Morphological properties of tectal neurons that project to the nucleus geniculatus lateralis, pars ventralis (GLv) and the surrounding ventral thalamus in chicks

Layer 10 neurons of the chick tectum were morphologically investigated. The layer 10 neurons displayed heterogeneous immunoreactivities to calcium-binding proteins (CaBPs). Calbindin (CB)-immunoreactive (ir) neurons had pyramidal or round somata, primarily found in layers 5, 9, and 13. Parvalbumin (PV)-ir neurons were of various shapes with small to large somata (109.7 ± 48.6 μm2) that were located mainly in layers 4 and 10. Calretinin (CR)-ir neurons had small to middle-sized somata (79.3 ± 9.7 μm2) located primarily in layers 10 and 13, and most of them were similar to typical radial cells in size and shape. Two distinct types of neurons that projected to the nucleus geniculatus lateralis, pars ventralis (GLv) and ventral thalamus were demonstrated in layer 10. Type 1 cells had small to middle-sized somata (74.3 ± 33 μm2), and each cell had a single apical dendrite that ramified into bush-like branches in layer 7. These cells corresponded to CR-ir neurons and radial cells in size and shape. Type 2 cells had larger somata (124.7 ± 52.6 μm2), and their shapes were pyramidal, polygonal, or oval. They had multiple obliquely ascending dendrites that ramified into bush-like branches in layer 7. These cells often appeared similar to PV-ir neurons.