The specification of the retino-tectal projection in the chick.

Abstract

AbstractThe specificity of the connections between the retina and the optic tectum has been studied in the chick by ablating between 15% and 75% of the optic cup (usually during the third day of incubation) and subsequently determining the distribution, within the tectum, of the synapses formed by the axons of the surviving ganglion cells. This was done towards the end of the incubation period, or shortly after hatching, either autoradiographically following the injection of a tritiated amino acid into the eye, or using a variant of the Nauta‐Gygax method after sectioning the optic nerve. In every case in which the initial retinal lesion was placed after Stage 12/13 (i.e., late on the second day of incubation) the surviving ganglion cells could be shown to have formed synapses in only a limited region of the contralateral optic tectum; and as far as could be determined from an examination of the cell loss in the isthmo‐optic nucleus (in which the centrifugal fibers to the retina have their origin) the remaining portion of the neural retina consistently projected only to the homotopic region of the tectum (i.e., the region to which it would normally have been expected to project). In several cases it was found that the axons had passed over a heterotopic region of the tectum in order to reach a more distant region in which they had formed synapses. After a lesion of the optic vesicle before Stage 11 the surviving ganglion cells appeared to innervate all parts of the tectum; since the earliest retinal ganglion cells are formed at Stage 11/12 it would seem that in the chick, as in Xenopus, there is a clear temporal co‐incidence between the withdrawal of the first ganglion cells from the cell‐cycle and the establishment of the topographic specification of the retino‐tectal projection.Preliminary studies of the projection of the retina upon the optic tectum, and of the tectum upon the isthmo‐optic nucleus, in four to five week old chicks, indicates that the organization of these connections (and hence, presumably, that of the centrifugal fibers from the isthmo‐optic nucleus to the retina) is essentially the same as that in the pigeon. However, there is a significant difference in the overall morphology of the isthmo‐optic nucleus in the two species: as can best be seen in parasagittal sections, the isthmo‐optic nucleus of the chick lacks a well‐developed “anterior limb,” so that the representation of the horizontal meridian lies close to the junction of the “body” of the nucleus and its “posterior limb”.