Immunocytochemical Localization of Cellular Retinol Binding Protein in the Rat Retina

Cellular retinol binding protein (CRBP) was localized in the rat retina by means of light and electron microscopic immunocytochemistry. The peroxidase-antiperoxidase (PAP) method employed at the light microscopic level showed that CRBP is sharply localized to the retinal pigment epithelium (RPE). None was detectable in the epithelium of the pars plana or pars plicata of the ciliary body. Likewise, the photoreceptors were negative. Within the neural retina, the PAP method revealed a bilaminar staining pattern in the inner plexiform layer and staining within elements near the vitreal surface. Indirect immunoferritin electron microscopy demonstrated that CRBP is distributed uniformly within the RPE cytosol from basal infoldings to apical processes. The nucleoplasm also was stained, albeit, more lightly than the cytoplasm. Labeled elements in the inner plexiform layer and vitreal surface were identified as Müller cell processes and end feet, respectively. Interpretation of the results includes a dual role for CRBP in the RPE, namely its involvement in gene expression and transcytoplasmic transport of retinol.