Distribution, laminar location, and morphology of tectal neurons projecting to the isthmo-optic nucleus and the nucleus isthmi, pars parvocellularis in the pigeon (Columba livia) and chick (Gallus domesticus): a retrograde labelling study.

Retrograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L), fluorogold, fast blue, rhodamine labelled microspheres, and horseradish peroxidase (HRP) was employed to study the distribution, laminar location within the optic tectum, and morphology of tectal cells projecting upon the isthmo-optic nucleus (ION) and the nucleus isthmi, pars parvocellularis (Ipc), in the pigeon and chick. Following injections into the ION, all retrograde markers labelled tecto-ION neurons and their dendrites in the ipsilateral tectum. The cells were located within a relatively narrow band at the border between layers 9 and 10 of the stratum griseum et fibrosum superficiale (SGFS). Retrogradely labelled neuronal somata were different in both dendritic branching and shape; however, tecto-ION neurons generally possessed non-spiny radially oriented and multi-branched dendrites. The apical processes extended into the retino-recipient layers (2-7) of the SGFS and basal dendrites extended into layers 12-14 of the SGFS. Positive neuronal somata were observed throughout the rostro-caudal extent of the optic tectum. The average distance between adjacent tecto-ION neurons varied from one region to another. Specifically, retrogradely labelled cells were more numerous in the caudal, lateral, and ventral tectum, and less numerous at rostro-dorsal levels. Approximately 12,000 tecto-ION neurons were labelled within the ipsilateral optic tectum following either PHA-L or fluorescent dye injections. While the regional distribution of tecto-Ipc neurons was not examined, the morphology indicated that the cells had a single radially oriented dendritic process. Therefore, the apical dendrites are more restricted than those of tecto-ION cells. Moreover, the dendrites were spiny and arborized within layers 3, 5, and 9 of the ipsilateral optic tectum. The axon of tecto-Ipc cells arise from the apical process as a shepherd's crook and descend into the deep layers of the optic tectum. These results indicate that 1) tecto-ION and tecto-Ipc neurons are possibly monosynaptically activated by retinal input, 2) tecto-ION neurons are heterogeneous in morphology, and 3) there is a differential distribution of the tecto-ION neurons throughout the rostro-caudal extent of the optic tectum, suggesting a greater representation of the caudo-ventral portion of the optic tectum within the ION. The discussion primarily concerns the organization of the retino-tecto-ION-retinal circuit in light of the distribution and morphology of tecto-ION neurons within the optic tectum.