Pattern formation is the process by which cells are spatially arranged into elaborate biological structures. While significant advances have been made in understanding how different cell types become determined, less is understood about how cells are configured into the characteristic shapes found in many organs. Signal transduction networks under genetic control are known to regulate cell shape and form through the cytoskeleton. On the other hand, sporadic descriptive analysis over the past century has suggested that cells can be configured by mechanisms following physical principles. The notion that cells organize themselves within structures that minimize their total surface area is a recurring one.

Within each Drosophila ommatidium, four cells form a circular plate that acts as both the floor of the simple lens and the roof of the underlying chamber of photoreceptors. These cells are called cone cells. Two pigment cells form the walls of the lens, shaping and optically insulating it. This pattern is evident near the apical surface of the retina. Morphogenesis of the pattern occurs over several days of juvenile life. The pigment cells enlarge and begin to surround the cone cells, displacing other cells from contact. Eventually they ensheath the cone cells and make contact with each other. Meanwhile, the cone cells expand and rearrange their contacts with each other. The net result is a transparent circular plate that functions as an aperture for focused light to transmit from lens to photoreceptors. The mechanism guiding this patterning is poorly understood.