Specificity of neural connections in the retinotectal system.

The existence of a retinotopic map raises the question of how such a map is formed during development. Since little is known about molecular mechanisms which may be involved in the formation of retinotopy, the structural aspects of this developmental process were studied to a greater detail in the chick. In the retina the following patterns of growth can be observed: (1) Ganglion cell formation starts in the centre and spreads towards the periphery. Thus, each cell is labelled by both its position and its time of origin. (2) Axons tend to grow towards the optic fissure by the shortest possible route, and more peripheral axons whose perikarya are born later than that of central ones tend to lie next to the inner limiting membrane. A regular change in the pattern of fibres, which we call transformation, occurs at the optic fissure. The incremental rings are transformed into crescents lying ventrally in the optic stalk at early stages of development. In the middle of the optic nerve retinotopy is re-established. Another transformation occurs shortly behind the chiasm. Except for these transformations, fibres run parallel to each other even across the chiasm. Thus, fibres arriving at the optic tectum are well organized. It is concluded that a multiplicity of factors contribute to the formation of the retinotectal map at any site of the visual pathway. The analysis of these factors may indicate how topographically organized projections develop.