Binding of a Marker for Immunoglobulins to the Surface of Rabbit Testicular, Epididymal, and Ejaculated Spermatozoa

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Abstract
The Cowan I (CI) stain of Staphylococcus aureus was used to detect the presence of Ig on rabbit spermatozoa. The acrosomal and postacrosomal regions of 86 .+-. 4.5% of motile ejaculated spermatozoa were labeled with CI bacteria. Wood 46 strain bacteria, which lack protein A, labeled very few spermatozoa. Binding was blocked by pretreating CI bacteria with rabbit IgG. Spermatozoa were collected from the efferent ducts of the testis and the caput and caudal regions of the epididymis by micropuncture to avoid contamination by serum Ig. Only 5 .+-. 1.8% of efferent duct sperm bound CI bacteria, 84 .+-. 7.1% of caput sperm and 40 .+-. 10.4% of caudal sperm were labeled. The hypothesis that the decrease in labeling of cauda samples could be attributed to the masking of binding sites by luminal fluid components was tested on ejaculated sperm; significantly fewer were labeled in the presence of seminal plasma. Neither the exposure of ejaculated samples to serum nor the contamination of micropuncture samples with blood affected the percentage of spermatozoa labeled by bacteria. Apparently an Ig-like molecule is produced in the caput epididymidis or becomes exposed on caput sperm. This ligand may then be masked by substances secreted into the lumen distal to the caput epididymis.