Objectives for this
Exercise
The male reproductive system consists of the testes and their associated ducts to conduct sperm to the outside, also of various male accessory glands which produce non-sperm components of the ejaculate, and that portion of the urethra which is used for the transport of semen. It also includes the intromittent organ, the penis.
The testes are "exocrine" and endocrine in nature. They produce both a cellular "secretory product" in the form of sperm, and a the hormone testosterone as a true endocrine secretion. These two products come from different regions of the testes, which we'll examine separately.
The testes of most mamall are located extra-abdominally in the scrotum. This is actually a temperature controlling device. In most mammals spermatogenesis proceeds evenly and smoothly at an optimal temperature slightly below the core body level. The role of the scrotum is to maintain that temperature. When the ambient temperature drops, the testes are pulled up by muscular action towards the warmth of the body cavity, and when it rises, the muscle relax, allowing the testes to descend and remain cool. Almost all terrestrial mammals have extra-abdominal testes.
Start with slide 51. The outer border of the testis is demarcated by a thick band of dense collagenous connective tissue, the tunica albuginea. The tunica albuginea has a good many blood vessels running through it, including some rather large arteries. This is to be expected, since the inner portions of the testis require good circulation, and the blood supply has to come through the outer tunic. From the tunica albuginea, small septa subdivide the cavity of the testis into smaller compartments. Septation is usually incomplete in domestic animals.
To see the tunica albuginea, click here.
The bulk of the testicular tissue is the seminiferous tubules. There's a phenomenal total length of these: in humans the collective total of the 800-1600 tubules is about 600 meters, and the figure is much higher for large animals like boars and bulls. The outer wall of each seminiferous tubule is a single layer of boundary tissue cells similar in some ways to smooth muscle. They're sometimes called myoid cells or peritubular contractile cells, and at least in some species (e.g., rodents) they are known to be contractile, though it hasn't been proven of all animals. The contractions appear to be spontaneous, and not to involve any nervous stimulation. Between the seminiferous tubules--in the interstitial region--you will also find blood vessels of the peritubular capillary network, and the interstitial cells, discussed below.
For a closeup view of the architecture of the tubules, click here.
The lining cells of the tubules comprise the seminiferous epithelium, and there are several cell types. Some of them are the source of sperm and some are there to sustain the production.
The Sertoli cell or sustentacular cell (Enrico Sertoli, 1842-1910, an Italian histologist) is easily identifiable in H&E preparations by its nucleus. This cell type is (as the name implies) not part of the sperm cell line, but a sort of "nurse" cell that sustains the proper environment for spermatozoa to develop.
The Sertoli cells sits on the outermost portion of the tubule, right up againts the bounadry. Its nucleus will be slightly separated from the tube border. The nucleus is pale-staining, with an oval to pyramidal shape and one or two prominent nucleoli. Although these cells have a very extensive and branching cytoplasmic structure, little of the Sertoli cell's true extent can be seen in the light microscope. The extensive branching nature of the cell's cytoplasm and its role in isolating the haploid components of the system from the rest of the body were not understood before the advent of the EM.
To see an example of the Sertoli cell, click here
The sustentacular cell has as its role the nurturing and care of developing spermatocytes (see below). The "branches" of the "tree" enfold the forming sperm through all stages of the development cycle, eventually releasing them into the lumen. But the process of sperm development presents some difficulties. After all, haploid spermatocytes and spermatids are genetically different from the diploid spermatogonia (see below). Hence, there exists a possibility that the immune system, given access to these haploid cells, might mount an immune reaction to them. Furthermore, the very earliest stages of sperm development require a different set of physiologic conditions to the later stages. The sustentacular cells have as one of their chief roles the prevention of autoimmune reactions and the maintenance of two separate compartments in which the different stages of the sperm cycle can exist under optimal conditions.
The base of the sustentacular cell sits on the outer wall of the seminiferous tubule, right against the boundary cell layer. The cell's lowest "branches" reach out to those of other sustentacular cells. When the processes make contact, they fuse to form long occluding junctions and effectively separate the tubule into two compartments; one is "below" (or outside of) the fused lower processes (the adluminal or basal compartment), and the other is "above" them, on the side open to the tube (the luminal compartment). These two compartments have different physiologic environments. The blood vessels alongside the boundary of the seminiferous tubules carry nutrients and remove wastes, but this arrangement means they can't contact the materials in the luminal compartment. The barrier presented by the occluding junctions thus prevents autoimmune reactions; blood borne components of the immune system are denied access to the haploid stages of sperm development by this blood-testis barrier. Simultaneously the barrier permits the maintenance of different environments for different stages of sperm development.
As spermatocytes develop they need to move into the luminal compartment. The appropriate sustentacular cells grow new processes which "undermine" the cells that are ready to move "up." After this has happened, the junctions "above" it are broken, and presto! the cell is now--without having passed through any membranes--no longer in the compartment accessible to the blood. Neat trick, huh?
The seminiferous epithelium contains spermatogonia, the stem cells for the production of sperm. Spermatogonia are basally located (i.e., at the periphery of the tubule) and can be identified by their densely stained, round nuclei. These are diploid cells. Some of the spermatogonia remain undifferentiated, divide, and continue to give rise to new generations of spermatogonia; others undergo meiosis, becoming haploid, and are morphologically differentiated into sperm in the process of spermiogenesis.
Primary spermatocytes are the initial product of spermatogonial maturation; you will see these as large cells with a round nucleus filled with clumped chromatin material. Many of them should be seen in various stages of the meiotic cycle. Although secondary spermatocytes are undoubtedly present, this is a very short lived stage, and you are unlikely to find one of these rare cells; if you did it would be very difficult, given the quality of this slide, to tell it from a primary. By this time meiosis is usually complete, and secondary spermatocytes are haploid.
The secondary spermatocytes transform themselves into spermatids, located much closer to the lumen of the tube. These should be easy to identify. Spermatids will be seen in different stages of differentiation into matured sperm; many are chunky and squarish looking cells with nuclei beginning to get quite dense. In some parts of the section you'll see tails beginning to develop. You may also see stages in which the excess cytoplasm is being cast off. A "tailed spermatid" is a fairly common sight.
To see nearly-mature sperm about to be cast off into the tubule, click here.
In the interstitial regions between the seminiferous tubules (i.e., outside of them) you'll see small groups of the cells which produce the male steroid hormone testosterone. These are interstitial cells (or Leydig cells for Franz von Leydig, 1821-1908, a German anatomist). They're cuboidal in shape and present in groups of 2-20 cells. You should be able to make these cell out without difficulty on slide 51. They have been covered in detail in the exercise on endocrine organs.
Slide 241 is a testis from a hamster. Because this was made from a small testis, all of it fits on the slide, so the epididymis is also present as two crescentic areas on each side of the testis. The epididymis is a region of collection, maturation, and storage, to which sperm are transferred after they complete their morphogenesis. We'll get back to it later; for now let's look at some of the structures "upstream" of it.
On slide 241, between the epididymis and the testis there's a clear area. At low power, you should see that a portion of the tunica albuginea is "extended" into this space. There's a set of small ducts and an artery surrounded by veins in the same region. Now examine this area at about 10x; and you'll immediately notice that the portion of the duct system closest to the testis is lined by simple cuboidal epithelium.
Here is an example of the rete testis.
The rete testis is the beginning of the outlet ducts. It receives sperm directly from the seminiferous tubules and passes them on to the next portion of the "drainage." The rete testis is sort of like the channels through a river delata: tortuous and winding and eventually coalescing into a few main channels.
The rete testis opens into seven or eight efferent ducts. These coiled ducts are lined by a pseudostratified columnar epithelium of peculiar appearance, varying in height from place to place, giving the duct a "scalloped" appearance. Many of the epithelial cells lining the duct, especially the taller ones, are ciliated. These are true cilia, i.e., kinocilia, which have the typical internal structure and which are capable of movement. The cilia help propel the not-yet-fully-matured and still immotile spermatozoa along their journey.
To see an example of the efferent ducts, click here.
Now let's go back and examine the epididymis itself. The "tubules" in it are really one very long, monstrously coiled tubule (up to 200 yards long in some species). Usually the lumen of this tube is filled with sperm that have been released from the seminiferous tubules "upstream."
For a low power view of the epididymis, click here
Sperm are stored and matured in the epididymis, prior to ejaculation. The lining of the epididymis is pseudostratified columnar, but here they're adorned with "stereocilia" rather than true cilia. Stereocilia are unfortunately misnamed. They are not cilia at all, but instead are just enormously long microvilli. They have no internal structure and do not move; they're a specialization for increased surface area.
The epididymis is a secretory organ. It produces glycoproteins needed to coat the sperm and "capacitate" it, i.e., make it functionally capable of effecting fertilization. Once the sperm have been resident in the epididymis for some time they are fully matured both morphologically and physiologically. The epididymal cells have a very well-developed Golgi apparatus, since they are actively involved in glycosylation reactions. You can see this Golgi apparatus in good preparations quite easily, though on slide 241 you may have some trouble.
Click here for closer views of the epididymal epithelium.
The ductus deferens may be considered the extra-testicular continuation of the cauda of the epididymis, and it's the portion of the tract which conducts sperm to the urethra for their ultimate release into the Grand Adventure that awaits them.
The ductus deferens is seen on slide 704 in both longitudinal and cross section. The thickness of the wall in proportion to the cross section is clearly visible. The mucosal lining is thrown up into folds which run parallel to the long axis of the organ. The lining epithelium is, as before, a stereociliated pseudostratified columnar type. The organ as a whole is surrounded by a CT envelope through which blood vessels and nerves pass.
Click here to see a section of the ductus deferens.
Slides 211 and 709 show the spermatic cord, the suspensory structure of the testis. It consists of the principal artery supplying blood to the organ, which runs through a plexus of veins, the nervous supply, and the ductus deferens; the whole is bound together with CT.
Note in this slide the profusion of blood vessels. The very close association of arteries and veins in the cord permits efficient countercurrent heat exchange between the two; thus incoming arterial blood is pre-cooled to keep the temperature of the testes at the ideal point for spermatogenesis, about two degrees below normal body temperature.
Click here for a cross section of the spermatic cord's blood vessel plexus.
Not shown in this slide but also part of the spermatic cord is the cremaster muscle, a skeletal muscle which can be contracted to draw the testes up towards the abdomen.
The ductus deferens carries sperm to the urethra, and the sperm are discharged through that portion of the urethra which passes through the penis. Slide 751 shows a penis cut in cross section.
The urethra is located slightly below the center of the section, and in this slide (which was taken near the tip) is lined with stratified squamous epithelium; more proximally you would ordinarily see urinary ("transitional") epithelium.
Immediately surrounding the urethra are the blood sinuses of the corpus spongiosum, which in this slide are still filled with blood. Dorsally you can see the much larger blood sinuses of the corpus cavernosum. The latter are involved in the mechanism of erection. A very dense CT wall demarcates the corpus cavernosum, and septa divide it into chambers enclosing the erectile tissue. Careful examination of the blood sinuses in these areas will reveal that they are lined with simple squamous epithelium, as is the rest of the cardiovascular system.
There is one structure on slide 707 worth special notice: the os penis, found in most non-primate mammals. This triangular shaped bone is located dorsal to the urethra, and ventral to the corpus cavernosum, and its internal spaces are filled with marrow. There is a large artery on either side of the os penis. The longitudinal section on this slide demonstrates the surface skin of this region: stratified squamous, only very slightly keratinized.
Having expended so much effort on the means of production and distribution, it's probably just as well to take some time to look at the final product all this machinery is designed to make: sperm. Perhaps more than any other cell, the spermatozoon is best exemplified by a mechanical analogy.
In essence, a spermatozoon is little more than a DNA torpedo. Everything that is not absolutely essential to the overriding mission of "getting there firstest with the mostest" has been stripped away, and the analogy to a torpedo is quite appropriate. There is a powerplant, a propulsive mechanism, and--of course--a warhead that does the damage.
To see an example of sperm, click here.
Sperm are produced in almost unbelievable numbers. The typical human male produces something on the order of 6 million sperm per gram of testicular tissue; a measly rat about three times that. The all time champion sperm machine is the pig testis; a boar cranks out about 23 million sperm per gram, for a total production of 16 billion sperm per day!
You have sperm smears on slides 212 and 934. Slide 212 is from a dog. Select an area near the edge of the smear where the sperm are not concentrated too thickly, and you will be able to make out a considerable amount of detail. (This is one of the rare situations in normal histology where an oil-immersion lens is useful, so give way to your instincts and use it.)
You should be able to make out the acrosomal cap in regions where the staining has been good (there is a considerable degree of variation in this slide's stain). Dog sperm have heads which are flattened dorsoventrally, but when seen from the top, they appear to be rounded. The acrosomal region covers about two thirds of this head, and a dark/light transition back near the point of attachment of the tail marks its boundary.
The connecting piece of the tail is the region immediately behind the head, and is very short. The next part of the tail is the midpiece. This portion is covered by the mitochondrial sheath, and is noticeably thicker than the rest of the tail. Behind that is the principal piece, separated from the mitochondrial region by the annulus, which you will probably not see. The transition from principal piece to end piece in the tail is not so marked as from midpiece to principal piece, but you may be able to discern it. Don't be surprised to see two headed or two tailed sperm; such abnormalities are fairly common, and in some individual animals they may represent a very substantial proportion of the number of sperm produced.
Slide 934 is a sperm smear, probably from a rat. The divisions of the tail are hard to see, but the really interesting thing about this slide is the shape of the heads. They are described as "scimitar shaped" in texts, a scimitar being the sort of curved broadswords favored by Barbary Pirates and Algerine corsairs in the early 19th century. This shape is characteristic of sperm from all of the rodents of which I am aware.
To see an example of rat sperm, you can either find a rat or you can click here. Take your pick.
Horses have sperm with heads shaped like broadhead arrows, as do humans. Dogs have paddle shaped sperm heads. The shapes presumably reflect some sort of structural adaptation to the challenges faced by a sperm in its quest, but why there should be so much species variation is anybody's guess.
Sperm are merely one component of the ejaculate, and the male accessory glands produce the rest. In histological sections, most of these look much alike. Of these glands, by far the most important to clinicians is the prostate gland. Prostatic tumors (both benign and malignant) are very common in older dogs (and in men) and they frequently cause difficulty in urination.
The major landmark in this section is the urethra (which, by the way, is lined with a nice example of urinary epithelium). The gland itself takes the form of secretory regions disposed radially around the urethra. As you can readily appreciate, having all that stuff around the urethra could present problems if a tumor were present; it would occlude the urethral lumen just as one might flatten a soda straw.
To see an example of this gland, click here.
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