Tectothalamic visual projections in turtles: their cells of origin revealed by tracing methods.

In two species of turtle (Emys orbicularis and Testudo horsfieldi), retrograde and anterograde tracer techniques were used to study projections from the optic tectum to the nucleus rotundus (Rot) and to the dorsal lateral geniculate nucleus (GLd). The ipsilateral Rot received the most massive tectal projections, stemming from numerous neurons located in the stratum griseum centrale (SGC). These neurons varied in size and shape, many of them having a wide zone of dendritic arborization within both the (SGC) and the stratum griseum et fibrosum superficiale (SGFS). Projections from the tectum to the GLd were ipsilateral, were extremely scarce, and arose from a small number of neurons of various shapes situated in the SGFS; these cells were, as a rule, smaller than those projecting to the Rot. For the most part, these neurons were radially oriented, with rather restricted dendritic arborizations in the most superficial sublayers of the SGFS; smaller numbers of projection neurons were horizontally oriented, with long dendrites branching throughout the layer. Some neurons located in the stratum griseum periventriculare (SGP) projected to both the Rot and the GLd. Most of these neurons had dendritic arborizations within the retinorecipient zone of the SGFS. We were unable to rule out the possibility that some cells projecting to the GLd were situated in the SGC. Both the GLd and the main body of the Rot did not contain neurons projecting to the optic tectum. Thalamic neurons projecting to the tectum were observed in the ventral lateral geniculate nucleus, the intergeniculate leaflet and the interstitial nuclei of the tectothalamic tract, and the nucleus of the decussatio supraoptica ventralis. The question of whether variation in the laminar organization of the tectorotundal and tectogeniculate projection neurons in reptiles, birds, and mammals may be related to different degrees of differentiation of the tectal layers is discussed.