Calcium-Binding Proteins and Cytochrome Oxidase Activity in the Pigeon Entopallium: A Comparative Analysis of Interspecies Variability as Related to the Discussion on Avian Entopallium Homology

We report the results of our studies of the distribution patterns of calcium-binding proteins (parvalbumin, PV, and calbindin, CB) and metabolic activity (cytochrome oxidase, CO) in the pigeon entopallium—the telencephalic projection field of the tectofugal visual system. These characteristics were comparatively analyzed in different avian species in the light of the recent revision of entopallial projections’ nomenclature (Krützfeldt and Wild, 2005). We demonstrate that in the pigeon neuropil both high PV immunoreactivity and CO activity as well as lower CB immunoreactivity are confined to the core region of the entopallium (E). The latter contains cells immunoreactive (ir) to PV and CB and having a heterogenous repertoire: small/medium-sized granular and large multipolar cells. They overlap in E and partly colocalize within the same cell, but differ in the internal (Ei) and external (Ex) portions by distribution density and labeling intensity. CO activity was identified in both cellular morphotypes. Sparse PV- and CB-ir cells were found in the perientopallium (Ep). The interspecies variability of PV and CB immunoreactivity, described in the avian entopallium by other authors, indicates its dependence on the adaptive functional specialization which underlies selective expression of these calcium-binding proteins. The above as well as the pertinent literature data are discussed in the wake of the current discussion on homology of the avian entopallium, supporting the idea of the existence in sauropsid amniotes of the ancestral precursor of the mammalian extrastriate visual cortex.