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

The loss of MITF function in the MITF^mi/mi mouse affects not only RPE differentiation, but also the development of rod photoreceptor outer segments. Our data indicate that opsin immunoreactivity is detected in the cell membrane and along the ONL/RPE border of developing MITF^mi/mi rod photoreceptors and that rod outer segment morphogenesis is initiated. Although molecules associated with the outer segment continued to be expressed, outer segments did not elongate and develop stacked organized discs perpendicular to the RPE. The MITF^mi/mi RPE also failed to form apical microvilli and lacked the apical network of the phosphoprotein ezrin seen in wild type tissue. The MITF^mi/mi RPE basal surface was loosely organized and retained ezrin labelling which indicated some degree of differentiation. The correlation seen in our data suggest that there may be a link between the failure of the RPE apical domain to form and lack of rod outer segment elongation.