The topographic projections of the retina upon the optic tectum and ventral lateral geniculate nucleus (GLv) of the chick were investigated by making small intraretinal injections of 3H-proline. The retinotectal projection pattern was similar to that described for the pigeon. The retinal projection to the GLv was also topographic and was restricted to the outermost lamina of the nucleus. The anteroposterior retinal axis was reversed in the GLv relative to its orientation in the tectum but the superoinferior axis was oriented identically in both. Furthermore, the posterior retina had an enlarged area of projection in the GLv similar to the enlarged area of retinotectal projection for the "red field" found in pigeons. The tectogeniculate projection was topographic and was confined to the outermost geniculate lamina. The second-order retinotopic map made by the tectogeniculate projections was in register with the retinogeniculate projection. Although the retinal and tectal projection areas were coextensive in the outermost geniculate lamina, the grain density distributions peaked at different points along a radial path through the geniculate laminae. Injections of HRP into the optic tectum led to very light retrograde labeling of a small population of GLv cells topographically corresponding to the tectogeniculate projection zone of the injection site. The data suggest that the chick GLv is comparable to the GLv of other non-primate mammals.
The chick brain is a useful model system for studying the ontogeny and phylogeny of neural circuitry, especially that of the visual system. In this study the distribution of cells and processes showing GABA-like immunoreactivity (GABA+) in the diencephalon and mesencephalon of the posthatch chick was determined immunohistochemically with a polyclonal antibody to GABA and compared with the results of similar studies in mammals. Most of the small GABA+ cells were found in the chick visual centers such as the nucleus lateralis anterior, suprachiasmatic nucleus, ventral lateral geniculate, optic tract, dorsolateralis anterior pars lateralis, lentiformis mesencephali, ectomammillary nucleus, area pretectalis, and the optic tectum. Large GABA+ cells were found in the following nuclei: reticularis superior, posteroventralis thalami, subpretectalis, isthmi pars magnocellularis, interstitio-pretectosubpretectalis, mesencephalicus lateralis pars dorsalis. These large cell-containing nuclei receive projections from visual or auditory centers. GABA+ axons were found throughout the diencephalon and mesencephalon but were especially prominent in the ansa lenticularis, fasciculus medialis longitudinalis, and optic tract. The distribution of GABA+ cells in the chick is more widespread than in rodents and exhibits an increased association with the visual centers suggesting a correlation with the specialized visual requirements of the bird.
The 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."The past two decades have witnessed a significant revival of interest in the nervous systems of submammalian forms. To some extent this reflects the continuing concern of many anatomists and PhYsiologists in comparative neurology per se, and in the light which comparative studies may throw on the evolution of the verebrate tention that has been paid to Certain sys-J. COMP. NEUR., 155: 127-164. 127tems, particularly in the brains of fish, amphibians and birds, clearly stems from the realization that these may provide m...
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