The ultrastructure of the principal cells and intraepithelial leucocytes in the initial segment of the rat caput epididymidis was examined with the electron microscope. Specializations of the principal cells associated with absorption include numerous endocytic invaginations of the cell surface, numerous coated vesicles and multivesicular bodies in the apical cytoplasm. It was demonstrated that particulate tracers are taken into the cells and sequestered in secondary lysosomes and multivesicular bodies. Morphological features consistent with secretory activity are also found in the principal cells and include numerous cisternae of rough endoplasmic reticulum with a flocculent grey content and an extremely well-developed Golgi apparatus. The speculation that the principal cells are actively secretory despite the absence of secretory granules formed in the Golgi and of a visible mechanism for release of the product at the cell surface is discussed.
The “halo cells” in the epididymal epithelium were also examined and it is shown that many of these cells are not typical migratory lymphocytes. Chief among the differences are their granule-containing multivesicular bodies and more abundant endoplasmic reticulum. Nonetheless, it is conceivable that the halo cells are lymphocytes and that the conditions they encounter as they leave the circulation and enter the epididymal epithelium may stimulate morphological changes. The possible immunological significance of these observations is discussed.