Protein G-gold immunoelectron microscopy of colon carcinoma: the effect of tumor differentiation on carcinoembryonic antigen immunostaining. 
The distribution of carcinoembryonic antigen (CEA) in human colonic cancer cells was compared to normal colonic epithelial cells by the recently introduced protein G-gold immunoelectron microscopy method.  Immunostaining for CEA was obtained in glutaraldehyde-fixed tissues, but tissues fixed in osmium tetroxide failed to retain CEA immunoreactivity.  In normal colonic mucosa CEA was found on the brush border and in the apical vesicles of the absorptive cells.  The labeling was most intense in the cells lining the upper crypts and luminal surfaces.  Mucin granules of goblet cells were also labeled.  Well-differentiated adenocarcinoma cells showed cytoplasmic and apical labeling sites similar to those of normal colonic epithelial cells and in addition labeling of the basolateral membranes.  The latter sites were more intensely labeled in poorly differentiated cancer cells whereas the intracytoplasmic reactivity was less intense compared to well-differentiated tumor cells.  One case of poorly differentiated adenocarcinoma showed no labeling.  These results indicate that the cellular site and intensity of CEA expression are closely associated with cellular differentiation in colonic cancer cells.  The findings are consistent with the hypothesis that the biosynthesis and processing of CEA are different in colonic cancer cells as compared to normal colonic epithelial cells.