrete androgen-binding protein (which traps testosterone at high concentration inside the tubule), inhibin and, in the foetus, anti-Müllerian hormone. Most importantly, adjacent Sertoli cells are linked by occluding tight junctions that form the blood–testis barrier, dividing the epithelium into a basal compartment (spermatogonia, early primary spermatocytes) and an adluminal compartment (later spermatocytes, spermatids, sperm). The barrier shields haploid germ cells, which express novel surface antigens, from immune surveillance.

Interstitium — Leydig cells

Outside the tubule, in the loose connective tissue between coils, sit the eosinophilic clusters of Leydig (interstitial) cells. They are polygonal cells with abundant smooth ER, tubular-cristae mitochondria and lipid droplets — the classic ultrastructure of a steroid-secreting cell — and they convert cholesterol to testosterone under pituitary LH control. The testosterone diffuses into the adjacent tubule (where Sertoli-bound ABP holds it locally) and into the bloodstream.

Sertoli cells are the directors of the seminiferous tubule. To describe them well, give the examiner their structural identity first, then move through their functions, then build the blood–testis barrier on top of that scaffold, and finally state why the barrier matters.

Structure on light and electron microscopy

Each Sertoli cell is a tall, irregular, columnar cell that stretches from the basement membrane all the way to the tubular lumen. Its lateral surfaces are deeply indented to cradle developing germ cells at every stage. The nucleus is characteristically pale, ovoid or triangular, often basally placed, and carries a prominent central nucleolus — on H&E this pale nucleus near the basement membrane is the diagnostic clue. Ultrastructurally the cytoplasm is rich in smooth endoplasmic reticulum, mitochondria, lysosomes (for phagocytosis) and bundles of intermediate filaments.

Functions

Their first job is mechanical and metabolic support: germ cells, once they cross into the adluminal compartment, lose direct access to blood and depend entirely on Sertoli cells for lactate, amino acids and growth factors. Sertoli cells also phagocytose the residual bodies shed by spermatids during spermiogenesis, recycling their components. They carry FSH receptors and respond by secreting androgen-binding protein (ABP) into the tubular lumen — ABP traps testosterone diffusing in from the Leydig cells and holds it at intratubular concentrations roughly 100 times serum levels, which is essential for spermatogenesis. They also secrete inhibin (which feeds back negatively on pituitary FSH) and, in the foetal testis, anti-Müllerian hormone, which causes regression of the paramesonephric (Müllerian) ducts and so commits the embryo to a male internal genital tract. Finally, after spermiogenesis they participate in spermiation, releasing mature sperm into the lumen.

The blood–testis barrier

Just above the spermatogonia, adjacent Sertoli cells are welded together by extensive tight (occluding) junctions. This continuous ring of junctions creates the blood–testis barrier and divides the seminiferous epithelium into two compartments. The basal compartment, between the basement membrane and the junctions, contains the spermatogonia and the earliest primary spermatocytes; it freely communicates with the interstitial fluid. The adluminal compartment, above the junctions, contains later primary spermatocytes, secondary spermatocytes, spermatids and spermatozoa, and is isolated from blood-borne molecules and immune cells. As germ cells progress, the tight junctions transiently open beneath them and re-form above, so the cell moves from basal to adluminal compartment without ever breaching the barrier.

Significance

Two consequences flow from this arrangement. Immunologically, meiotic and post-meiotic germ cells express novel surface antigens that the immune system has never encountered (and is not tolerant of); the barrier hides them and prevents an autoimmune orchitis. Physiologically, the barrier lets Sertoli cells maintain a unique adluminal micro-environment — high testosterone, high potassium, controlled pH — that is required for meiosis and spermiogenesis. Clinically, breakdown of the barrier (mumps orchitis, testicular trauma, ruptured vasectomy site) is a recognised cause of anti-sperm antibody formation and immunological infertility.

Spermatogenesis is the continuous, ~74-day-long process by which a diploid stem cell sitting on the basement membrane of the seminiferous tubule becomes a haploid, motile spermatozoon released into the lumen. It has three sequential phases — spermatocytogenesis (mitotic phase), meiosis, and spermiogenesis (morphological phase) — and the entire sequence is choreographed by Sertoli cells with hormonal support from FSH and intratubular testosterone.

Phase 1 — Spermatocytogenesis

The process begins with spermatogonia, diploid stem cells resting on the basement membrane. Three functional sub-types exist in humans. Type A-dark spermatogonia are the long-term reserve pool, dividing slowly. Type A-pale spermatogonia are the renewing pool: each mitosis produces one cell that stays on as A-pale and one that commits forward, becoming a type B spermatogonium. Type B cells undergo one final mitotic division, and their daughters detach slightly from the basement membrane and enter meiosis as primary spermatocytes. The whole spermatogonial phase is mitotic, diploid, and continuously replenishes the line throughout adult life.

Phase 2 — Meiosis

Each primary spermatocyte then undergoes the two meiotic divisions that reduce the chromosome complement from 46(2N, 4C) to 23(1N, 1C). Meiosis I takes a long time — about 22 days — because of an extended prophase in which homologous chromosomes pair (synapsis) and exchange material (crossing-over) during pachytene. This is why primary spermatocytes are the largest, most conspicuous germ cells on a slide; their thick condensed pachytene chromatin is unmistakable. Meiosis I yields two secondary spermatocytes, each haploid but still with sister chromatids (1N, 2C). These cells are short-lived: within a few hours they complete meiosis II — an equational division that separates sister chromatids — producing four haploid spermatids (1N, 1C) per starting primary spermatocyte. Because secondary spermatocytes exist so briefly, they are almost never seen on a histology slide, and their absence in a section is normal.

Phase 3 — Spermiogenesis

Spermiogenesis is the post-meiotic remodelling of a round, ordinary-looking spermatid into a streamlined spermatozoon. Crucially, no cell division is involved; everything is morphological. The Golgi apparatus assembles large pro-acrosomal vesicles that coalesce into a single acrosomal cap covering the anterior two-thirds of the condensing nucleus — this cap is essentially a giant modified lysosome loaded with acrosin, hyaluronidase and other hydrolases for fertilisation. One centriole grows the flagellar axoneme (9+2 microtubules). Mitochondria migrate and spiral into a helical sheath around the proximal flagellum, defining the midpiece. A microtubule scaffold called the manchette shapes the head and pulls cytoplasm rearward. Histones in the nucleus are replaced by small, highly basic protamines, condensing the chromatin into a dense, transcriptionally silent package. Finally, the excess cytoplasm is shed as a residual body, which Sertoli cells promptly phagocytose. The mature spermatozoon, still attached to Sertoli cells, is released into the lumen at spermiation.

Architectural notes and clinical anchor

Throughout the whole process, every germ cell sits in a lateral indentation of a Sertoli cell, which provides nutrients, removes waste and isolates the meiotic and post-meiotic cells behind the blood–testis barrier. Spermatogenesis proceeds in waves along the tubule and is temperature-sensitive: the scrotum holds the testis 2–3°C below core body temperature, which is why cryptorchidism (undescended testis) arrests the process and predisposes to seminoma. Disorders of the FSH–Sertoli–ABP axis or of LH–Leydig–testosterone signalling will block spermatogenesis at characteristic stages.

The Leydig (interstitial) cell is the testosterone-producing cell of the testis. To answer well, locate it, describe it on light then on electron microscopy, explain how it is regulated, and connect it to clinical pictures of androgen excess and deficiency.

Location

Leydig cells lie outside the seminiferous tubule, embedded in the loose interstitial connective tissue between the coiled tubules. They are almost always found in clusters next to capillaries and lymphatics — an arrangement that makes sense functionally, because the testosterone they make has to diffuse rapidly into both the adjacent tubule (where Sertoli-bound ABP traps it for spermatogenesis) and the bloodstream (where it masculinises the body and feeds back on the hypothalamic–pituitary axis). Their proximity to fenestrated capillaries also explains why they receive LH and cholesterol substrate so efficiently.

Light microscopy

On routine H&E, Leydig cells are large, polygonal, often clustered in groups of three to ten. The nucleus is round, vesicular and central, with one or two prominent nucleoli. The cytoplasm is strikingly eosinophilic and often described as foamy because of the many lipid droplets within it. In humans, a unique cytoplasmic finding is the presence of rod- or crystal-shaped crystals of Reinke; their function is unknown but their presence reliably distinguishes a human Leydig cell from any other interstitial cell.

Electron microscopy

Ultrastructurally, Leydig cells display the classic stigmata of a steroid-secreting cell: abundant smooth endoplasmic reticulum filling much of the cytoplasm (housing the cytochrome P450 enzymes that hydroxylate steroid intermediates); mitochondria with tubular rather than shelf-like cristae (housing P450scc, which cleaves cholesterol's side chain — the rate-limiting step); and prominent lipid droplets storing cholesterol esters that fuel steroidogenesis. There is relatively little rough ER, in keeping with the fact that steroids, unlike proteins, are not stored or packaged into secretory granules — they are made on demand and diffuse freely across the plasma membrane.

Function and regulation

The cardinal product is testosterone. Leydig cells carry G-protein-coupled LH receptors (the same hormone is called ICSH — interstitial-cell stimulating hormone — in the male). LH binding raises cAMP, activates protein kinase A, mobilises cholesterol into mitochondria via the StAR protein, and drives the cholesterol → pregnenolone → progesterone → androstenedione → testosterone pathway. Released testosterone diffuses into the adjacent seminiferous tubule (Sertoli cells convert some to dihydrotestosterone and oestradiol, and bind the rest to ABP), into local lymphatics, and into the bloodstream. Systemic testosterone is responsible for the development and maintenance of the male phenotype — deepening of the voice, beard, libido, muscle bulk, bone density — and feeds back negatively on hypothalamic GnRH and pituitary LH.

Clinical anchors

Leydig-cell pathology is a favourite of examiners. In Klinefelter syndrome (47,XXY) the seminiferous tubules are hyalinised and spermatogenesis fails, yet Leydig cells become hyperplastic — despite this, serum testosterone is low because the cells are dysfunctional, producing high LH, gynaecomastia and a eunuchoid habitus. Leydig-cell tumours, although rare, are functional and present with precocious puberty in boys or virilisation/feminisation in adults. Conversely, androgen insensitivity from receptor mutations leaves Leydig-cell output intact but renders peripheral tissues deaf to it.

This essay walks the examiner along the excurrent duct system after sperm leave the testis: the epididymis matures and stores them, the ductus deferens propels them at ejaculation, and the prostate contributes a large fraction of the seminal plasma they swim in. A working answer gives histology first, then function, then a brief clinical anchor for each.

Epididymis

The epididymis is a single, immensely coiled tube about 6 m long, folded into a comma against the posterior surface of the testis with a head (caput), body (corpus) and tail (cauda). Throughout, it is lined by a uniform pseudostratified columnar epithelium consisting of two cell populations: tall principal cells bearing very long, branched, non-motile stereocilia (giant microvilli, not true cilia) and short basal cells at the basement membrane. A thin sleeve of smooth muscle wraps the duct, increasing in thickness from head to tail.

Functionally the epididymis does two jobs. First, it matures sperm: as they transit over ~12 days, surface protein remodelling and membrane lipid changes give them progressive forward motility and the ability to bind and penetrate the zona pellucida (although final capacitation occurs later, in the female tract). Principal cells absorb most of the testicular fluid and secrete glycoproteins essential for this remodelling. Second, the epididymis stores mature sperm in the cauda until ejaculation, where rich smooth muscle drives sympathetic-induced contractions. Clinically, epididymitis in young men is usually due to Chlamydia trachomatis or Neisseria gonorrhoeae; in older men, E. coli from urinary reflux.

Ductus (vas) deferens

The ductus deferens picks up where the cauda ends, runs in the spermatic cord, traverses the inguinal canal, and ends in the ejaculatory duct. Its lining is similar pseudostratified columnar epithelium with short stereocilia, often thrown into longitudinal mucosal folds that give the lumen a star-shaped outline on cross-section. The diagnostic feature, however, is the wall: three thick concentric layers of smooth muscle — an inner longitudinal, a middle circular, and an outer longitudinal layer. This muscular wall is the thickest of any duct of comparable diameter in the body, and its sympathetic-driven peristaltic burst at ejaculation rapidly expels stored sperm into the urethra. Bilateral surgical interruption (vasectomy) is the basis of permanent male contraception.

Prostate

The prostate is a walnut-sized fibromuscular organ surrounding the proximal urethra below the bladder neck. It consists of 30–50 branched tubuloalveolar glands embedded in a dense fibromuscular stroma — the smooth muscle of this stroma contracts at ejaculation to squeeze the glands' contents into the urethra. The glandular epithelium varies from simple columnar to pseudostratified columnar. A characteristic finding inside the lumina is the corpus amylaceum: a concentric, lamellated, eosinophilic concretion of glycoprotein and cellular debris that accumulates with age and is a reliable visual fingerprint of prostatic tissue.

The glands are arranged in three concentric McNeal zones: a large peripheral zone (~70% of glandular volume) lying posteriorly — the seat of prostatic adenocarcinoma, palpable on digital rectal exam; a smaller central zone (~25%) around the ejaculatory ducts; and a small transition zone (~5%) surrounding the prostatic urethra — the seat of benign prostatic hyperplasia (BPH), whose nodules compress the urethra and cause hesitancy, frequency and retention. The prostate contributes roughly 30% of semen volume as a thin, slightly acidic, milky fluid containing prostate-specific antigen (PSA) — a serine protease that liquefies the seminal coagulum within 5–30 minutes — together with citric acid, fibrinolysin and zinc (antibacterial). Serum PSA is the standard screening marker for prostate cancer.

Seminal vesicle — brief note

Although not strictly part of the duct, the paired seminal vesicles empty into the ejaculatory duct alongside the ductus deferens. Their wall shows elaborately folded mucosa giving a honeycomb appearance and a pseudostratified columnar epithelium with yellow lipochrome pigment granules. They contribute the largest single fraction of the ejaculate (~60–70%): an alkaline, viscous, fructose-rich fluid (sperm energy), together with prostaglandins and the coagulation proteins that briefly gel the ejaculate.