Neuronal Recognition in the Retinotectal System
Regularity of association between cells is ubiquitous; it is the sine qua non of any organized system, but the nervous system shows a greater range and variety of regular and orderly cellular relationships than any other system. For example, glial cells and neurons are always found in association, and there are very many examples of exclusive association between neurons of specific types. If the same neurons are always found together, either lying in close proximity, forming nonsynaptic intercellular junctions, or making synaptic contact, the embryologist asks how such intercellular contacts developed. Is the intercellular relationship, regular as it may be, merely the result of a web of circumstances that cannot be traced directly to any single cell, or is the orderly relationship the direct result of the properties and activities of certain, identifiable cells? The question has some pragmatic interest—it is likely to be far more difficult to discover the mechanisms of cellular association if they are distributed through the developing system in time as well as space than if the mechanisms are intrinsic properties and functions of the associating cells. Such intrinsic mechanisms might therefore be expressed by cells that are experimentally isolated from the entire system in vitro or in cells that by transplantation are put into novel spatial and/or temporal contexts in the developing nervous system.
KeywordsRetinal Ganglion Cell Positional Information Optic Tectum Optic Nerve Fiber Retinal Axon
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