A fundamental feature of developing tissues and organs is generation of planar polarity of cells in an epithelium with respect to the body axis.

The Drosophila compound eye shows two-tier dorsoventral (DV) planar polarity. At the individual ommatidium level, the eight photoreceptors in each unit eye form a dorsoventrally asymmetric cluster. At the level of eye field, hundreds of ommatidia in the upper and lower halves of an eye are uniformly polarized dorsally or ventrally, respectively. This results in DV mirror symmetries about the equator. The uniform orientations of photoreceptor clusters over long distance in the eye field provide an excellent model for studying the genetic basis of long-range planar polarity.

Ommatidial DV polarity can be detected in third instar eye imaginal disc during the early stage of retinal differentiation. The foundation for this DV polarity pattern is laid much earlier in undifferentiated eye disc. The eye disc primordium is partitioned into the DV compartments of independent cell lineages. The chapter outlines key genetic events involved in early DV patterning and growth of eye disc, and its potential role in organizing long-range signaling for DV planar polarity during later differentiation of the eye.