Defects in the MITFmi/mi apical surface are associated with a failure of outer segment elongation

The loss of MITF function in the MITFmi/mi mouse affects not only RPE diffe rentiation, but also the development of rod photoreceptor outer segments. O ur data indicate that opsin immunoreactivity is detected in the cell membra ne and along the ONL/RPE border of developing MITFmi/mi rod photoreceptors and that rod outer segment morphogenesis is initiated. Although molecules a ssociated with the outer segment continued to be expressed, outer segments did not elongate and develop stacked organized discs perpendicular to the R PE. The MITFmi/mi RPE also failed to form apical microvilli and lacked the apical network of the phosphoprotein ezrin seen in wild type tissue. The MI TFmi/mi RPE basal surface was loosely organized and retained ezrin labellin g which indicated some degree of differentiation. The correlation seen in o ur data suggest that there may be a link between the failure of the RPE api cal domain to form and lack of rod outer segment elongation. (C) 2001 Acade mic Press.