Histology of the Testis

1. General Organization

2. Seminiferous Tubules

• Number: 250-1000 per testis
• Dimensions: ~150-250 µm in diameter; each tubule ~50 cm long (range 30-80 cm) and highly convoluted
• Shape: Each tubule forms a loop; both ends open near the mediastinum into short straight tubules (tubuli recti), which drain into the rete testis
• Wall: Lined by a complex stratified seminiferous (germinal) epithelium, resting on a basement membrane (basal lamina)

Tunica Propria (Peritubular Tissue)

3. Cells of the Seminiferous Epithelium

A. Sertoli Cells (Supporting/Sustentacular Cells)

• Tall columnar cells extending full thickness of the epithelium, from basal lamina to lumen
• Nuclei are large, ovoid/triangular, with prominent nucleoli; often indented - a key identifying feature on H&E
• Extensive lateral and apical cytoplasmic processes embrace surrounding spermatogenic cells
• Do not replicate after puberty
• Adjacent Sertoli cells are joined near their bases by tight junctions (occluding junctions) forming the blood-testis barrier (BTB)

Blood-Testis Barrier

• Basal compartment (below the junctions): contains spermatogonia and preleptotene spermatocytes; exposed to blood-derived substances
• Adluminal (abluminal) compartment (above the junctions): contains primary/secondary spermatocytes and spermatids; immunologically privileged, protected from autoimmune attack since meiotic/post-meiotic cells express novel antigens

Functions of Sertoli Cells

B. Spermatogenic Cells

Spermatogonia (basal compartment, resting on basal lamina)

• Type A dark (Ad): Reserve stem cells; serve as renewal stem cells; dark oval nuclei with fine chromatin
• Type A pale (Ap): Committed progenitors; pale oval nuclei; give rise to type B
• Type B: Round nuclei with peripheral heterochromatin; divide to form primary spermatocytes

Primary Spermatocytes

• Largest cells in the epithelium; most commonly seen in prophase I (long duration: ~22 days in humans)
• Recognized by their large size and prominent chromosomes (thread-like in leptotene, paired in zygotene, thick/crossing over in pachytene, short/condensed in diplotene)
• Undergo meiosis I → two secondary spermatocytes

Secondary Spermatocytes

• Smaller than primary; undergo meiosis II rapidly (~few hours)
• Rarely seen in routine sections due to brief existence

Spermatids (early and late)

• Early (round) spermatids: Small, round cells with round nuclei; located near lumen; product of meiosis II; haploid
• Late (elongated) spermatids: Elongated dark nuclei directed toward lumen; product of spermiogenesis

4. Spermatogenesis

1. Spermatogonial phase - mitotic renewal and commitment
2. Spermatocyte phase (meiosis) - reduction to haploid cells (spermatids)
3. Spermatid phase (spermiogenesis) - morphological transformation into spermatozoa

Spermiogenesis (Spermatid → Spermatozoon)

• Acrosome formation: Golgi apparatus produces proacrosomal granules that fuse into the acrosomal vesicle, flattening to cap the anterior nucleus
• Nuclear condensation: Chromatin condenses; nucleus elongates
• Flagellum formation: Centriole organizes the axoneme
• Mitochondrial sheath: Mitochondria migrate to surround the midpiece
• Cytoplasmic reduction: Excess cytoplasm shed as residual bodies, phagocytosed by Sertoli cells

Cycle of the Seminiferous Epithelium

5. Interstitial Tissue (Leydig Cells)

Leydig Cell Features

• Large, polygonal eosinophilic cells, occurring singly or in clusters
• Central round nuclei
• Cytoplasm rich in smooth ER (accounts for eosinophilia; site of steroidogenesis enzymes)
• Mitochondria with tubulovesicular cristae - hallmark of steroid-secreting cells
• Lipid droplets (cholesterol substrate)
• Lipofuscin pigment
• Distinctive rod-shaped crystals of Reinke (~3 × 20 µm) - pathognomonic for human Leydig cells; exact function unknown, likely a protein product

Function

• Produce testosterone in response to LH (= ICSH in males) from pituitary
• Also secrete INSL3 (stimulates testicular descent, promotes meiosis) and oxytocin (stimulates myoid cell contraction)
• Active in fetal life (stimulated by hCG), quiescent from ~5 months fetal life, then reactivated at puberty

6. Intratesticular Duct System

7. Temperature Regulation

• Pampiniform plexus - venous network around testicular artery in spermatic cord: countercurrent heat exchange
• Dartos muscle (scrotal skin) and cremaster muscle - relax/contract to adjust testicular position
• Scrotal sweat evaporation

Key Clinical Correlations

• Cryptorchidism: Failure of testicular descent; bilateral cases cause infertility if not corrected by 2-3 years (sustained 37°C damages spermatogenic cells). Markedly increases risk of germ cell tumors.
• Hydrocele: Excess fluid in tunica vaginalis cavity.
• Testicular germ cell tumors: 95% of testicular malignancies; arise after puberty; strongly associated with cryptorchidism.
• Leydig cell tumors: Rare, usually benign; contain Reinke crystals; cause precocious puberty in prepubertal boys or gynecomastia/feminization in adults.
• Aging: Progressive thickening of tunica propria, decreased spermatogenesis.