The development of neuronal connection patterns in the visual systems of insects

The retina of the compound eye contains a patterned spatial array of receptor cell groups whose symmetry and component numerical constancy allow the unique identification of individual cells. The axon connection patterns arising from the retina similarly are uniquely identifiable and relate by simple consistnet transpositons to the retinal position of their somata. The coincidence of these features and the epidermal origin or retinula cells provides a model system for the developmental analysis of neural patterns. It is known that the retina develops by a wave of determinative mitoses and subsequent cellular differentiation, transmitted inductively from a posteriorly situated differentiation centre, which passes over the prospective eye field independently of the growing axon connections with the optic lobe. The ganglion cells of the optic lobe are similarly produced in a temporal sequence but their subsequent differentiation depends on centripetal innervation and proceeds as a wave initiated in the retina. Axon growth between the retina and optic neuropiles occurs in this temporal sequence, appears largely non-exploratory and dependent on mechanical guidance. By contrast, individual axon connections within the neuropile form probabilistically by extensive filopod explorations between cones. Both growth between and growth within the neuropile depend upon features in the pattern of retinal development which are discussed.