Comparative physiology of the mammalian epididymis

The mammalian epididymis is a highly metabolically active organ which synthesizes and/or secretes sialic acids (sialomucoproteins), lipids, glycerylphosphoryl chlorine, carnitine and steroids. The secretory activity of the different segments of the epididymis varies and is affected by age, levels of circulating androgens, testicular fluid and its androgens and spermatozoa. The caput epididymidis of rat, hamster, bull, monkey and human show higher concentrations of total lipids and phospholipid phosphorous than the cauda epididymidis. During their transit through the epididymis, the spermatozoa lose phospholipids which may be due to their utilization as a substrate for energy. Glycerylphosphoryl choline is a secretory product of the caput epididymidis in the rabbit and is accumulated in the cauda epididymidis. Carnitine is present in high concentration in the cauda epididymidis of rat and bull; bovine spermatozoa acquire carnitine during their epididymal transit which may be involved in fatty acid oxidation. Sialic acids (free or bound to proteins as sialomucoproteins) are secreted by the epididymis of the rat, hamster, monkey and human. Bound sialic acid in the spermatozoa decreases markedly during their transit through the epididymis in the rat, hamster and monkey. An inverse relationship exists between levels of bound sialic acid in the luminal plasma and that of spermatozoa in the cauda epididymidis. The epididymis actively synthesizes steroids and probably secretes them. The epdidymis is also capable of metabolizing the androgens. Hormonal homeostasis for the maintenance of the functions of the epididymis may involve not only free testosterone and its metabolites but also estosterone bound to androgen binding proteins