Fine structure of the intratesticular exretory pathway in the guinea pig

Insights into differentiation and function of the transition region between the seminiferous tubule and rete testis

Seminiferous tubules physically connect to the rete testis through short segments called the transition region (TR). During fetal development, this specialized junction is considered the initial site where testis cords begin to form and to grow in length well beyond birth and into adulthood and form convoluted tubular cores. Mitotic activity of the Sertoli cell, the somatic cell of the epithelium, ceases before puberty, but modified Sertoli cells in the TR remain immature and capable of proliferation. This review presents what is known about this specialized region of the testis, with an emphasis on the morphological, molecular and physiological features, which support the hypothesis that this short region of epithelial transition serves as a specialized niche for undifferentiated Sertoli cells and spermatogonial stem cells. Also, the region is populated by an elevated number of immune cells, suggesting an important activity in monitoring and responding to any leakage of autoantigens, as sperm enter the rete testis. Several structure/function characteristics of the transition region are discussed and compared across species.

Reexamination of the morphology of the extratesticular rete and ductuli efferentes in the goat

The morphology of the extratesticular rete and ductuli efferentes was reexamined in serial cross sections collected from the entire mass of the efferent ductules and in longitudinal sections collected from the partially unraveled efferent ductules. The extratesticular rete forms a 3-4-mm-long sac-like dilatation, which, within the head of the epididymis, has a wide lumen (up to 4 mm) and gives off along its length numerous evaginations, which, in turn, make connections with the ductuli efferentes. The latter is a mass of 16-18 ductules lined by three types of nonciliated cells: type II cells are characterized by dense, periodic acid-Schiff (PAS)-positive granules; type III cells are characterized by empty-appearing, PAS-negative vacuoles; and type I cells lack both granules and vacuoles. The distribution of the three types of nonciliated cells varies along the length. Whereas only type I cells are present in the beginning portion of the efferent ductule, type II cells predominate in the middle portion and type III cells in the distal portion (near the epididymis). The transition from one cell type to the other type is gradual; thus there are short segments along the length that share characteristics first for type I and type II cells and then for type II and type III cells. These results demonstrate that different nonciliated cell types are not randomly distributed in the epithelium of the ductuli efferentes but, instead, gradually differentiate from type I to type II to type III cells along the length of each efferent ductule. Factors controlling this differentiation remain to be studied.

Transitional cells at the junction of seminiferous tubules with the rete testis of the rat: their fine structure, endocytic activity, and basement membrane

Transitional cells line the intermediate region of rat seminiferous tubules situated between the rete testis and the seminiferous epithelium proper. These tall elongated cells orient themselves in a downstream direction and converge on one another distally in the lumen of the rete testis where they form a distinct papillalike structure through which a narrow patent lumen is apparent. In addition to widely dispersed Golgi apparatus and mitochondria, these cells contain an abundance of microtubules, cisternae of endoplasmic reticulum, and a distinct lobulated nucleus showing clumps of chromatin and a prominent nucleolus. The endocytic activity of these cells was examined by employing adsorptive (cationic ferritin, concanavalin A ferritin) and fluid-phase tracers (native ferritin, horseradish peroxidase-colloidal gold complex, and concanavalin A ferritin in presence of alpha methyl-D-mannoside). Such tracers were injected separately into the lumen of the rete testis, and the animals were killed at 2, 5, 15, and 30 min and 1, 2, and 6 hr after injection. At 2 min, both adsorptive and fluid-phase tracers were found within coated and uncoated pits of the apical plasma membrane of these cells as well as in large, subsurface, uncoated spherical, C-shaped, and tubular membranous elements. At 5 min the tracers were seen in endosomes of different sizes; while at 15 min and 30 min, pale and dense multivesicular bodies of small and large sizes, respectively, were labeled. At 1-hr and longer time intervals secondary lysosomes became labeled. While both fluid-phase and adsorptive tracers followed the same pathway and fate, binding to the apical and lateral plasma membranes of the transitional cells and to the membrane delimiting coated and uncoated pits was observed only with the adsorptive tracers. These results demonstrate that the transitional cells are actively involved in both fluid-phase and adsorptive endocytosis, which may play an important role in modifying the composition of the luminal fluid. The transitional cells of the distal zone of the intermediate region rest on an elaborate basement membrane (BM) complex which includes a thin BM immediately underlying these cells, a thick distal layer of BM, and strands of BM spanning the distance between the two in the form of a loose anastomotic network. Use of antisera against heparan sulfate proteoglycan, laminin, and type IV collagen revealed the presence of all three components within all areas of the BM complex. In the meshes of the anastomotic BM network, extracellular vesicles were observed.(ABSTRACT TRUNCATED AT 400 WORDS)

The ductuli efferentes of the goat: a morphological study

The morphology of the ductuli efferentes of the goat was compared with that of other animal species, especially with that of the bull. Both in the goat and in the bull, it forms a slightly brownish, bulbous mass of highly convoluted ductules (18-19 in the goat, 13-16 in the bull) that occupies approximately one third of the first limb of the caput epididymidis. The epithelium consists of two major cell types, ciliated and nonciliated, and a few intraepithelial lymphocytes and macrophages. The nonciliated cells can be further divided into three types: type II cells are characterized by specific granules that are dense, mostly homogeneous, eosinophilic, osmiophilic, and positive to periodic acid-Schiff (PAS) staining; type III cells are characterized by specific vacuoles that appear empty and do not stain with PAS, eosin, or osmium; type I cells lack both granules and vacuoles. These morphological, histochemical, and tinctorial differences observed among nonciliated cell types suggest that they are probably independent cell types rather than functional stages of one cell type. All three cell types are endowed with absorptive features such as microvilli, pinocytotic vesicles, and subapical vacuoles, but a higher differentiation of these features in type III cells suggests their greater role in the absorption of testicular fluid. Whether granules of type II cells and vacuoles of type III cells are absorptive and/or secretory remains unresolved from the available data. However, some circumstantial evidence, as presented in the discussion, supports the latter possibility.

The connection between the seminiferous tubules and the rete testis in the domestic fowl (Gallus domesticus). Morphological study

The tubules connecting the seminiferous tubules proper to the rete testis in the fowl were studied with the aid of light and electron microscopy. The material examined ultrastructurally was fixed by vascular perfusion through the thoracic aorta. The seminiferous tubules were joined to the rete testis in three different ways; they were either linked by a terminal segment and a tubulus rectus, by a terminal segment only, or opened directly into the rete cavities. The terminal segment of the seminiferous tubules was lined with columnar cells (modified Sertoli cells). These cells were characterized by having an indented nucleus with a prominent nucleolus, many mitochondria, a sizable Golgi apparatus, electron dense bodies and many cytoplasmic protrusions into the lumen. Intraepithelial lymphocytes as well as macrophages in the lumina of the terminal segment, the tubuli recti and the rete testis were also observed. Myoid cells were found in the boundary tissue of the terminal segment.

The mammalian tubuli recti: ultrastructural study

The ultrastructure of the tubuli recti was studied in the testes of sexually mature bulls, boars, rams, goats, rabbits and rats fixed by vascular perfusion. The tubuli recti are lined with a simple epithelium that varies in height, from squamous to tall columnar according to the species and the region. The cells are characterized by extensive lateral and tortuous basal plasma membranes and a luminal border with microvilli. Tight junctions and desmosomes are found in the upper half of the lateral borders. The Golgi apparatus is sizable and associated with it are coated vesicles and many smooth vesicles concentrated towards the luminal border. A distal segment of the tubuli recti is found in bulls only and is characterized by a high epithelium which is thrown into folds giving the lumen a festooned appearance. It is suggested that the epithelial cells of the tubuli recti are involved in fluid exchange and in the removal of unwanted spermatozoa.

The mammalian rete testis--a morphological examination

The fine structure of the rete testis was examined in several primates, domestic animals and rodents. The rete testis consists of a series of interconnected wide channels lined with a simple cuboidal to columnar epithelium, resting on a thick basal lamina. Beneath the basal lamina dense bundles of collagen fibrils and a few blood vessels, lymphatics or nerve tissue are found. The epithelial cells are characterized by large, deeply indented nuclei, spherical or short rod-shaped mitochondria, supranuclear Golgi profiles, some cisterns of rough endoplasmic reticulum, free ribosomes and numerous micropinocytotic vesicles in the ectoplasmic regions. Smooth endoplasmic reticulum, secretory granules, lysosomes or other types of dense bodies are rarely seen. The apical surface of the cells bears numerous microvilli and a single very long flagellum which is presumed to be motile. Ajoining lateral cell membranes exhibit a juxtaluminal tight junction, elaborate interdigitations and desmosomes. The basal plasma membrane is highly irregular greatly increasing its surface area of contact with the underlying interstitium. The nuclei of the rete epithelial cells contain pale-staining, spherical structure, 2 mum in diameter, composed of circularly oriented fine filaments. The significance of the nuclear structures remains unknown. Thorotrast was injected into the lumen of the hamster and rat rete testis and 30 minutes later the proximal portion of the excurrent duct system of the testis was prepared for electron microscopy. Whereas the ductuli efferentes and first part of the epididymis possessed numerous apical vesicles filled with the thorotrast, this electron opaque substance was rarely found in the epithelium of the rete testis. Thus, incorporation of particulate matter into the lining cells of the rete from its lumen is apparently less active than in the epithelium of the ductuli and epididymis. Vascularly introduced intercellular tracer compounds such as lanthanum nitrate or horseradish peroxidase did not enter the lumen of the rete testis from the interstitium. The tracer molecules appeared to be blocked by the juxtaluminal tight junction separating adjacent epithelial cells. This latter observation suggests that a blood-testis barrier exists at the level of the rete testis epithelium. Although physiological studies have indicated that the composition of fluid secreted in the seminiferous epithelium is considerably modified in the rete testis, the present morphological study does not provide additional evidence to support a secretory or absorptive function for this region of the excurrent duct system of the testis.