Learning Objectives

• Describe the histological organization of the testis
• Outline the process of spermatogenesis in the germinal epithelium of the seminiferous tubule
• Explain the structural and functional significance of the blood-testis barrier.

Testes

Like the ovaries, testes are a source of gametes and steroid sex hormones. Each testis is surrounded by a thick connective tissue coat called the tunica albuginea. The bulk of the gland contains seminiferous tubules that are the site of sperm production. The gland is divided into lobules by thin septa that radiate from the tunica albuginea. Each of these lobules contains between one and four seminiferous tubules.

Upon exiting a seminiferous tubule, sperm traverse the rete testis, ductus efferentes, and convoluted epididymis, visible on the dorsal aspect of the testis here.

Seminiferous Tubules

Seminiferous tubules are the site of sperm production and consist of a germinal epithelium surrounding a central lumen. Uncommitted cells, called spermatagonia, reside along the basement membrane of the germinal epithelium and and function as stem cells for sperm production. As spermatagonia mature into sperm, they migrate upwards through the epithelium and are finally released into the lumen.

The interstitium surrounding the germinal epithelium contains two notable cells. Myoid cells surround the tubules and generate rhythmic contractions to propel spermatozoa and fluid. They also synthesize collagen and other fibers of connective tissue. Leydig cells produce testosterone and localize in clumps.

Spermatogenesis

Many stages of sperm development can be seen in a histological section of the seminiferous tubule. At the base of the germinal epithelium are spermatogonia which can divide to produce more spermatogonia or primary spermatocytes that are committed to developing into sperm.

Primary spermatocytes are located in the middle region of the seminiferous tubule. These cells have a prolonged prophase in meiosis I that allows for extensive time for crossovers to develop between homologous chromosomes. After completing meiosis I, primary spermatocytes give rise secondary spermatocytes.

Secondary spermatocytes proceed rapidly through meiosis II to produce spermatids. The short life-span of secondary spermatocytes means they are rarely found in the germinal epithelium.

Spermatids are the products of completed meiosis and are haploid. They remain connected to one another by cytoplasmic bridges which result from incomplete cytokinesis. The bridges mediate sharing of cytosolic components and communication between spermatids and allow for synchronous maturation. Note the condensed nucleus.

Spermatids mature into spermatozoa through a process called spermiogenesis that consists of the following changes:

• An acrosome, containing hydrolytic enzymes, develops from the Golgi and overlies the dense, elongated nucleus.
• A flagellum grows from the side opposite to the acrosome and extends into lumen of the seminiferous tubule.
• Mitochondria aggregate around the flagellum.
• The bulk of the cytoplasm is cast off as a residual body, leaving only a thin rim of cytoplasm around the nucleus.

Sertoli Cells

Sertoli cells extend from the basement membrane of the germinal epithelium to the lumen of the tubule and envelope the developing sperm cells. Sertoli cells are joined to one another by junctional complexes that form the blood-testis barrier. This barrier separates the germinal epithelium into a basal compartment and an adluminal compartment. Spermatogonia reside in the basal compartment but subsequent stages of sperm development reside in the adluminal compartment.

The blood-testis barrier serves three important functions in sperm development:

• Allows Sertoli cells to create a unique chemical environment in the lumen of the tubule to facilitate sperm development
• Prevents passage of cytoxic agents into the seminiferous tubules
• Prevents sperm from entering the blood stream and triggering an immune reaction to sperm. Generation of antibodies against sperm could reduce their ability to swim or fertilize an egg.

Sertoli cells synthesize androgen-binding protein (ABP) that keeps testosterone levels high in the seminiferous tubules. They also produce growth factors that support sperm cells and spermatogenesis. Follicle-stimulating hormone (FSH) increases production of ABP and growth factors. Inhibin, produced by Sertoli cells, reduces production of FSH.

Leydig Cells

Leydig (interstitial) cells are located within the loose connective tissue surrounding the seminiferous tubules. They appear pale due to their high cholesterol content and often contain crystals of Reinke , which have an unknown function. Leydig cells produce testosterone which is responsible for male secondary sexual characteristics and the maintenance of the germinal epithelium. Leutenizing hormone (LH), released by cells in the anterior pituitary, stimulates production of testosterone in Leydig cells.

Rete Testis

The rete testis connects the seminiferous tubules to the ductus efferentes. It is lined by ciliated cuboidal epithelial cells that also contain microvilli. The activity of the cilia helps to move the spermatozoa along the tube, as they are immobile until they reach the epididymis. The microvilli absorb excess materials, including protein and potassium, from the seminal fluid.

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