Comparative analyses of the neuron numbers and volumes of the amygdaloid complex in old and new world primates

The goals of this dissertation are to identify if neuroanatomical characteristics of the amygdaloid complex (AC) - nuclei volumes, cell numbers, and cell densities - vary across non-human primate (NHP) species and to consider which AC regions might influence the development of social and emotional behavior. NHP comparisons at the level of individual amygdaloid nuclei have not been carried out in non-ape species. Chapter I summarizes what we currently know about the structure of the AC in NHP and reviews the functions of individual nuclei. Thirteen Old and New World primates, representing five species, were analyzed using standard stereological methods for nuclei volume and neuron number in 26 AC subdivisions. Chapter II explains the methods used to evaluated volumes and measure cell numbers. This paper focuses specifically on four AC nuclei (accessory basal, basal, central, and lateral). Chapter III presents my findings, which are three-fold: 1) the central nucleus volume is negatively allometric with the total amygdala volume while the accessory basal, basal, and lateral nuclei are isometric or changing at the same rate as the whole AC; 2) the central nucleus neuron numbers are increasing at a slower rate than the neurons in the three nuclei of the basolateral complex; 3) neuron density remains consistent across all nuclei evaluated. My data indicate that the central nucleus may have evolved differently from the three other amygdala nuclei I have evaluated. The isometric relationship between the neuron numbers and volume of the nuclei of the basolateral complex and the AC suggest that these nuclei evolved together. Chapter IV discusses some evolutionary interpretations of the data and speculates upon what these changes in AC structure could have on social behavior