Definition and connections of the entopallium in the zebra finch (Taeniopygia guttata).

In birds the entopallium (formerly known as the core region of ectostriatum) is the major thalamorecipient zone, within the telencephalon, of the tectofugal visual system. Here we sought to redefine the entopallium in the zebra finch, particularly with respect to a laterally adjacent zone, known as the perientopallium (formerly known as the periectostriatal belt), and to determine its projections. We show that the entopallium can be defined by the almost complete overlap of dense terminations of thalamic rotundal afferents and intense cytochrome oxidase activity and parvalbumin immunoreactivity. The perientopallium, on the other hand, can be defined by relatively sparse projections from nucleus rotundus, a calretinin-positive plexus of nerve fibers, and weak cytochrome oxidase activity and parvalbumin immunoreactivity. Within the entopallium, medial and lateral parts can be distinguished on the basis of cell packing density, differential patterns of parvalbumin immunoreactivity and cytochrome oxidase activity, and different projections. We show that the entopallium projects laterally and diffusely to the perientopallium and nidopallium (formerly the neostriatum) and specifically and densely to a teardrop-shaped nucleus in the ventrolateral mesopallium (formerly known as the hyperstriatum ventrale), here called MVL (abbreviation used as a proper name). This latter projection arises predominantly from medial parts of the entopallium, which also receives a reciprocal projection from MVL, and projects to the lateral striatum. These findings suggest that the entopallium can be divided into medial and lateral parts having different functions, one of which is to provide for an extratelencephalic outflow from the medial part, via the lateral striatum. The findings also challenge the idea that informational flow through the various stations of the telencephalic tectofugal visual system is largely sequential and, together with findings in the chicken (Alpar and Tömböl), suggest instead that further substantial projections to telencephalic visual areas in birds can arise independently from both E (entopallium) and Ep (perientopallial belt). Copyright 2003 Wiley-Liss, Inc.