Spermatozoa achieve functional maturity during their transit through the epididymis and this maturation process is accompanied by changes in the composition and proteins of their surface. The addition of secretory products from the epididymis to the plasma membranes of the spermatozoa is considered to be a prerequisite for the acquisition by the spermatozoa of the capacities for forward motility and ovum recognition. An antibody was purified from an antiserum raised in the rabbit against fluid from the cauda epididymis of the mouse. This antibody, in combination with fluorescein isothiocyanate-conjugated goat anti-rabbit antibody, was used to demonstrate a progressive increase in the synthesis and secretion of antigens along the length of the epididymis. Immunoaffinity chromatography of [35S]methionine-labelled proteins, synthesized by segments of the epididymis maintained in vitro, showed that the predominant protein synthesized by the cauda, but not by the caput, epididymis, migrated on electrophoresis with an apparent Mr of 26,000. This same protein was the major antigen found on the plasma membrane of cauda spermatozoa that had been radioactively labelled with the non-penetrating probe isethionyl [1-14C]acetimidate.

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