Postembryonic development of the insect visual system.

The developing eye of the cockroach, Periplaneta americana, and the locust, Schistocerca gregaria, was chosen for an experimental investigation of factors involved in the development of nervous tissue and the formation of ordered nerve connections. Grafting experiments between eye colour mutants of P. americana showed that the retina develops postembryonically from a region of head epidermis which differs from other larval head epidermis in being competent to respond to a recruiting signal produced by adjacent eye tissue. Recruited cells divide and segregate into clusters which differentiate into ommatidia. The specific fate of individual cells is probably assigned according to their position within the cluster. Quantitative analysis of cell proliferation and death in the compound eye of S. gregaria under normal and experimental conditions showed that the retina develops normally in the absence of the optic lobe. In the absence of innervation from the retina, lamina ganglion cells are generated normally but do not differentiate and subsequently degenerate. Experiments on S. gregaria in which spatial or temporal relationships between retina and lamina were altered, showed that axons tend to grow along the paths of immediately adjacent axons, with no indication of specificity for their normal target sites. The following model was proposed for the development of the retina-lamina projection. Formation of the retina by recruitment of epidermis ensures that sensory axons develop in an orderly spatiotemporal sequence. Newly-formed axons grow along preceeding ones and, in turn, provide a conducting tract for suceeding axons, so that the orderly pattern of axon outgrowth is maintained in the pattern of axon arrival in the lamina. As both retina and lamina grow anteriorly, new axons arrive among new lamina cells. Lamina cells innervated by retinal axons differentiate into discrete synaptic units; excess, uninnervated cells degenerate. The relevance of the model to the initial formation of the projection, and to the formation of other nerve connections in insects, was discussed.