Ultrastructure of the mouse epididymal duct with special reference to the regional differences of the principal cells.

It is generally known that spermatozoa aquire the capacity for fertilization during passage through the proximal region of the epididymal duct and are then stored in the distal region of the duct (BEDFORD, 1975 ; HAMILTON, 1975). The mouse epididymal duct has been divided into five segments (I-V) by light microscopy ; Segments I, II and III constitute the head of the epididymis, Segment IV the body, and Segment V the tail; Segments I, II and III seem to belong to the proximal region, and Segments IV and V to the distal region (TAKANO, 1980). In this electron microscope study, we have examined the regional differences of the principal cells of the epididymal duct to understand the functional significance of each segment. The principal cells decrease in height distalwards from Segments I to V. The nucleus is situated basally in the cells, with a well developed Golgi apparatus located above it. The endoplasmic reticulum in Segments I, II and III is generally vesicular, and is distributed throughout the cytoplasm. The ribosomes attached to the endoplasmic reticulum increase in number from Segments I to III. In Segments IV and V, flattened rough endoplasmic reticulum is seen in the basal cytoplasm of the cells. The multivesicular bodies are usually located in the supranuclear cytoplasm. They are large in Segment II and frequent in Segments II and IV. The dense bodies are specific in appearance for the cells in each segment. They are seen in the supranuclear cytoplasm in Segment I and in the inf ranuclear cytoplasm in Segments III, IV and V. Few dense bodies are found in Segment II. The apical cytoplasm contains coated and non-coated vesicles. These vesicles consist of large and small types in Segments I, II and III. They are of the small type in Segments IV and V. The luminal surface membrane has coated or non-coated invaginations between stereocilia. The findings suggest that the principal cells have secretory and absorptive functions specific for each segment. Discussion shall be made on the possibility that the secretion of the specific epididymal substances may provide the fertilizing ability to spermatozoa and the absorption pertains to the testicular fluid, epididymal secretions and substances bound to spermatozoa.