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Lab 16. Male Reproductive Tract

Functionally, the principal structures of the male reproductive system are:

  1. the testes where the gametes (spermatozoa) and the male sex hormones are formed;
  2. the duct system, which conveys the spermatozoa to the exterior;
  3. exocrine glands which, along with the glandular epithelium lining the duct system, supply spermatozoa with their fluid medium and contribute to their maturation; and
  4. the penis which, in addition to being a duct, contains erectile tissue that facilitates the deposit of sperm in the female genital tract.

Objectives

  1. to understand the structures of the male reproductive system at the histological level.
  2. to recognize and distinguish the different parts of the male reproductive tract.
  3. to understand the process of spermatogenesis and appreciate the importance of supporting cells.

Testis

Gametes are produced in the testis within long, twisted seminiferous tubules which make up the bulk of the organ. The process of gamete production and maturation is called spermatogenesis. The primitive sex cells of spermatogonia are diploid and divide mitotically to produce large numbers of differentiated daughter cells called primary spermatocytes and undergo meiosis (reduction division, or maturation division). The resulting cells, secondary spermatocytes, have half the somatic chromosome number, but each chromosome is still composed of two chromatids. The secondary spermatocytes rapidly enter prophase, and the chromatids finally separate, producing haploid daughter cells called spermatids. The spermatids undergo spermiogenesis, i.e., morphological transformations resulting in spermatozoa without further cell division.

The endocrine cells of the testis called Leydig or interstitial cells, are in clusters of various sizes between the seminiferous tubules. Their secretion, testosterone, influences spermatogenesis as well as all male secondary sex characteristics. In the genital tract, it promotes development and controls the secretory activity of the duct system and its associated glands.

At the mediastinum testis, the seminiferous tubules end as narrow, short, straight tubules (tubuli recti) lined with columnar Sertoli-like cells and these tubules enter the rete testis, a labyrinth of channels. Here sperm passing from the tubules come to a network of large, irregular spaces lined with cuboidal or squamous epithelium. The rete testis opens into the 10 or 15 ductili efferentes.

The testis, slide #94, (examine both odd and even-numbered slides), is covered with a capsule of dense fibro-elastic connective tissue, tunica albuginea, which may be seen on one side of the testis fragment in your slide. The capsule is covered with mesothelium equivalent to visceral peritoneum, but only portions remain on the tissue. The connective tissue capsule has a dense outer portion and a less dense inner portion which contains blood vessels.

The surface of the testis not covered with mesothelium is the region of the mediastinum testis where the seminiferous tubules converge to become excretory ducts and where blood vessels and nerves enter the gland. Most of the slides were cut near the mediastinum so that the epithelium-lined labyrinthine channels of the rete testis may be seen in the dense collagenous C.T. In some slides, particularly in #94 odd, seminiferous tubules can be seen opening into (or becoming) tubuli recti lined with columnar epithelium; these are very short, and the epithelium soon becomes the squamous to cuboidal epithelium of the rete testis.

Mediastinum Testis
with seminiferous
tubules leading into
Tubuli recti

Tubuli recti leading
into the rete testis

 

Connective tissue septa penetrate into the testis in a pattern radiating from the mediastinum to the tunica albuginea, dividing it into about 250 conical spaces called lobules. Each lobule is tightly packed with a few (1–4) long, twisted seminiferous tubules which are U-shaped in the sense that both ends Open into tubuli recti. The outer covering of the seminiferous tubules consists of specialized limiting membrane covered by closely packed and circularly oriented flattened cells which are small, modified smooth muscle elements (myoid cells); there is also abundant ground substance and fine fibrillar elements between these cells. Within the tubules, the sex cells and Sertoli cells constitute the seminiferous epithelium. The cell boundaries of the various cells are not clearly seen in histological sections, except near the edge of the lumen, and the various cells in the seminiferous epithelium are identified by differences in nuclear morphology and localization. First find Sertoli cells. Their nuclei are usually oval and located midway between the limiting membrane and the lumen. They are pale-staining with prominent, often acidophilic nucleoli and their nuclear membrane may be deeply infolded at one or two points. E.M. studies have shown these cells to have a slender pillar-like shape with irregular outlines due to compression by adjacent cells. Sertoli cells may have an exocrine function since they secrete androgen-binding proteins into the lumen of the tubules. They are generally thought to contribute to the biochemical maturation of spermatozoa which, at one stage of spermiogenesis, can be seen in groups just above Sertoli cell nuclei. Find such groups in your section.

Spermatogenesis begins at the limiting membrane of the tubules and proceeds toward the lumen. Nuclei of spermatogonia, therefore, lie nearest the limiting membrane of the tubules. The nuclei of primary spermatocytes are round and larger than those of spermatogonia, and they soon enter the very long meiotic prophase during which the nuclei become very large. Thread-like chromosomes are visible in these nuclei.

   

Secondary spermatocytes contain half the somatic number of chromosomes and half the amount of DNA found in the prophase nuclei of the primary spermatocytes and hence, they have smaller nuclei. They quickly divide to produce spermatids and so are few in number. The student need not identify them. The resulting haploid spermatids have spherical nuclei at first and then undergo morphological changes. Their nuclei, as well as cytoplasm, become condensed and eventually form the typical sperm head structure. For a summary of the morphological changes accompanying spermiogenesis, see your textbook. Try to find spermatid nuclei around the area of the tubule lumen; they are more numerous and smaller than those of secondary spermatocytes.

   

Find the interstitial (Leydig) cells in the connective tissue between seminiferous tubules. They are large, irregularly shaped cells with a round nucleus. Their nuclei may be either dark or pale, and in the latter case they usually contain one or two prominent nucleoli; their cytoplasm may contain pigment or crystalloid material.

Duct System

The duct system of the male reproductive tract begins at the mediastinum testes (hilus) and consists of the ductuli efferentes, the epididymis, the vas deferens with its ampulla, and the ejaculatory duct. The epithelial lining of the duct is secretory and in some regions absorptive and under the control of androgens. A large fraction of the fluid leaving the testis with spermatozoa is reabsorbed by the epithelia of the ductuli efferentes and the upper portion of the epididymis. Spermatozoa leaving the testis appear morphologically complete but are unable to fertilize eggs. Further maturation takes place as they pass slowly through the epididymis (several weeks).

The ductuli efferentes are 10 to 15 small coiled ducts with a few layers of smooth muscle in their walls. The epithelial lining of the ductules is pseudostratified ciliated and consists of alternating groups of columnar and cuboidal cells. The convoluted ductules empty into a single larger tube, the ductus epididymis. This is an extremely long (4-6 meters) tortuous tube with a thin muscle coat. An outer fibrous tunic binds the whole mass to form the epididymis. The duct is lined with pseudostratified columnar epithelium consisting of rounded-basal cells and tall columnar cells with stereocilia. The epididymis is divided into head, body, and tail, the duct being smallest and most convoluted in the head where reabsorption of fluids predominates. Towards the tail, the duct gradually straightens and enlarges, acquiring a thicker muscular coat that becomes continuous with the vas deferens. Spermatozoa are stored in the body and tail, however the ionic medium maintained by the epithelium is such that they are non-motile and conserve stored energy.

The vas deferens (ductus deferens) is a tube with an extremely thick muscular wall. It is bound by C.T. with blood vessels, nerves and lymphatics to form the spermatic cord which passes from the scrotum into the abdominal cavity. The lumen of the vas deferens has an irregular outline due to longitudinal folds of the mucosa, and it is lined by a pseudostratified epithelium that is lower than that of the ductus epididymis. Some cells have stereocilia, but the latter tend to disappear toward the ampulla. The ampulla is a fusiform dilation of the vas in the abdominal cavity, just before it receives the duct of the seminal vesicle. The muscle coat becomes thinner and the lumen larger, and the longitudinal folds of mucosa are tall and intricately branched to form a network of partitions. The epithelium is clearly secretory with glandular outpocketings extending deep into the muscle layer. After receiving the duct of the seminal vesicle, the vas is reduced in diameter and becomes the ejaculatory duct which empties into the prostatic urethra. There are numerous folds and glandular pockets, but these are less prominent than in the ampulla; the epithelium is simple or pseudostratified columnar, becoming transitional near the urethra.

Examine ductuli efferentes in slide #95 (from a 15 year-old subject). Numerous sections through these ducts may be seen, and the outline of their lumen has a scalloped appearance due to the presence of alternating tall and low cells. Under high magnification, it may be seen that both columnar and cuboidal cells can have true cilia (i.e., motile), and ciliated cells have darker staining cytoplasm than the non-ciliated cells. Both columnar and cuboidal cells can be pale-staining and contain vacuoles that appear to be secretory and absorptive. Beyond the epithelium is a basement membrane followed by a layer of circular smooth muscle two to three cells thick. Outside the smooth muscle is the interstitial connective tissue in which these tubules are embedded. Note the arterioles and venules in the interstitial connective tissue.

With the scanning lens, locate a group of tubules with a much taller, very regular epithelium near the large dilated ductules; this is the beginning of the coiled ductus epididymis lined with a pseudostratified columnar epithelium with stereocilia. A layer of smooth muscle cells, 2 or 3 layers thick, is apposed to the basement membrane of this epithelium.

Examine figures in your text, then study the vas deferens (ductus deferens) and some associated spermatic cord structures in slide #96. Some slides have the junction of the duct of the seminal vesicle with the ampulla of the vas deferens. Under low magnification, note the ratio of wall thickness to lumen diameter in the vas deferens and examine the orientation of the muscle coats; outer and inner longitudinal coats of variable thickness and circular coats in between the longitudinal coats become thinner and disappear near the ejaculatory duct. This is the most heavily muscled tube in the body, and its function is related to ejaculation rather than sperm maturation. On slide #96 odd, an acidophilic layer just beneath the mucosa consists of a mixture of fibroblasts and small smooth muscle cells. The epithelium is pseudostratified columnar with stereocilia but most of the cells have lost their stereocilia during preparation.

Glands

Seminal Vesicles

Each of the paired seminal vesicles is an irregular highly convoluted tube, 10-15 cm. long, bound up into a compact mass by highly vascular connective tissue. These slides show several sections of the same tube bound together by intervening vascular connective tissues. The walls of the tube itself have three coats: a mucosa made up of epithelium with underlying loose elastic connective tissue, a thin muscular coat, and a fibrous coat. The epithelium is pseudostratified non-ciliated columnar, and it is thrown into branching folds of extremely complicated patterns.

Examine slide #99. In even-numbered boxes (from an adult), the lumen contents consist mainly of desquaminated, dying cells. The epithelium appears cuboidal or even squamous and mucosal folds have been flattened out so that only a few remain. This gland reflects a decline of androgen levels. However, sperm are present in the lumen. In odd-numbered boxes (from a 15 year old boy), the epithelium is pseudostratified, and the mucosal folds are large and abundant. Identify the smooth muscle in the tube walls.

The Prostate Gland

As the urethra leaves the bladder, its lumen becomes U-shaped. At this point many tubulo-alveolar glands surround and open into the urethra; these glands are embedded in fibro-muscular tissue and constitute the prostate gland. The glands are in three groups: the short mucosal glands directly beneath the opening throughout the urethral epithelium, a few submucosal glands with relatively short excretory ducts, and the large main prostatic glands with long ducts embedded deep in the connective tissue and making up the bulk of the prostate.

The prostate gland seen in slide #97 does not include the urethra, and therefore, primarily has the large main prostatic glands. There is a dense, very abundant stroma containing small bundles of smooth muscle. The glands are originally tubular in shape but gradually acquire large, round or irregular swellings so that they become irregularly tubulo-alveolar. Examine the glandular epithelium in several areas of your slides; it varies from low cuboidal to columnar and tends to be lower in alveoli containing many desquamated cells. The lumen of some secretory units contains concretions, which are masses of carbohydrate and other materials, deposited around cell fragments. Concretions become more abundant with age.

   

The ducts of the prostatic glands converge toward the urethra, and many have a highly-folded distensible lumen wall. The urethral mucosa (slide #98 even) is highly folded, and its epithelium is usually stratified columnar. Mucosal glands are seen immediately adjacent to the urethra while a few submucosal glands are found a little further away. Main prostatic glands are present on this slide. Ducts of the submucosal and main prostatic glands are seen in some slides, but not all. Note the numerous lymphatic channels in the connective tissue around the urethra. (Some of the slides may contain few glandular elements and be more representative of the membranous portion of the urethra, i.e., the part of that emerges from the prostate). Slide 98 odd, does not include the urethra, but in some slides, the ejaculatory duct or sections through seminal vesicle may be seen on one side, and a portion of the bladder with its transitional epithelium on the other.

Penis

As the urethra emerges from the prostate, it pierces the fasciae of the urogenital diaphragm. This is the "membranous" urethra. It then becomes surrounded with erectile tissue which consists of a network of large irregular venous sinuses in connective tissue. This is the "cavernous" part of the urethra. Many small serous and mucous glands open into the urethra.

The erectile tissue immediately surrounding the urethra is the corpus cavernosum urethrae, or corpus spongiosum. In the penis, this body becomes smaller in diameter, and there is an adjacent, much larger body of erectile tissue called the corpus cavernosum. The blood spaces of these cavernous bodies are supplied by highly coiled arteries which uncoil and allow a rapid passage of large volumes of blood to fill and distend the sinuses and cause erection of the penis. These sinuses are venous channels and do not serve for exchanges or phagocytosis; they should not be confused with sinusoids.

Examine a cross section of the penis, slide #100, fetal penis in even-numbered boxes, and adult castrate penis in odd-numbered boxes. These slides are present in only a limited number of boxes. The outer stratified squamous epithelium has been removed except for some areas on the castrate penis. Under the epidermis there is a network of loose elastic connective tissue containing blood vessels and nerves. The smaller corpus spongiosum (corpus cavernosum urethrae) is surrounded by a dense connective tissue capsule and is located under a median indentation of the large corpus cavernosum. Note the stratified squamous epithelium of the urethra and the endothelium of the blood sinuses of the erectile tissue.

The large corpus cavernosum is surrounded by a dense, fibrous capsule which sends in medial septa, giving the structure a bi-lobed appearance in cross-section. The fibrous capsule limits expansion of the corpus cavernosum, and provides the turgidity which occurs when the blood spaces are filled. Examine the structure of its erectile tissue and note that the spaces are larger towards the central, deeper region. Compare with the corpus cavernosum around the urethra, where the sizes of the sinuses tend to be more uniform. The cavernous bodies are supplied and drained by the numerous vessels seen at their periphery.