The nucleus pretectalis principalis: A pretectal structure hidden in the mammalian thalamus.

A defining feature of the amniote tecto-fugal visual pathway is a massive bilateral projection to the thalamus originating from a distinct neuronal population, tectal ganglion cells (TGCs), of the optic tectum/superior colliculus (TeO/SC). In sauropsids, the thalamic target of the tecto-fugal pathway is the nucleus rotundus thalami (Rt). TGCs axons collateralize en route to Rt to target the nucleus pretectalis principalis (PT), which in turn gives rise to bilateral projection to the TeO. In rodents, the thalamic target of these TGCs afferents is the caudal division of the pulvinar complex (PulC). No pretectal structures in receipt of TGC collaterals have been described in this group. However, Baldwin et al. (Journal of Comparative Neurology, 2011;519(6):1071-1094) reported in the squirrel a feedback projection from the PulC to the SC. Pulvino-tectal (Pul-T) cells lie at the caudal pole of the PulC, intermingled with the axonal terminals of TGCs. Here, by performing a combination of neuronal tracing, immunohistochemistry, immunofluorescence, and in situ hybridization, we characterized the pattern of projections, neurochemical profile, and genoarchitecture of Pul-T cells in the diurnal Chilean rodent Octodon degus. We found that Pul-T neurons exhibit pretectal, but not thalamic, genoarchitectonical markers, as well as hodological and neurochemical properties that match specifically those of the avian nucleus PT. Thus, we propose that Pul-T cells constitute a pretectal cell population hidden within the dorsal thalamus of mammals. Our results solve the oddity entailed by the apparent existence of a noncanonic descending sensory thalamic projection and further stress the conservative character of the tectofugal pathway.

Anatomical organization of the visual dorsal ventricular ridge in the chick (Gallus gallus): Layers and columns in the avian pallium.

The dorsal ventricular ridge (DVR) is one of the main components of the sauropsid pallium. In birds, the DVR is formed by an inner region, the nidopallium, and a more dorsal region, the mesopallium. The nidopallium contains discrete areas that receive auditory, visual, and multisensory collothalamic projections. These nidopallial nuclei are known to sustain reciprocal, short-range projections with their overlying mesopallial areas. Recent findings on the anatomical organization of the auditory DVR have shown that these short-range projections have a columnar organization that closely resembles that of the mammalian neocortex. However, it is unclear whether this columnar organization generalizes to other areas within the DVR. Here we examine in detail the organization of the visual DVR, performing small, circumscribed deposits of neuronal tracers as well as intracellular fillings in brain slices. We show that the visual DVR is organized in three main laminae, the thalamorecipient nucleus entopallium; a dorsally adjacent nidopallial lamina, the intermediate nidopallium; and a contiguous portion of the ventral mesopallium, the mesopallium ventrale. As in the case of the auditory DVR, we found a highly topographically organized system of reciprocal interconnections among these layers, which was formed by dorsoventrally oriented, discrete columnar bundles of axons. We conclude that the columnar organization previously demonstrated in the auditory DVR is not a unique feature but a general characteristic of the avian sensory pallium. We discuss these results in the context of a comparison between sauropsid and mammalian pallial organization.