An unusual population of pyramidal neurons in the anterior cingulate cortex of hominids contains the calcium-binding protein calretinin

In the context of an on-going comparative analysis of primate neocortex evo lution, we describe the occurrence and distribution of a previously unrecog nized group of pyramidal neurons, restricted to the superficial part of lay er V in the anterior cingulate cortex of hominids and characterized by immu noreactivity to the calcium-binding protein, calretinin. These neurons were rare in orangutans, more numerous in gorillas and common chimpanzees, whil e humans had the highest numbers. These calretinin-containing pyramidal cel ls were not observed in the cingulate cortex of any other primate or mammal ian species. This finding, together with other recent observations on the h ominoid cingulate cortex, is interesting when considering primate neocortic al evolution, as it indicates possible adaptive and anatomical modification s in a cortical region critical for the integration of many aspects of auto nomic function, vocalization, and cognitive processes. (C) 2001 Published b y Elsevier Science Ireland Ltd.