Immunolocalization of apolipoprotein E in the testis and epididymis of the rat

Apolipoprotein (apo) E, a 35-kDa protein found on the surface of several lipoproteins, has been detected in many peripheral tissues and is postulated to function in facilitating the transfer of cholesterol/lipids between cells. We examined the expression of apo E in the testes and epididymides of juvenile rats (21 days old), prepubescent rats (34-36 days old) and sexually mature rats (75-80 days old). Apo E was localized by means of a polyclonal rabbit anti-rat apo E antibody and standard immunocytochemical techniques. Strong positive staining for apo E was observed in the interstitial space of the tests from all rats. Apo E-containing cells were identified as Leydig cells through use of a 3 beta-hydroxysteroid dehydrogenase antibody on serial sections. In contrast to sexually mature rats, the two younger groups of rats also showed positive staining for apo E within the seminiferous tubule associated with Sertoli and germ cells. In addition, strong positive staining for apo E was observed in the stroma of the caput, corpus, and cauda epididymides from rats of all ages. This study demonstrates that apo E is differentially localized during development of the tests, suggesting a regulatory and/or cholesterol transport role.