Generation of Projections in the Developing and Regenerating Nervous System
The development of the retino-tectal projection in birds, amphibia and fish is an example of the generation of spatially ordered connections between neurons in one area and target cells in another area of the nervous system. Various mechanisms, including pre-existing order of fibers in the nerve, order in time of arrival of fibers in the target area, and fiber-fiber interactions may contribute to the spatial order of connections. This does not suffice, however, since partial rotation of target tissue causes partially rotated maps [l] and partial destruction of early retinal rudiments lead to non-innervated parts of the tectum even if these are overgrown by fibers connecting elsewhere . It rather appears that main determinants for projections are spatially distributed chemical components in the tissue of origin and in target tissue. We propose a model for the formation of such projections in which components of growing axons which are spatially graded with respect to position of the neuron of origin, and graded components in the target tissue, interact and cooperate in generating a guiding parameter p; in the simplest case, p is the concentration of a substance which guides the fiber in the direction of maximal slope until a minimum of p is reached. Relatively simple kinetics (involving production of p and its inhibition) suffice for reliable projections. Effects of regulation depending on fiber density can be superimposed on the basic mechanism for projection. Expansion and compression of the map occurring after ablations of parts of the retina or the tectum can be simply explained on this basis.
KeywordsMold Retina Kelly
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