The male reproductive system consists of testis, ducts, associated glands and the sexual organ or penis. The main function of the male reproductive system is to produce and release sperm for sexual reproduction. Furthermore, it is an endocrine gland releasing androgen hormones like testosterone, which is responsible for the secondary sexual features that result in sexual dimorphism.
Testicles (or testes) are oval structures suspended inside a bag of tissue known as scrotum, located outside the abdominal cavity. Because it is an expansion of the peritoneum, scrotum contains two layers of mesothelium and liquid serum between them secreted by the mesothelial cells. This aqueous solution is a lubricant the allows the movement of the testicle. These two layers of mesothelium are known as tunica vaginalis.
Tunica albuginea is a layer of connective tissue coating the testis, located under the tunica vaginalis. Below the tunica albuginea, there is a layer with abundant fibroelastic connective tissue, with scattered smooth muscle cells, and an internal layer enriched in blood vessels known as tunica vasculosa. In the back part of the testis, tunica albuginea gets thicker and forms the mediastinum testis. Several connective tissue walls or septa extend from mediastinum testis to the anterior part dividing each testicle in pyramidal compartments known as lobules. Lobules are connected between each other through discontinuities of the septa. Each lobule contains from 1 to 4 seminiferous tubules, smooth connective tissue with blood vessels, nerves and interstitial cells such as Leydig cells, which produce the testosterone hormone.
Seminiferous tubules are convoluted, about 0.2 µm in diameter, and from 30 to 70 cm in length in humans. One of the ends of the tubule may be closed or connected with the tubule of other testicular lobule. They are more straight toward the posterior part of the lobule, where the seminiferous tubules from the different lobules fuse to form a testicular network or rete testis.
The germinal epithelium forms the majority of the wall of the seminiferous tubules. It contains the male germinal cells or spermatogonia, as well as scattered somatic cells known as the Sertoli cells. These somatic cells do not divide after sexual maturation. Germinal cells constitute most of the germinal epithelium and proliferate before and during the adult life. They can divide by mitosis to produce new germinal cells, or by meiois to produce male gametes or spermatozoids. Meiosis is essential for sexual reproduction because it results in haploid cells that after spermiogenesis are differentiated into sperm.
Each seminiferous tubules are covered by a peritubular layer of fibrous connective tissue and myoid cells (smooth muscle cells). Sperm is produced in the seminiferous tubules and is moved away by contractions of these smooth muscle cells.
The male reproductive system consists of several ducts that produce, store and drive the sperm from the seminiferous tube to outside the body.
Straight segments of the seminiferous tubules form the vertices at the posterior part of the testicular lobules. The sperm produced in the other parts of the seminiferous tubules are collected in these segments, which are short, have no germinal cells, and their walls are made up of Sertoli cells. The posterior straight segments of the testicular lobules are fused with each other to form an anastomosed network of tubes known as rete testis, located in the mediastinum testis, which is a mass of connective tissue that project into the testis from the tunica albuginea. The tubes of this labyrinth show a cuboidal epithelium.
From 15 to 20 efferent ductules leave the dorsal part of the rete testis and enter the head of the epididymis where they become very convoluted and surrounded by vascular connective tissue, forming what is known as vascular cones. In the head of the epididymus, efferent ductules converge to form the epididymis duct, which constitutes most of the structure known as the body of the epididymis. Epididymis is a very long and convoluted duct where sperm is stored. It shows a pseudostratified epithelium coated by basal lamina and connective tissue. There is a thin layer of smooth muscle cells under the connective tissue that compresses the duct by peristaltic contractions. A layer smooth muscle cells is also surrounded the efferent ductules.
The epididymus becomes the ductus deferens, which drives the sperm from the scrotum to the inguinal region, coursing the lateral wall of the pelvis toward the urethra. Ductus derferens has thick walls and a narrow lumen. It is made up of pseudostratified epithelium, a basal lamina, a thin lamina propria, and a poorly delimited submucosa. There is also a well developed smooth muscle layer divided into three sublayers. An adventitia layer coates the muscle layer. Near to the end of the ductus deferens there is an enlargement known as ampulla.
Ejaculatory duct is the a short terminal duct that extends from the ampulla of the ductus deferens, crosses the prostate, and ends in the urethra. The epithelium of the ejaculatory duct is columnar or pseudostratified.
Glands associated to the male reproductor systm are seminal, prostate and bulbourethral glands (Cowper's glands).
The seminal vesicles are long structures situated behind the prostate. Their excretory ducts join the ductus deferens and together form the ejaculatory duct. The pseudostratified epithelium of the seminal vesicles is a secretory epithelium that produces and releases the seminal fluid. This solution contains many substances like fructose for sperm feeding and prostaglandins that influence part of the female reproductive system. The activity of the epithelium is modulated by testosterone.
Under the epithelium, there is a layer of connective tissue highly irrigated by blood vessels. Epithelium and connective tissue form the mucosa. Under the mucosa there is a profusely folded layer known as submucosa. The crests of these folds, which are projected into the mucosa as well, may be fused in their apical parts leaving chambers of variable sizes. Thus, the lumen of the seminal vesicle is very irregular and looks like a honey-comb when observed at low magnification. Covering the submucosa, there is a layer of smooth muscle. The adventitia is the most external layer.
The prostate is a complex gland that in humans consists of 30 to 50 compound tubuloalveolar glands releasing their content into the prostatic urethra. These glands are surrounded by fibromuscular stroma. The prostate is divided into four regions of different size: the transition zone that coats the urethra, central zone that surrounds the ejaculatory ducts, peripheral zone, which is the largest part, and the non-glandular fibromuscular stroma occupying the anterior part.
Prostate shows three levels of organization. There are small glands in the mucosa and submucosa of the urethra. The major glands are located in the surroundings, included in fibrous conective tissue that also contains smooth muscle cells, and altogether forms the largest part of the prostata. Externally, there is a capsule of fibroelastic tissue with some smooth muscle cells. The prostatic lisquid solution contains enzymes like fibrinolisins that help to decrease the vicosity of the semen.
Bulbourethral glands, or Cowper's glands, are small glands located behind the urethra. They are compound tubulo-alveolar glands that release their content into the urethra. The secretory parts consist of cuboidal or columnar simple epithelium, surrounded by connective tissue containing striated muscle cells. The connective tissue sends tissue expansions to form septa between the secretory parts of the glands. Secreted substances are mostly lubricant and releasing is independent from ejaculation.
Penis is the male reproductive organ. It is organized in two dorsal parts known as corpora cavernosa and one ventral part or corpus spongiosum, which contains the urethra. There is connective and fibroelastic tissues covering these three structures froming the tunica albuginea that provides strength and support. Corpora cavernosa is a network of large anastomosed blood cavities that are filled during penis erection. The cavities are coated by smooth muscle. Glans is the distal end of the penis, which is covered by the foreskin known as prepuce.