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Abstract:
Connectivity in the nervous system depends on the subdivision of developing neural tissue into physico-chemically and functionally different parts, and the establishment of spatially specific connections between them. Presumably, self-organization of tissues requires positional information encoded in spatial concentration patterns. Short range autocatalytic processes coupled to longer range inhibition effects, are capable of the de novo generation of spatial distributions, showing self-regulatory properties characteristic of developmental biology. Such mechanisms are suggested to determine pattern formation in the two dimensions of cell sheets, as it occurs in the developing nervous system of higher organisms, generating segments with sharp boundaries as well as graded distributions. On the other hand, the third dimension - the arrangement of layers in multilayered sheet - results from contact mediated cell to cell interactions, cell migration, and, possibly, the time course of differentiation of various cell types. With respect to neural connectivity, the generation of projections (such as the retino-tectal projection) poses a challenging problem because, logically, few positional markers (and, therefore, few genes) suffice to specify connections for a very large number of neurons, and nature probably makes use of this logical possibility. Its analysis, however, is mathematically more involved than simple lock-and-key theories of connections. According to transplantation studies, positional markers on the target are essential; directional cues operating across considerable distances indicate that graded distributions are involved. Theoretical analysis has shown that two antagonistic graded effects on the target area are required in each dimension to specify internal target positions in the tectum and that retinal positional markers must exert a modulating influence, possibly by influencing the relative contribution of the two antagonistic markers in target tissue. Mechanisms by which gradients guide axonal growth could but need not be confined to forces of adhesion: any directional cue, however slight, may be enhanced within the axonal growth cone (in analogy to chemotactic guidance of cells), leading to growth in a defined direction, ft is postulated that graded distributions are an essential cue in primary path rinding, though undoubtedly further effects (involving direct or indirect fiber-fiber interactions and, at later stages, functional sharpening) also contribute to projections and their regulation.
