12.2 Testes

The adult male testes are paired, somewhat flattened, oval-shaped organs found in the scrotal sac. Each testis is covered by a tough and fibrous tissue layer, the tunica albuginea (see diagram in image 12.2a). The tunica albuginea partitions the testis into almost 300 wedge-shaped lobules. Each lobule contains one to three tightly convoluted seminiferous tubules which function in spermatogenesis, or sperm production. Sperms are produced at the rate of thousands per second throughout the life of a healthy, sexually mature male. The diagram in slide 1 shows the coiled seminiferous tubules surrounded by loose connective tissue (seen as lighter green color inside each lobule). Contained within this connective tissue are aggregations of endocrine cells called interstitial cells of Leydig (seen as small green dots), which produce the hormone testosterone. Capillaries and lymphatic vessels transverse the connective tissue in close association with the interstitial cells so that male hormones can easily exit the testicular tissue.

Image 12.2b illustrates the testis; note the seminiferous tubules (labeled "c" in image 12.2b), the tunica albuginea (labeled "a" in image 12.2b) and the ductus epididymis (labeled "b" in image 12.2b). Once the sperm are produced, they move through the seminiferous tubules to the rete testis, the efferent ductules and then the ductus epididymis as they progress through stages of maturation. Image 12.2c shows the rete testis. The arrow in image 12.2b points into the lumen of the tubule. Image 12.2d is a closer examination of the epithelium of the rete testis and image 12.2e shows the lumen of the efferent ductules. The sperms are transported from the testis to the epididymis via these efferent ductules.

12.3 Seminiferous Tubules

The seminiferous tubules are the functional units of the testis. The germinal cells, called spermatogonia, are in contact with the basement membrane. Spermatogonia undergo both mitosis and meiosis to produce the primary spermatocytes, the secondary spermatocytes, and the spermatids. Forming the walls of the seminiferous tubules are nurse cells, or Sertoli cells, which produce and secrete nutrients to the developing spermatozoa embedded between them. The spermatozoa are formed, but not fully matured, by the time they reach the lumen of the seminiferous tubules. Between the seminiferous tubules are specialized endocrine cells called interstitial cells (cells of Leydig). The function of these cells is to produce and secrete the male sex hormones.

Image 12.3a is a transverse section through the seminiferous tubules (labeled "ST" in image 12.3a). Note the tunic of fibrous connective tissue, and stratified epithelium (labeled "SE" in image 12.3a). The fibrous tunic contains cells with characteristics of smooth muscle cells. It is thought that these cells may be responsible for the contractile movements commonly observed in the seminiferous tubules. The stratified epithelium of the testis consists of two cells types: germ cells in different maturational stages of spermatogenesis and Sertoli cells, which are closely associated with the germ cells during their development. Maturing sperm cells can be seen in the lumen of the tubules, showing their aggregated tails (labeled "Ta" in image 12.3a).

Image 12.3b is a section of seminiferous tubules, showing spermatozoa in the lumen.

12.4 Seminiferous Tubule Epithelium

A transverse section of a seminiferous tubule is shown in image 12.4a. Note that the spermatogonia (labeled "Sg" in image 12.4a) are located at the periphery of the tubule, in close association with the tubule wall. Some of the spermatogonia become mature sperm; others maintain (by mitosis) the population of spermatogonia for further cycles of sperm production. Note the next advanced stage of cell, the primary spermatocytes (labeled "Sc" in image 12.4a) which undergo the first meiotic division to become the smaller secondary spermatocytes (seen as smaller cells right above and in direct contact with the primary spermatocyte). These secondary spermatocytes then undergo the second meiotic division to become the spermatids (labeled "Sd" in image 12.4a) which then differentiate into spermatozoa (labeled "Ta" in image 12.4a). The nurturing Sertoli cells (labeled "Se" in image 12.4a) are also identified.

Image 12.4b presents another example of a cross-section of a seminiferous tubule. Note the spermatogonia cell at the tip of the pointer. The tubule wall is labeled "a" in image 12.4b. Primary spermatocytes are labeled "b" in image 12.4b. Secondary spermatocytes are located in zone "c" of image 12.4b. Mature spermatozoa form in area "d" in image 12.4b. Image 12.4c shows Sertoli cells (cells which nurture the developing spermatozoa). Image 12.4d shows the interstitial cells (cells which secrete male hormones).

12.5 Spermatozoa

Spermatozoa are produced or matured at the rate of about 300 million per day and, once ejaculated, have a life expectancy of about 48 hours within the female reproductive tract. A spermatozoon is a very small cell that is highly adapted for reaching and penetrating a female ovum. A typical spermatozoon is divided into a head region (labeled "HR" in image 12.5a) and a tail region. The tail region is further divided into a neck, middle piece (labeled "MP" in image 12.5a), principal piece (labeled "PP" in image 12.5a), and end piece (labeled "EP" in image 12.5a). Within the head is found the nuclear material and the acrosome, which contains enzymes (hyaluronidase and proteinases) that effect the penetration of the sperm cell into the ovum. Numerous mitochondria in the midpiece carry on the metabolism that provides energy for locomotion. The tail, a typical flagellum, propels the sperm along its way.

Image 12.5b shows the epididymis which contains spermatozoa. The mass of spermatozoa (labeled "a" in image 12.5b) is shown to the right of the epithelium. The epithelium of the ductus epididymis is a ciliated columnar type (labeled "b" of image 12.5b). Finally, the darker red area to the left, is a connective tissue supporting layer for the epithelium (labeled "c" in image 12.5c). Although the testes produce the spermatozoa, the epididymis has the major responsibility for the storage of these cells until ejaculation, and it also finalizes the functional maturity of these gametes. If the sperms are not ejaculated, they usually loose their functional capability to fertilize an egg after a month and are then reabsorbed by the body.

12.6 Ductus Epididymis

After passing from the seminiferous tubules, the rete testis, and efferent ductules of the testes, the spermatozoa enter the epididymis, which is the principal storage organ of the male genital tract. The epididymis (as seen in image 12.6a) consists of a single, highly coiled tube, called the ductus epididymis. Image 12.6a has a label pointing to two lumen (labeled "Lu" in image 12.6a). Dense aggregations of spermatozoa (labeled "Sp" in image 12.6a) are frequently observed within the lumen. The epididymis is lined by pseudostratified columnar epithelium which is supported by a basal lamina. The basal lamina is surrounded by smooth muscle fibers that produce peristaltic waves of contraction along the longitudinal axis of the epididymis so as to propel the spermatozoa along its length. The interstices of the coiled portions of the epididymis are occupied by connective tissue (labeled "CT" in image 12.6a) that is rich in blood capillaries. Image 12.6b illustrates the tubular aspect of epididymis.

12.7 Epididymis Epithelium

A lateral view of the wall of the epididymis is shown in image 12.7a The pseudostratified epithelium (labeled "Ep" in image 12.7a) lining the epididymis characteristically possesses long microvilli (labeled "Mv" in image 12.7a) that were initially termed stereocilia because of their close resemblance to cilia. For size comparison, a tail (labeled "Ta" in image 12.7a) of one of the many spermatozoa found in the epididymis is observed next to the microvillus border. Residual cytoplasm (labeled "RC" in image 12.7a) is also present at the distal end of the tail. The cell borders (labeled "CB" in image 12.7a) of the epithelium appear as ridges. The vacuolated interior of the fractured cells may represent the location of intracellular organelles, such as the Golgi apparatus or lysosomes, which are known to be prominent in these cells. Also visible is the layer of smooth muscle (labeled "SM" in image 12.7a) underlying the epithelium. Image 12.7b also illustrates the epididymis epithelium where the arrow points to the stereocilia of the epithelium.

12.8 Ductus Deferens

Much of the ductus deferens is located within a structure known as the spermatic cord. The spermatic cord extends from the testis to the inguinal ring and consists of the ductus deferens, spermatic vessels, nerves, cremaster muscle, Iymph vessels, and connective tissue. A transverse section of this long, thick-walled ductus deferens is shown in image 12.8a. The majority of the tubular wall consists of three layers of smooth muscle (labeled "SM" in image 12.8a), which are spirally arranged around the longitudinal axis of the ductus deferens. The presence of smooth muscle is responsible for powerful peristaltic contractions that serve to propel spermatozoa along the length of the vas deferens during the process of ejaculation. The smooth muscle is surrounded peripherally by an adventitia (labeled "Ad" in image 12.8a) layer consisting of connective tissue through which blood vessels (labeled "BV" in image 12.8a), lymphatic vessels, and nerves travel in a course parallel to the tube. The inner surface of the ductus deferens is lined with a pseudostratified ciliated columnar epithelium (labeled "Ep" in image 12.8a) supported by an underlying lamina propria (labeled "LP" in image 12.8a) rich in elastic fibers. The lumen (labeled "Lu" in image 12.8a) of the ductus deferens is narrow. Image 12.8b shows another section of the ductus deferens, with the arrow pointing to the lumen.

12.9 Seminal Vesicles

The seminal vesicles are convoluted, club-shaped glands about 5 cm (2 in.) long and are positioned immediately posterior to and at the base of the urinary bladder. They secrete a sticky, slightly alkaline, yellowish substance, which serves as a fluid medium to sperm movement and longevity. The secretion from the seminal vesicles contains a variety of nutrients, including fructose, a monosaccharide that provides sperm with an energy source. It also contains citric acid, coagulation proteins, and prostaglandins. The discharge from the seminal vesicles makes up about 60% of the volume of semen. Histologically, the seminal vesicle appears as a mass of cuts embedded in connective tissue. The extensively coiled mucosal layer breaks the lumen into numerous intercommunicating spaces that are lined by pseudostratified columnar and cuboidal secretory epithelia (referred to as glandular epithelium). Blood is supplied to the seminal vesicles by the branches from the middle rectal arteries. The seminal vesicles are innervated by both sympathetic and parasympathetic fibers. Sympathetic stimulation causes the contents of the seminal vesicles to empty into the ejaculatory ducts of their respective sides.

image 12.9a shows a view into the interior of the distal end of the seminal vesicle and reveals the large outpocketings (labeled "Ou" in image 12.9a) formed by invaginations of the mucous membrane. The topography of the interior surface in this region appears highly folded (labeled "Fo" in image 12.9a), and the secretion that normally fills the lumen of the seminal vesicle was removed during tissue preparation. Image 12.9b shows a section of seminal vesicle. Image 12.9c is a magnification of this section. View images 12.9d, 12.9e, and 12.9f to see ever increasing light microscopic magnifications of the inner invagination of the seminal vesicle. image 12.9f gives a close-up view of the columnar epithelium which is typical of the inner lining of this organ.

12.10 Prostate Gland

The prostate gland is a single, doughnut-shaped gland about the size of a walnut. It is located directly below the urinary bladder and surrounds the superior portion of the urethra (image 12.10a). Its purpose is to produce and secrete an alkaline fluid into the prostatic urethra. This prostatic secretion constitutes 13 to 33% of the semen volume and its main role is to activate the motility of the sperms. Image 12.10b shows a microscopic view into the prostate, showing the numerous cavernous chambers into which the secretion takes place. Note the arrow pointing to a prostatic accretion, a solid-like condensation of secretory material. Image 12.10c is a close-up of the secretory epithelium in the prostate, which is typically columnar. This epithelium is very active mitotically and hence is a common source of cancer and glandular hypertrophy. Prostatic cancer is quite common in men and dangerous if not detected early. Because it can be asymptomatic for a long while, the cancer cells can spread to other parts of the body, such as bone.

12.11 Penis

The penis is a tubular organ designed to penetrate deep into the female vagina in order to safely deposit spermatozoa. The cross-sectional view of the penis in image 12.11a illustrates the three primary internal structures: the two corpus cavernosa (located side-by-side towards the top of the section and surrounded by a dense black layer of connective tissue) and the smaller corpus spongiosum (located below and medial to the cavernosa bodies). All three cavernous bodies are surrounded by connective tissue (outer light gray area) and loose skin (the outermost black line). Note the dorsal veins located in the top portion of the outer connective tissue sheath. The cavernous bodies are composed mainly of erectile tissue and when distended by blood serve to provide the hydrostatic pressure that makes the penis hard. Note that the erectile tissue contains numerous vascular spaces that can become engorged with blood. The penis is divided into a proximally attached root, an elongated tubular shaft and a distal cone-shaped glans.

Image 12.11b illustrates the vascular spaces found in the erectile tissue and also shows the urethra (labeled "a" in image 12.11b) imbedded in the corpus spongiosum (labeled "b" in image 12.11b).

12.12 Urethra

The model in image 12.12a shows the complicated passage of the male urethra from the bladder to the tip of the penis. Structures such as the urinary bladder (labeled "D" in image 12.12a) with its draining urethra (labeled "E" in image 12.12a) can be identified. Note how the urethra is joined by the ejaculatory ducts imbedded in the prostate gland (labeled "C" in image 12.12a). The ductus deferens (labeled "A" in image 12.12a) and the seminal vesicle (labeled "B" in image 12.12a) connect to the ejaculatory duct. Follow the urethra as it enters the bulb of the penis (labeled "F" in image 12.12a) and proceeds through the entire corpus spongiosum of the penis. The corpus spongiosum is seen passing beneath the corpus cavernosum (labeled "G" in image 12.12a) which is, in turn, anchored to the pubis bone (labeled "H" in image 12.12a). Also identify in this model the testicle (labeled "I" in image 12.12a) with its attached epididymis (labeled "J" in image 12.12a) positioned in the outer scrotal sac and also note the position of the rectum (labeled "K" in image 12.12a) with its terminal anal sphincter (labled "L" in image 12.12a) surrounding the anus.

The male urethra has three anatomical parts and shows three different kinds of epithelia along its length. The prostatic urethra which is encased by the prostate gland is lined with transitional epithelium as previously described in the bladder. The membranous urethra which extends from the prostate down to the bulb of the penis has a stratified columnar epithelium, as does the penile urethra which extends through the corpus spongiosum of the penis. In the distal enlarged portion of the penile urethra, called the "fossa navicularis", stratified squamous epithelium prevails. Also attached along the penile urethra are numerous branched tubular glands, called the glands of Littre, which secrete mucus. An example of a typical diagrammatic view of the cross section of the penile urethra showing the stratified columnar epithelium and these glands can be seen in image 12.12b.