Higher power of rete testis, the narrow cavernous channels lying in the dense connective tissue. You will notice a couple of thin-walled, blood-filled blood vessels also coursing in the same region. Sperm produced in the seminiferous tubules leave the testis by way of the rete, which ultimately converges on about twelve efferent ducts.
Low power view of seminiferous tubules. These tubules are very long and tightly coiled, so each one is cut many times in any given section of the testis. Blood vessels and interstitial cells of Leydig lie in the connective tissue stroma between the tubules.
Detail of wall of seminiferous tubule, showing stages of spermatogenesis. The large, vertically oriented nucleus at the base on the left belongs to a Sertoli cell; notice that sperm heads (somewhat out of focus) are clustered deep down near this nucleus. The horizontally flattened nuclei along the base of the tubule belong to spermatogonia, the continually multiplying, diploid germ cells. A few primary spermatocytes, with the dark, condensed chromosomes undergoing prophase of meiotic division, lie just above the spermatogonia. Above the spermatocytes are the round, relatively small, haploid spermatids, which would occupy the rest of the layers up toward the lumen.
Drawing of stages in the differentiation of sperm directly from spermatids. This process is called spermiogenesis.
A = a still-rounded spermatid, with an acrosomal body beginning to form in the region of the Golgi apparatus. At the opposite pole of the nucleus lie the centrioles, one of which begins to spin out a long cilium (or flagellum).
B = an elongating spermatid, with an acrosomal cap now forming over the top of the nucleus. The flagellum is longer and the centrioles are oriented perpendicular to each other. The centriole related to the flagellum is comparable to the basal body of ordinary cilia.
C = further development of the acrosomal cap and beginning pinching off of excess cytoplasm, thanks to the formation of a filamentous manchette, nuclear ring, and annulus.
D = further condensing of the nuclear chromosomal material and separating off of the excess cytoplasm. Notice that the intercellular bridge connecting this spermatid to its neighbor is still intact.
E = the cytoplasmic mitochondria have now collected along the proximal portion of the flagellum and are thus conserved (for energy purposes) when the residual cytoplasm is cast off. The flagellum has meanwhile developed a complex fibrous sheath which surrounds a central core of microtubules arranged in the nine plus two arrangement typical of cilia.
Scanning EM of differentiated sperm in the top photos, and a drawing below of a spermatozoan as seen in transmission EM. In the SEM views:
HR = head region, with acrosomal cap nearly covering the nucleus. This cap contains hyaluronidase, which will be released at the time of fertilization as an aid to breaking down membranes around the egg.
NK = neck region, where the centrioles lie
MP = middle piece, where the mitochondria congregate
PP = principal piece of the flagellum
EP = end-piece of the flagellum
Additional labels of importance in the TEM drawing include:
Ac = acrosomal cap
N = nucleus
Ne = extension of the nuclear envelope
Ce = centriole
M = mitochondria
ODF = outer dense fibers of the flagellum
FS = fibrous sheath of the flagellum
F = flagellar core containing the 9 + 2 arrangement of microtubules (otherwise known as the axoneme).
Mt = microtubules as seen in cross-section
Cross-cuts of the three main parts of the flagellum are shown. Notice that a plasma membrane covers the entire sperm; in other words, this is a highly differentiated single cell. The flagellum is not motile when sperm leave the testis, but maturation takes place during the long stay in the epididymis. Actually, sperm are not fully capacitated for fertilization until they encounter the fluids of the vagina.
Diagram of the relation between Sertoli cells and the cells undergoing spermatogenesis in the wall of the seminiferous tubule. Best seen to the left is a tall columnar Sertoli cell, with a very indented, irregular cell outline. Its nucleus lies near the base. The spermatogenic cells lie in the indentations of the Sertoli cell, apparently taking some nourishment from the columnar cell. The spaces where the spermatogonia, spermatocytes. and spermatids would lie are empty here, but you'll notice that there are connecting channels from space to space because these cells are incompletely separated during meiosis and are connected with each other by intercellular bridges, much as if in a chain of paper dolls. As the spermatids differentiate into sperm, their heads indent the Sertoli cells from the top and may push quite deeply down toward the nucleus, never breaking the Sertoli cell membrane but just being supported and protected in this way. Notice, near the base of the Sertoli cell, that the cell spreads out horizontally to meet its Sertoli cell neighbors and that bands of tight junctions develop between them (see arrows). These form the so-called "blood testis barrier", which provides an immunologically privileged environment for all the cells lying above it (toward the lumen). The diploid spermatogonia, which are genetically like most of the cells of the body, lie in the basal compartment, where they are exposed to all contents of the blood capillaries that surround the seminiferous tubule. Spermatocytes and spermatids, on the other hand, lie in the adluminal compartment, above the barrier, and are shielded from some substances in the blood. Remember, too, that these meiotically active and eventually haploid cells are genetically different from the rest of the body and would be recognized as "foreign" if not protected from blood and tissue fluids.
Wall of seminiferous tubule. Along the base can be seen small dark nuclei of spermatogonia and large, pale, ovoid or triangular nuclei of Sertoli cells, each with a prominent nucleolus. Sperm heads are imbedded in folds of Sertoli cell membrane, rather deep within the tubule wall. Sperm tails are pointing toward the tubule lumen. Primary spermatocytes have large nuclei with the condensed chromosomes in prophase, near the base of the wall. The small, round nuclei toward the lumen belong to early spermatids. Pale pink cytoplasmic cast-offs from differentiating spermatids (becoming sperm) lie next to the lumen. Look again at the two Sertoli cells farthest to the left and notice how their cytoplasm meets near the base to form the basal and adluminal compartments on either side of the junction.
Efferent ducts with their irregular epithelial border. These are the only portions of the male reproductive tract with motile cilia on the lining epithelium. Cilia help to move the sperm along toward the epididymis.
Epididymis with pseudostratified columnar epithelium with stereocilia. (Stereocilia are structurally like microvilli rather than like true cilia. They do not move.) Each cross-cut of tubule shows some surrounding smooth muscle cells. Notice how very regular this epithelium is in height, making an unusually smooth apical line near the lumen. This is characteristic of epididymis. Compare this with the "scalloped" edge of efferent ducts in the previous slides.
Another view of epididymal wall, showing more clearly the basal and columnar cells of the pseudostratified epithelium. The stereocilia are long and pale (practically invisible here!). Circular smooth muscle in the outer wall is evident.
Ductus deferens with its proportionally small lumen and heavy muscular coat. The bulk of the smooth muscle is circular, but there is a thin inner longitudinal and a somewhat thicker outer longitudinal layer.
Prostate gland with both distended and infolded epithelial linings of the secretory portions. The saccular secretory tubules lie in a dense connective tissue framework (stained pink here). The prostate, along with the seminal vesicles and bulbo-urethral glands, contributes nourishing and lubricating fluids to the ejaculated semen.
Prostate stained with Mallory to show pink/lavender strands of smooth muscle lying in the blue connective tissue framework. Notice again that the secretory tubules are wide and sacculated and that the epithelium is thrown into folds.
Penile urethra (corpus spongiosum), showing the typical dorso-ventral flattening of its lumen. The mucosal lining is highly folded. The bulk of the wall is composed of erectile tissue consisting of convoluted trabeculae surrounded by irregular vascular spaces (mainly venous sinuses) which are lined with endothelium. Some of the spaces are empty here; some are filled with dark red blood.