This evidence suggests that 2D6 antigen is secreted in a relatively dilute and soluble form by classic exocytotic pathways and is then concentrated in the luminal fluid by selective transport of water in the reverse direction. Gold particles in the lumen are usually associated with small electron-dense aggregates, and there is no indication that they are contained within or attached to membrane-bound vesicles (epididymosomes), which are thought to be formed by apocrine secretion. The latter process is characterized by the presence of apical blebs several microns in length protruding into the lumen from the surface of principal cells, and even after 30 years of detailed morphological studies on the epididymis, they remain a contentious phenomenon.
A long established view is that apical blebs are artifacts of the fixation and embedding procedures commonly used in electron microscopy. Recently, Hermo and Jacks have argued that apical blebs in the epididymis are not fixation artifacts but are manifestations of genuine apocrine secretion and are the probable origins of ep-ididymosomes that have been described in the lumen of the rat and bull epididymis. This long-standing and vexatious problem remains to be resolved. Although the present ultrastructural analysis was limited in scope, apical blebbing was not observed in any electron micrographs of the distal caput epithelium.