ELECTRON-MICROSCOPIC IMMUNOLOCALIZATION OF THE 18-KILODALTON AND 29-KILODALTON SECRETORY PROTEINS IN THE MOUSE EPIDIDYMIS - EVIDENCE FOR DIFFERENTIAL UPTAKE BY CLEAR CELLS

In previous studies we reported the synthesis, secretion, and immunolo calization at the light microscopic level of two mouse epididymal prot eins, MEP 7 and MEP 10 [Rankin et al. (1992b), Biol. Reprod., 46:747-7 66]. MEP 7 is the mouse homologue of the rat metalloproteins, AEG/D an d E, and MEP 10 is the mouse homologue of the rat retinoic acid bindin g proteins, B and C. We now describe the immunolocalization of MEP 7 a nd MEP 10 in the mouse epididymis at the electron microscopic level. M EP 7 was localized in the Golgi apparatus, in small electron-lucent se cretory vesicles, and on microvilli of the principal cells from the di stal caput epididymidis to the cauda. The luminal contents were also i mmunoreactive in these regions of the epididymis. Although some gold p articles were associated with the sperm surface, there was no selectiv e concentration of these particles. In addition, MEP 7 was localized i n large (600 nm) supranuclear endocytic vesicles and in infranuclear l ysosomes. MEP 10 immunoreactivity was also seen on the microvilli of t he principal cells of the distal caput and corpus and the luminal cont ents from the distal caput to the cauda epididymidis. There was no ass ociation of gold particles with the sperm surface. In contrast to MEP 7, there was no detectable MEP 10 immunoreactivity on the organelles o f the principal cells involved in protein secretion or endocytosis. Cl ear cells also demonstrated immunoreactivity to MEP 7 and MEP 10. Howe ver, the intensity of immunolabeling, and the number of clear cells la beled, was greater with MEP 10 than MEP 7. In the case of MEP 7, the g old particles were located on the large supranuclear endocytic vesicle s and on some infranuclear lysosomes, from the proximal corpus to the middle cauda, while in the case of MEP 10, gold particles were predomi nantly present in infranuclear lysosomes from the distal caput to the middle cauda. These results suggest that the principal cells are invol ved in both the secretion and endocytosis of MEP 7. The MEP 10 and MEP 7 proteins present in the lumen of the mouse epididymis are endocytos ed from the lumen and degraded in the clear cells. However, the proces s of endocytosis by the clear cells of these two proteins appears to b e different. (C) 1995 Wiley-Liss, Inc.