How should brain nuclei be delineated? Consequences for developmental mechanisms and for correlations of area size, neuron numbers and functions of brain nuclei.

The measurement of the size of brain areas and their neuron numbers depends on the delineation of the boundaries of a brain area. The comparison of cytoarchitectural, cytochemical and connectional delineation of a brain area, such as the vocal control nucleus HVC (nucleus hyperstriatalis ventralis, pars caudalis) of songbirds, shows that the estimated size of a nucleus depends on the delineation criterion. In correlation, the cytoarchitectural, cytochemical and projection properties of the same brain area change independently both during development and in adulthood. This needs to be considered in cross-species, intersexual, developmental and temporal comparisons of brain areas. The combination of cytoarchitectural criteria with area-specific cytochemical and connectional markers to delineate the boundaries of a brain nucleus gives new insights into neural plasticity and parcelation of brain areas.