Histology of the Male Reproductive System
Testes: Structure, function, and cellular architecture
The testes have two primary functions: spermatogenesis (production of sperm) and steroidogenesis (synthesis of androgens, mainly testosterone). Testosterone is essential for sperm production, the development of male secondary sexual characteristics, and embryonic male development. The testes are covered by dense irregular connective tissue and a serosal surface, the tunica vaginalis, which is derived from a simple squamous mesothelium. The tunica albuginea is a thick layer of dense irregular connective tissue that forms septa which extend into the testicular parenchyma and divide the bulk of the testes into lobules. These septa prevent twisting of the tubules and help organize the tissue.
The scrotum is a separate sac from the testis to allow movement and reduce friction. The bulk of the testicular tissue is the seminiferous tubules, where spermatogenesis occurs and where testosterone is produced by Leydig cells located in the interstitial tissue between tubules. The epididymis lies posterior to the testis and stores immotile sperm until maturation and ejaculation; during this time, sperm acquire motility. If not ejaculated, sperm may be reabsorbed in the epididymis. The tunica vaginalis is composed of a simple squamous epithelium, while the tunica albuginea beneath forms the dense connective tissue capsule.
Seminiferous tubules and their surrounding structures
Seminiferous tubules are extremely long (up to ) and have a variable, often wide diameter. The tunica albuginea invaginates to form septa, partitioning the tubules into lobules and preventing twisting of these long tubes. The tubules are lined by a true epithelium consisting of Sertoli cells (support cells) with interspersed spermatogenic cells. Sertoli cells are typically described as a simple or tall columnar epithelium; they rest on a basement membrane, with a lamina propria beneath. In the lamina propria, peritubular myoid cells (contractile smooth-muscle-like cells) lie Just outside the basement membrane; these cells contract to move the developing germ cells and fluid toward the epididymis.
Leydig cells reside in the interstitial tissue between seminiferous tubules. They have a pale, bubbly cytoplasm due to lipid droplets and are eosinophilic in appearance; they produce testosterone throughout life and also secrete insulin-like substances (notably described here as insulin type three). The epithelium of the seminiferous tubules is supported basally by Sertoli cells, whose elongated pale nuclei and cytoplasmic extensions extend through the epithelium. Sertoli cells are essential for forming the blood–testis barrier and for nourishing differentiating spermatogenic cells.
Blood–testis barrier and Sertoli cell function
The blood–testis barrier is formed by tight junctions between Sertoli cells, creating a basal compartment containing spermatogonia and a luminal compartment containing later-stage germ cells. This barrier is tripartite: a physical barrier restricting large molecule movement, an immunological barrier preventing immune cell access and modulating cytokine passage, and a physiological barrier enabling selective transport of nutrients and metabolites. Sertoli cells supply nutrients and create an environment rich in factors that support germ cell development while maintaining high testosterone levels in the lumen via Sertoli–Leydig interactions and secretory products.
The barrier is explicit in diagrams: spermatogonia rest on the basement membrane; primary and secondary spermatocytes (diploid to haploid through meiosis) appear in the mid-epithelium; spermatids (haploid) are closer to the lumen and attach to Sertoli cells via junctions during development. After meiosis, spermatids are released into the lumen as spermatozoa (sperm) after spermiogenesis, a process that occurs mainly within the seminiferous tubules before sperm are moved to the epididymis.
Spermatogenesis: stages and cellular progression
Spermatogenesis begins with spermatogonia, diploid cells that rest on the basement membrane and undergo mitosis to produce primary spermatocytes. Primary spermatocytes then undergo meiosis I to form secondary spermatocytes, which quickly complete meiosis II to yield haploid spermatids. Spermatids are closest to the lumen and are initially non-motile; they are attached to Sertoli cells via junctions and are eventually released as mature spermatozoa after spermiogenesis. Spermiogenesis involves condensation and remodeling of the nucleus, formation of the acrosome, elongation and remodeling of the cytoplasm, and the development of a long flagellum. The entire process from spermatogonia to mature sperm takes about in humans.
During spermiogenesis, the nucleus becomes highly condensed and highly electron-dense due to dense heterochromatin; mitochondria reorganize around the developing flagellum (midpiece) to provide energy for motility, and a cap-like acrosome forms from the Golgi-derived vesicles at the anterior part of the nucleus. The flagellum grows from cytoplasm, forming the long tail of the sperm with a canonical axoneme structure of microtubule arrangement. The cytoplasm of developing spermatids is largely phagocytosed by Sertoli cells via junctional interactions, leaving the mature sperm anchored to the Sertoli cells by cytoplasmic bridges during development. Once properly formed, spermatids detach and are transported toward the lumen by fluid movements produced by Sertoli and myoid cells so that they can reach the epididymis for final maturation and storage.
In cross-section, spermatogonia appear as large cells with prominent nuclei resting on the basement membrane; spermatocytes show heterochromatic, somewhat patchy nuclei within the mid-epithelium; spermatids show elongated, densely stained nuclei near the lumen with developing axoneme and flagellum visible in higher magnification images. Electron micrographs reveal the Sertoli cell nucleus with extensive cytoplasmic extensions that traverse the epithelium, forming the blood–testis barrier and supporting germ cell development. The germ cells, particularly after meiosis, are located on the luminal side of Sertoli cells and progressively mature into sperm that enter the lumen for transport to the epididymis.
Blood vessels encircle seminiferous tubules; myoid cells lie just outside the basement membrane and help propel the lumenal fluid and cells toward the epididymis. Leydig cells in the interstitium are lipid-rich and contribute to/testosterone production; occasional insulin-like substances may also be produced here.
Epididymis: maturation and storage of sperm
The epididymal ducts are highly coiled, forming a long single duct that is about in total length, yet typically ~ at the posterior aspect of the testis in cross-section. The epididymis is lined by pseudostratified columnar epithelium with tall stereocilia (long microvilli) that dramatically increase surface area to reabsorb fluid and nutrients, aiding in sperm maturation and storage. The lamina propria contains abundant smooth muscle cells arranged in layers to facilitate peristaltic contractions that move sperm along toward the vas deferens during ejaculation. The nuclei of epididymal epithelial cells are generally positioned basally, with the apical cytoplasm largely cytosol, a feature helpful for distinguishing epididymis histologically from seminiferous tubules.
The epididymis is responsible for acquiring motility and final maturation of sperm. Sperm stay immotile within the seminiferous tubules and are immotile as they move through the epididymal lumen until ejaculation; the epididymis can reabsorb sperm if ejaculation does not occur.
Spermiogenesis: details of final sperm maturation
Spermiogenesis transforms round spermatids into motile, streamlined sperm. The nucleus condenses and becomes highly condensed (dense chromatin). A flagellum forms with a core axoneme of the microtubule arrangement, surrounded by mitochondria that cluster in the midpiece to provide energy for motility. An acrosome forms at the apex of the nucleus to house enzymes necessary for fertilization. The cytoplasm is reduced as Sertoli cells phagocytose excess cytoplasm; cytoplasmic bridges between spermatids are dissolved, releasing mature sperm into the seminiferous tubule lumen.
Cross-sectional and electron micrographs show: elongated, dense nuclei of spermatids near the lumen; a cap-like acrosomal cap; the midpiece with mitochondria; a long anterior tail (flagellum) that grows from the cytoplasm; and Sertoli cell processes in contact with the spermatid to facilitate maturation and eventual detachment.
Prostate gland: structure, function, and clinical notes
The prostate is a tubuloalveolar (alveolar) gland with ducts opening into a large, aerated lumen. The glands have a connective tissue capsule with dense fibromuscular stroma, including a rich smooth muscle component important for expelling prostatic secretions during ejaculation. The alveolar lumens are typically wide, and the epithelium lines the lumens in a simple columnar to cuboidal height, which can vary along the gland. In histological sections, the epithelium may appear to vary in height depending on the region.
Prostatic secretions are alkaline and contribute to the semen by liquefying the ejaculate and helping to dissolve cervical mucus. The secretion from the prostate contains enzymes such as the prostate-specific antigen (PSA). PSA is used clinically as a biomarker for prostate pathology; elevated PSA levels can indicate benign prostatic hyperplasia or cancer, though interpretation must consider clinical context and other tests.
As men age, concretions (lump-like gelatinous bodies) can form within the prostatic lumen; these are benign and common with aging. Histologically the smooth muscle and collagen fibers in the surrounding stroma contribute to the contractile function that expels prostatic secretions at ejaculation. The prostate gland is often compared histologically with the mammary gland (lookalike gland in the female reproductive system) due to similar histological features, a point to be explored when examining next week’s breast tissue.
The penis: anatomy, histology, and function
The penis serves dual functions: conduct urine via the urethra and deliver semen during ejaculation. Histologically, the penis contains three erectile bodies: two dorsal corpora cavernosa (corpora cavernosa) and one ventral corpus spongiosum. The urethra runs within the corpus spongiosum. All three erectile bodies are surrounded by tunica albuginea, a dense fibroelastic connective tissue that binds the corpora together and with which the corpora engorge during erection.
External to the tunica albuginea lies the fasciae ( Bucks fascia) and then the thin skin. The skin of the penis is loosely attached, allowing stretch during erection and flaccidity. Proximally, the penile urethra is lined by transitional epithelium; as it extends toward the glans, the urethral epithelium changes to stratified squamous. The corpora cavernosa contain numerous vessels with wide lumens and a circular arrangement of smooth muscle in the tunica media; the cavernosa are densely supported by the dense irregular connective tissue of the tunica albuginea and surrounding collagen and elastin fibers. The corpus spongiosum has a somewhat different histology: fewer vessels and a thinner tunica albuginea around it, allowing the urethra to remain patent during erection.
In cross-section, the penile urethra is flattened and surrounded by the spongiosum; close to the base, the epithelium is transitional, while nearer the glans, it may be stratified squamous. The corpora cavernosa are supplied by helicine arteries that dilate during arousal, under parasympathetic control, and become engorged with blood to produce tumescence and rigidity. The corpus spongiosum remains less engorged than the cavernosal bodies to avoid obstructing the urethral lumen during ejaculation.
Erectile function and ejaculation: autonomic control and histology
Erection is a vascular, neural event driven by the parasympathetic nervous system. Parasympathetic stimulation causes the release of acetylcholine and nitric oxide (NO). NO stimulates smooth-muscle relaxation, particularly of the helicine arteries that supply the corpora cavernosa, leading to dilation and rapid filling of the erectile spaces with blood. This engorgement, in combination with the constriction of the tunica albuginea, reduces venous outflow and results in tumescence and rigidity of the penis.
The sympathetic nervous system drives ejaculation. A surge in sympathetic activity causes contraction of smooth muscle in the vessels of the erectile tissues (and in the ducts and glands along the reproductive tract, including the epididymis and prostate), pushing sperm and glandular secretions into the urethra and toward ejaculation. Simultaneously, venous outflow is reduced transiently, but rapid ejaculation requires coordinated contraction and then a rapid reduction in inflow as the distal relaxation occurs and the penis returns to a flaccid state.
In the context of semen, ejaculation depends on the coordinated activity of the epididymis, ductus deferens, seminal vesicles, prostate, and bulbourethral glands, which contribute fluids to semen. If a vasectomy is performed, sperm are no longer present in the ejaculate, but other fluids from the ducts, prostate, seminal vesicles, and bulbourethral glands continue to be produced, resulting in an ejaculation that lacks sperm.
Semen: composition, physiology, and clinical relevance
Semen is the mixture of sperm from the epididymis and semen from the accessory glands: the seminal vesicles (rich in fructose and prostaglandins), the prostate (enzymes and liquefaction), and the bulbourethral glands (lubricating mucus and alkaline protection). The mucous and alkaline secretions help neutralize vaginal acidity and facilitate sperm transport. The prostatic enzymes (including PSA) aid liquefaction of semen after ejaculation, enabling sperm to move freely.
A practical clinical note: PSA is a marker used to screen for prostate pathology, but its accuracy and interpretation depend on patient age and condition; elevated PSA can reflect cancer, benign prostatic hyperplasia, or prostatitis. Concretions in the prostatic lumen are common in older men and indicate aging rather than pathology. Semen contains sperm plus fluids from the ducts and glands; the fluids alone would still be produced in the absence of sperm (e.g., after vasectomy), though the semen would lack sperm.
Lookalike glands and integration with the female reproductive system
There are histological lookalikes between the prostate and the female mammary gland, which will be explored in the next session after the female reproductive histology is covered. The mammary gland and prostate share certain histological characteristics that can be used to compare tissue architecture and understand reproductive organ histology across sexes.
Quick recap: key numerical and structural references to remember
Seminiferous tubules: up to long with septa forming lobules via tunica albuginea.
Epididymal duct length: approximately ; coiled to form a long duct.
Time for sperm maturation: about .
Axoneme structure in sperm tails: microtubule arrangement.
Major components of semen: sperm + seminal vesicle fluids (fructose, prostaglandins) + prostate secretions (enzymes, liquefaction) + bulbourethral mucus.
Erectile tissue: corpora cavernosa (two dorsal) and corpus spongiosum (ventral) with tunica albuginea encasing each.
Neurovascular control: parasympathetic NO-mediated vasodilation for erection; sympathetic activity drives emission and ejaculation.
This concludes the comprehensive walkthrough of the male reproductive histology covered in the lecture. We touched on architecture, cellular players (Sertoli, Leydig, myoid cells), barriers, germ cell development, epididymal maturation, prostate anatomy and pathology markers, penile erection physiology, and semen composition, with clinical notes where relevant. We’ll practice identifying these features in slides during the practical session next week.