The multiple factors determining retinotopic order in the growth of optic fibres into the optic tectum.

Evidence is presented to support the conclusion that normally functioning optic nerve fibre terminal arborizations are open to continuous modification of their location and that they are capable of large scale gradual movement across the optic tectum in lower vertebrates. The termination of optic fibres at precisely defined tectal locations during normal embryonic development does not appear, in view of this and other evidence, to be due to any restrictions imposed by specializations distinguishing terminal sites themselves. However, there is clear evidence that, on the basis of possibly very simple specializations acquired as part of their embryological origin at particular locations in the retina, growing optic fibres actively and continuously select specific routes to be followed through intervening nervous tissue which eventually lead them to predictable and at least approximately appropriate terminal regions in the tectum. It is proposed that terminals move into and maintain fully retinotopic order as a result of direct interactions between fibres themselves based on features correlated with the retinal proximity of their cells of origin. This may involve further use of specializations due to related embryological origin: correlations in nerve impulse activity among neighbouring retinal ganglion cells may serve to stabilize most favourable terminal combinations. It is argued that fibres are subject to multiple influences which contribute to their orderly growth and that the demands made on the embryological differentiation of nervous tissue can thereby be considerably reduced.