Mechanism of Cell Shape Determination in Teleost Retinal Cones

Abstract

Cell shape change plays a fundamental role in morphogenesis and cell movement. Other papers in this symposium have described several examples in which cell shape change contributes to eye development. In this review, I wish to call attention to special aspects of cell shape change pertinent to photoreceptors. Unlike the other examples in this symposium, photoreceptor morphogenesis entails much more complex shaping than the elongation and apical constriction of columnar epithelial cells. During morphogenesis a part of the photoreceptor cell extends into the subretinal space and somehow sculpts itself into the complex and specific shapes of inner and outer segments. Similarly, the unique shapes of the rod and cone axons and synapses develop within the outer nuclear layer. Thus we must look to mechanisms of shape determination more local and more specific than the growth pressures and packing considerations which influence epithelial morphogenesis. Though we do not yet fully understand how such detailed, specific shaping is achieved, we are now beginning to comprehend the basic mechanisms which cells employ to change their shapes. Active cell shape change may be viewed as the result of two basic cellular processes: contraction and elongation. By the subtle combination of these two processes, even intricate cell shapes can be achieved. In the following discussion, I will consider possible mechanisms of contraction and elongation separately. They may not, however, be mutually exclusive.