Histology: Male Reproductive System Flashcards
- The testes have both exocrine and endocrine functions: They function as exocrine glands that produce sperm, the male germ cells. As endocrine glands, they synthesize and secrete steroid and glycoprotein hormones, including testosterone. Fluids secreted by the testis and other structures of the male genital ducts provide a medium for transport, nutrition and maturation of the sperm. Steroid hormones, primarily testosterone and dihydrotestosterone are essential for the regulation of growth and development of spermatozoa and for the maintenance of accessory reproductive glands. These hormones also function in the development and maintenance of secondary sexual characteristics and, to some extent, sexual behavior.
- In the adult human male, each testis is an ovoid structure approximately 4.5 cm in length, 2.5 cm in width and 3 cm in diameter. The testes are held away from the body within the scrotum because spermatogenesis can only occur 2-3 degrees below core temperature (core temp = 37 degrees C). The arrangement of blood vessels in the pampiniform plexus contributes to regulating temperature in the scrotum relative to body temperature: the testicular artery is very convoluted as it nears the testis and is surrounded by a venous plexus. This allows for counter-current exchange of heat from artery to surrounding veins, and this permits the cooling of arterial blood prior to its entry into the testis.
•Within the scrotum, the testes are invested anteriorly and laterally by a peritoneum-derived serous sac, the tunica vaginalis. This structure protects the testes by allowing them to slide easily within the scrotum. Each testis is encased in a dense fibrous capsule, the tunica albuginea. The tunica albuginea thickens posteriorly and projects into the interior of the testis forming the mediastinum testis. Blood vessels, lymphatics and nerves enter and exit the testis at the mediastinum. From the mediastinum, thin connective tissue septa divide the testis into about 250 lobules. Each lobule contains one to four highly convoluted seminiferous tubules. Spermatozoa (sperm cells) are produced within the seminiferous tubules.
•The coiled seminiferous tubules are lined with a stratified “germinal” epithelium composed of two cell types: Sertoli, or support cells and spermatogenic cells. Sertoli cells form the epithelium within which the spermatogenic cells undergo mitotic and meiotic divisions to produce spermatozoa. The outer wall of the seminiferous tubule is composed of three to five layers of modified fibroblasts called myoid cells. These cells exhibit rhythmic contractions which aid in the movement of spermatozoa and fluids through the seminiferous tubules.
•Each Sertoli cell extends from the basement membrane to the lumen of the tubule. Tight junctions present between lateral processes of adjacent Sertoli cells divide the seminiferous tubule into a basal compartment and an adluminal compartment. Spermatogonia are located in the basal compartment. Primary and secondary spermatocytes and spermatids are located at increasingly higher levels of the adluminal compartment. Spermatozoa are released into the lumen of the tubule.
The functions of Sertoli cells include:
- Support: provide physical support for the spermatogenic cells and their release into the seminiferous lumen.
- Protection: they form the Blood-Testis Barrier.
- Nutrition: they control the movement of metabolites and nutrients across the seminiferous epithelium and transfer nutrients to developing spermatozoa.
- Phagocytosis: they phagocytose cast off cytoplasm during spermiogenesis as well as degenerating spermatogenic cells.
- Secretion: they synthesize and secrete androgen binding protein to concentrate testosterone in the seminiferous tubules and proximal genital duct system.
•Spermatogenesis is a multistep process in which spermatogonia develop into spermatozoa. It occurs in three phases: spermatocytogenesis, meiosis and spermiogenesis.
- Spermatocytogenesis: the production of spermatogonia through mitotic divisions.
- Meiosis: Two successive meiotic cell divisions yield 4 haploid spermatids from each spermatogonia that enters meiosis.
- Spermiogenesis: the cytodifferentiation of spermatids into spermatozoa.
Spermatocytogenesis: Spermatogonia are diploid germ cells that reside in the basal compartment of the seminiferous tubule. Spermatogonia divide mitotically to give rise to additional spermatogonia. A subpopulation of spermatogonia enter first meiosis and become primary spermatocytes. These leave the basal compartment and enter the adluminal compartment by passing through Sertoli cell tight junctions.
Meiotic Phase: In this phase, the diploid primary spermatocyte undergoes two successive cell divisions (Meiosis I and II) that yield 4 haploid spermatids. The first meiotic division produces secondary spermatocytes from primary spermatocytes. Secondary spermatocytes quickly enter meiosis II to produce spermatids.
Spermiogenesis is the process by which spermatids differentiate into spermatozoa. During this process the acrosome (a specialized lysosome) forms, the flagellum develops, the nucleus changes shape as the chromatin condenses, and excess cytoplasm is shed. A centriole is involved in the formation of the axoneme, the central core structure of the flagellum composed of microtubules in a 9 + 2 configuration. As the spermatid elongates, excess residual cytoplasm is discarded. The spermatozoon is then released from the seminiferous epithelium. The residual bodies of shed cytoplasm are phagocytosed by Sertoli cells.
•The mature sperm is approximately 60 µm in length and consists of a head, a midpiece and a tail. The head is flattened and arrow-shaped. The acrosomal cap occupies approximately two-thirds of the anterior portion of the head, which also includes the very condensed haploid nucleus. At the time of fertilization, the acrosomal membrane fuses with the sperm plasma membrane releasing hydrolytic enzymes. These enzymes disaggregate the cells of the corona radiata and dissolve the zona pellucida of the ovum. The spermatozoan axoneme extends through the midpiece and into the tail. The middle piece includes a sheath of helical mitochondria that provide ATP needed for flagellar motility. The flagellum or tail is the long motile process that propels spermatozoa from their site of deposition in the vagina up into the uterine tube where fertilization takes place. Fructose from seminal vesicles is the energy source that allows sperm mitochondria to produce the ATP necessary for sperm motility.
- The connective tissue stroma surrounding the seminiferous tubules, in addition to a rich vascular supply, nerves and lymphatic vessels, contains interstitial Leydig cells. The interstitial cells of Leydig are endocrine cells that synthesize the male sex hormone testosterone. The synthesis and release of testosterone is under pituitary control via Luteinizing Hormone (LH). Sertoli cells, in response to Follicle Stimulating Hormone (FSH) from the anterior pituitary, synthesize and secrete androgen binding protein. Androgen binding protein concentrates testosterone in the seminiferous tubule. The elevated level of testosterone is required for spermatogenesis.
- The interstitial tissue surrounding the seminiferous tubules contains blood and lymphatic vessels, nerves, loose connective tissue and Interstitial cells of Leydig. Within the first trimester of fetal development, Leydig cells differentiate and secrete testosterone, necessary for the development of the male reproductive tract. Following this period, the Leydig cells involute until puberty. Beginning again at puberty, Leydig cells synthesize and secrete androgens in response to LH released from the pituitary gland. Ultrastructurally, Leydig cells possess an abundance of smooth endoplasmic reticulum, a typical feature of steroid-secreting cells (e.g. adrenal cortex).
- The genital ducts include straight tubules (aka tubuli recti), rete testis, epididymis, ductus deferens and ejaculatory ducts. There are two complete duct systems, one for each testis, and these empty into a common urethra within the prostate gland. The straight tubules are short conduits that drain the seminiferous tubules to the rete testis. They are actually somewhat coiled on themselves, which gives the appearance of beads on a string in histological sections. The transition from seminiferous tubule to straight tubule is distinct in that there is a narrowing of the tubule, a loss of spermatogenic cells and a distinct change in the epithelium. The proximal straight tubules are lined by Sertoli-like cells that transition to simple columnar or cuboidal epithelial cells distally.
- The rete testis, located in the mediastinum testis area, is a network of interconnected channels lined with simple cuboidal epithelium. The cells possess a single cilium and a few short microvilli. The rete testis converges into 12 to 20 efferent ductules which leave the testis. The efferent ductules possess an irregular lumen whose epithelium consists of both ciliated and nonciliated cuboidal cells. The ciliated cells move the tubule fluid and the as yet non-motile spermatozoa towards the epididymis. The efferent ductules drain into the single, highly convoluted epididymis.
•Spermatic cord. The testes are suspended within the scrotum by the spermatic cord, which includes skeletal muscle (cremaster muscle), the ductus deferens, arteries, pampiniform plexus of veins, nerve fibers and lymphatic vessels. The cremaster muscle, whose fascicles originate in the internal oblique muscle of the abdominal wall, raises and lowers the testes within the scrotum in response to cold and warm ambient temperatures respectively.
•The epididymis is lined by a pseudostratified columnar epithelium supported by a thin lamina propria surrounded by circular smooth muscle. It is within the epididymis that the spermatozoa mature and develop the capacity for motility (though they do not become motile until they encounter secretions of the seminal vesicles). Acquisition of this capacity requires binding of glycoproteins of epididymal origin to specific regions of the spermatozoan plasma membrane. The epididymal epithelium is composed of two cell types, principal cells and basal cells. The principal cells are tall, columnar cells possessing very long microvilli called stereocilia on their luminal surface. The principal cells are involved in both secretory and absorptive activity. The structurally simple basal cells are stem cells for the epithelium. From the proximal to the distal end of the epididymis, the smooth muscle increases from a single layer to three layers (inner longitudinal, middle circular and outer longitudinal) similar to the ductus deferens.
- The ductus deferens (the old name was vas deferens, a misnomer since it is not a vessel) is a continuation of the epididymis and is a more or less straight tube with an internal mucosa, a very thick muscularis and an external adventitia. Its function is to convey spermatozoa from the epididymis to the urethra. The mucosa consists of a pseudostratified columnar epithelium with stereocilia. The columnar epithelium and associated lamina propria exhibit folds that project into the lumen. The very prominent muscular wall is organized into inner longitudinal, middle circular and outer longitudinal layers.
- The ductus deferens ends at the prostatic urethra. Prior to entering the prostate the ductus deferens dilates forming the ampulla. The seminal vesicle joins the duct at the distal portion of the ampulla. The segment of the ductus deferens that traverses the prostate to the prostatic urethra is termed the ejaculatory duct. Because the ejaculatory duct is embedded in the fibromuscular stroma of the prostate gland, it does not possess a muscularis.
- The genital glands consist of the seminal vesicles, the prostate and the bulbourethral glands. The seminal vesicles contribute the majority of the ejaculate – approximately 55%; the prostate contributes approximately 30%, with the remainder coming mainly from epididymis and bulbourethral glands.
- The seminal vesicles are paired long, sac-like structures that lie between the posterior bladder and the rectum. They are essentially diverticula of the ductus deferens. The highly folded mucosa consists of pseudostratified cuboidal to columnar epithelium interspersed with short, rounded basal cells. The muscularis consists of an inner circular and an outer longitudinal layer. An adventitia with abundant elastic fibers surrounds the muscularis. The thick glandular secretion of seminal vesicles is rich in fructose, prostaglandins and proteins. Like the prostate, the secretory activity of the seminal vesicles is androgen-dependent. Fructose provides the energy source for sperm motility. The fructose concentration of semen is an assay for the activity of the seminal vesicles.
•The prostate is the largest of the accessory glands and surrounds the prostatic urethra. It consists of both fibromuscular and glandular components. The glandular portion of the prostate is composed of three regions (mucosal, submucosal and main prostatic glands) that are arranged somewhat concentrically around the prostatic urethra. The prostate secretes a slightly acid fluid rich in proteolytic enzymes, citric acid and acid phosphatase (although overall pH of semen is alkaline). The proteolytic enzymes such as fibrinolysin aid in liquefaction of the semen. The glandular portion is composed of 30 to 50 branched tubuloalveolar glands. Ducts from these glands converge into ducts that empty into the prostatic urethra. The thick fibromuscular capsule as well as the stroma of the prostate contains smooth muscle fibers, fibroelastic tissue and blood vessels.
•In older men, prostatic concretions or corpora amylacea normally occur in some of the glandular alveoli. These are small spheres of glycoproteins that are often calcified.
•Benign prostatic hyperplasia (“BPH”) is a benign enlargement of the mucosal glands of the paraurethral prostate that commonly occurs with age. Some degree of BPH is present in most men over 50, and can be the cause of urinary “flow” problems in many men past middle age. Severe problems may require a “TURP”, a transurethral resection of the prostate. Unlike BPH, adenocarcinoma of the prostate (prostate cancer) usually involves the peripherally-located glands. It is this region of the prostate that is palpated during a rectal exam.
•The bulbourethral glands (aka Cowper’s glands) are compound tubuloalveolar glands with an epithelium ranging from columnar to low cuboidal. The paired excretory ducts of the bulbourethral glands empty into the bulb of the urethra. The glands secrete mucous during sexual arousal, and this serves as a lubricant for the urethra prior to ejaculation.
- The penis consists of three cylindrical erectile bodies surrounded by elastic connective tissue and covered by thin skin. Two dorsally-situated corpora cavernosa, fused at their medial surfaces, lie adjacent to the longer corpus spongiosum. The corpus spongiosum, which contains the “spongy” portion of the urethra, ends distally as the bulbous glans penis. At their root, the corpora cavernosa diverge as crura (legs) that are tightly bound to the connective tissue coverings of the ischiopubic ramus. The corpus spongiosum has an enlarged proximal portion called the bulb which is covered by a sheet of skeletal muscle, the bulbospongiosus muscle. Contraction of this muscle during orgasm exerts forward compression leading to the ejaculation of semen.
- All three of these cavernous bodies receive an abundant blood and nerve supply. A thick collagenous capsule, the tunica albuginea, encircles each of the three erectile bodies and is most prominent around the corpora cavernosa. The erectile tissue is composed of irregular endothelium-lined vascular spaces that are separated by trabeculae of dense fibroelastic tissue and smooth muscle.
- The male urethra is subdivided into three regions: a prostatic portion, a membranous portion and the “spongy” (penile) urethra. The prostatic portion is lined by transitional epithelium. The membranous urethra and proximal spongy portion are lined by pseudostratified to stratified columnar epithelium. Stratified squamous epithelium lines the terminal portion of the urethra.
- Erection: Blood is normally shunted away from the vascular spaces of the erectile tissues because of the high vascular tone of the small arteries that supply them (helicine arteries). Erection results when parasympathetic stimulation “relaxes” arterial smooth muscle leading to engorgement of the erectile tissue. Expansion of the erectile tissue leads to compression of the efferent veins by the tough tunica albuginea further restricting drainage from the erectile tissue. Loss of erection results from sympathetic constriction of the arterial smooth muscle.
- Ejaculation includes emission and ejection phases. Emission refers to the movement of semen into the urethra by the sympathetic stimulated contraction of smooth muscle of the genital ducts and glands in response to sexual arousal.
- Ejection: The expulsion of semen from the urethra results from rhythmic contraction of the bulbospongiosus (skeletal) muscle. The sensations associated with ejaculation constitute the orgasm.
•The penis is innervated by spinal, sympathetic and parasympathetic nerves. Numerous sensory nerve endings are distributed throughout the penis, especially the glans. The sympathetic and parasympathetic nerves supply the smooth muscle of the erectile tissue trabeculae and the blood vessels.
