Ingrowth and ramification of retinal fibers in the developing optic tectum of the chick embryo.

Onset, temporal sequence, and pattern of ingrowth of retinal fibers into the developing optic tectum of the chicken were investigated with histological procedures including the Golgi technique. Invading fibers could first be detected by stage 34 (eight days of incubation) at a specific locus which is the central area of the optic tectum. Compared to other tectal regions the central area is distinguished at this time by its advanced cytoarchitectural development and by the maturation of dendrites of radial cells located within superficial laminae. Immediately after their arrival at the central area some fibers can be observed invading the outer tectal layers and forming side branches. These observations permit the conclusion that fibers do not wait at their termination site for several days, as has been suggested earlier. Retinal axons start to invade the tectum at the site which is most advanced in its structural development. This early maturation of neurons in a specific tectal region might be a sufficient explanation for the central retinal fibers connecting to neurons of this area, which, propter hoc, is called the central tectal area.