The major aims of this investigation were, first, to shed light on the mechanism of secretion of epididymal glycoproteins; second, to examine the mechanism of binding of specific glycoproteins to the sperm surface; and third, to elucidate their disposition within the plasma membrane. For this purpose we selected a McAb known as 2D6 that has been shown to recognize a major glycoprotein antigen on rat spermatozoa of 24 kDa. This antigen is not detectable on testicular cells but is present in epididymal secretions and binds to the tail plasma membrane of maturing spermatozoa as they pass through the distal regions of the duct. Here
Morphological Evidence for Secretion of Epididymal Glycoproteins
The current light-microscopic and ultrastructural investigations of 2D6-reactive antigens in the epithelium of the epididymis have shed more light on the mechanism of glycoprotein secretion. 2D6 immunoreactivity first appears in the apical cytoplasm and microvilli of principal cells in the distal caput epididymidis, a noticeable feature being the abrupt transition between this region and the preceding proximal caput and initial segment. 2D6 immunoreactivity is also present in the epithelium throughout the distal regions of the duct, although it is somewhat obscured in the cauda epididymidis because of the very strong reaction in the lumen. At the ultrastructural level, gold-labeled 2D6 antigen is associated with the Golgi cisternae and small membrane-bound vesicles found in the apical cytoplasm. These vesicles are frequently confluent with the surface membrane, and their contents are of a similar electron density to the glycocalyx on the microvilli, which label heavily with gold particles.