The Nature of the Nerve Fibre Guidance Mechanism Responsible for the Formation of an Orderly Central Visual Projection
Theories about the formation of patterned nerve connections have generally required a detailed, highly specific recognition between individual nerve fibres and their targets, or other fibres, based on unique matching cytochemical cues. A re-examination of the published data, together with new data from experiments on the retinotectal system of lower vertebrates, suggests that a considerable amount of the data can be accounted for by more simple mechanisms such as the maintenance of nearest neighbour relationships between growing fibres through the non-specific process of fasciculation and the contact guidance of fibres to their termination sites. These simple morphogenetic mechanisms are insufficient to explain the fibre selectivity in tissue exchanges between different tectal sites, or the reorientation of the optic projection after rotation of the fibre array or its target tissue. High degrees of retinal and tectal differences need not be invoked to account for the results of graft exchanges since the selectivity of the fibres is of a low order. Non-specific factors such as fibre-fibre competition, varying densities of fibre subpopulations and variable accessibility of different fibres to different tectal regions could account for the findings. The data presented suggest that fibres may respond to the same polarity cue throughout the tectum but that on occasions highly ordered projections are formed without reference to these cues. The mechanisms responsible for polarity control remain speculative but might involve generalized properties, such as tissue maturity, which could act cumulatively along the length of the optic pathway to orient the fibres, and not solely in their target tissue.
KeywordsOptic Nerve Optic Tectum Polarity Control Optic Projection Contact Guidance
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