THE CYTOPLASMIC DROPLET OF RAT EPIDIDYMAL SPERMATOZOA CONTAINS SACCULAR ELEMENTS WITH GOLGI CHARACTERISTICS

The cytoplasmic droplet of epididymal spermatozoa is a small localized outpouching of cytoplasm of the tail of unknown significance. EM reve aled flattened saccular elements as the near exclusive membranous comp onent of the droplet. Light and electron microscopic immunolabeling fo r Golgi/TGN markers showed these saccules to be reactive for antibodie s to TGN38, protein affinity-purified alpha2,6 sialyltransferase, and anti-human beta1,4 galactosyltransferase. The saccules were isolated b y subcellular fractionation and antibodies raised against this fractio n immunolabeled the saccules of the droplet in situ as well as the Gol gi region of somatic epithelial cells lining the epididymis. The isola ted droplet fraction was enriched in galactosyltransferase and sialylt ransferase activities, and endogenous glycosylation assays identified the modification of several endogenous glycopeptides. EM lectin staini ng in situ demonstrated galactose and N-acetyl galactosamine constitue nts in the saccules. Endocytic studies with cationic and anionic ferri tin as well as HRP failed to identify the saccules as components of th e endocytic apparatus. Epididymal spermatozoa were devoid of markers f or the ER as well as the Golgi-associated coatamer protein beta-COP. I t is therefore unlikely that the saccular elements of the droplet part icipate in vesicular protein transport. However, the identification of Golgi/TGN glycosylating activities in the saccules may be related to plasma membrane modifications which occur during epididymal sperm matu ration.