The bladders of rats maintained on a vitamin A-deficient diet develop patches of cornified epithelium that are histologically comparable to epidermis, and also areas in which there is a stratum granulosum but no stratum corneum. In the electron microscope, a marked similarity was observed between the fine structure of cornified bladder epithelium and that of skin. Prickle cells, with a marked fibril/desmosome relationship were found, and the dense keratohyalin-like areas in the stratum granulosum were associated with fibrils and/or ribosomes. There is known to be a close association between ribosomes and keratohyalin in both skin and oral mucous membrane, and a similar association was observed here in the cornified areas of bladder epithelium. In the noncornified areas, no organized cytoplasmic fibrils, prickle cells, or stratum corneum was found, but the stratum granulosum contained dense keratohyalin-like material. This material was not associated with fibrils and was apparently derived from aggregates of morphologically altered ribosomes. It is suggested that the formation of fibrils and the formation of keratohyalin may be mutually independent but normally coincident processes in keratinizing squamous epithelia. It is also suggested that in these vitamin A-deficient animals, the fibrils in the cornified bladder develop from the cytoplasmic filaments seen in normal transitional epithelial cells, and that keratohyalin is formed by enzymatic degradation of aggregated ribosomes. These results are related to an earlier suggestion that molecular keratin is probably synthesized in normal transitional epithelium, even though it is not usually cornified. © 1968 Academic Press Inc.
