The Development of Specificity of Retinal Central Connections: Changing Concepts

Abstract

The above-cited statement describes perhaps most elegantly the initial concept of how neurons may form synaptic connections. In the decades following Cajal’s observation, it was recognized that vertebrate neurons send their axons in the embryonic CNS to form highly specific point-to-point connections with specific types of target neurons at particular loci in the brain (for review, see Gaze, 1970; Jacobson, 1978; and Lund, 1978). For the present review, fundamental work apparently started with Sperry’s observation of the early 1940’s. Sperry (1943, 1944) found that when the optic nerve of lower vertebrates was cut or crushed, the optic fibers regenerated back to their original position. This conclusion was based upon behavioral testing of the animals following eye rotation and consideration of other surgical procedures. Later Sperry (1950) wrote “the optic fibers differ from one another in quality according to the particular locus of the retina in which the ganglion cells are located. The retina apparently undergoes a polarized field-like differentiation within the retina and the tectum during development, which brings about specification of the ganglion cells and the tectal cells.”

From the functional point of view the growth cone of the retinal ganglion cells may be regarded as a sort of club or battering ram, endowed with exquisite chemical sensitivity, with rapid amoeboid movements, and with a certain impulsive force, thanks to which it is able to press forward and overcome obstacles met in its way, forcing cellular interstices until it arrives at its destination.

Ramon y Cajal, 1899, pp. 544–545