Tissue and cell specificity of immobilin biosynthesis. 
The mechanisms for the initiation of sperm motility have been poorly understood until recently.  Immobilin is a novel mucin glycoprotein of high molecular weight found in the cauda epididymis of the rat that, at concentrations equivalent to those found in native cauda epididymal fluid, reversibly inhibits sperm motility.  In this study, immobilin was purified from rat cauda epididymal fluid to apparent homogeneity and used to generate polyclonal antibody in rabbits.  The antibody was characterized by immunoblotting, and immunofluorescence was used to localize immobilin in paraffin sections of components of the reproductive system of adult male rats.  Immobilin was not detectable in the efferent duct and was first detectable in the apical portion of some epithelial cells of the initial segment of the caput epididymis.  Immobilin was detectable intracellularly only in cells of the caput epididymis.  In the corpus and cauda epididymis immobilin was detectable only in the lumen of the tubules.  Immunoprecipitation of immobilin radiolabeled in vitro confirmed that immobilin biosynthesis in the adult rat is restricted to the caput epididymis.  Principal cells in the caput epididymis synthesize immobilin and secrete it into the lumen of the tubules to travel with the sperm into the cauda.