Structural features and functions of principal cells of the intermediate zone of the epididymis of adult rats.

Background: In the present study, principal cells of the intermediate zone of the epididymis, an area situated between the initial segment and proximal caput, were observed to present morphological features distinct from those of principal cells of other regions.

Methods: The epididymides of adult rats were fixed by perfusion with glutaraldehyde and embedded in Epon. Administration of fluid phase tracers was performed in the case of several animals. Localization of anti-SGP-2 and anti-immobilin antibodies in conjunction with light (LM) and electron (EM) microscope immunocytochemistry was also performed.

Results: In the LM and EM, the most distinctive feature of many principal cells of this zone was the presence of apically located vacuoles referred to as giant endosomes due to their large size and because they readily incorporated tracers introduced into the lumen of the epididymal duct and were acid phosphatase-negative. Giant endosomes, containing electron-dense granular patches, appeared to form by the progressive fusion of small, medium, and large endosomes. In the supranuclear region, multivesicular bodies (MVBs) and lysosomes were present. Although smaller in size than the giant endosomes, MVBs and lysosomes contained the electron-dense patches. It is suggested from morphological images that giant endosomes fragment into smaller units corresponding to MVBs which gradually transform into lysosomes. Experiments using anti-SGP-2 and anti-immobilin antibodies revealed gold particles over the Golgi apparatus and secretory vesicles (150-300 nm) of principal cells of this zone as well as the luminal contents indicative of secretion of these proteins. Interestingly, giant endosomes were also immunolabeled with both antibodies as were stereocilia, coated pits and vesicles, and endosomes of various sizes; lysosomes were minimally labeled. These results suggest that principal cells of the intermediate zone endocytose as well as secrete SGP-2 and immobilin. The internalized SGP-2 and immobilin may correspond to that secreted further upstream and that, possibly due to their short half-life and terminated function, are removed from the lumen of the duct. Principal cells of this zone secrete these proteins possibly to replenish that lost by endocytosis.

Conclusions: Principal cells of the intermediate zone contain giant endosomes. The presence of such large structures suggests that the early events in endocytosis is a slower process in principal cells of this zone as compared to other regions. The fact that these cells both secrete and endocytose SGP-2 and immobilin adds to the complexity of our understanding of how principal cells function along the length of the epididymis.