Adult structure and development of the human fronto-opercular cerebral cortex (Broca's region)

Broca's area encompasses opercular and triangular part of the inferior frontal gyrus, covered by Brodmann's areas 44 and 45, respectively. Recent neuroimaging studies have revealed that, in addition to classical language functions, Broca's area has novel and unexpected functions, serving as a likely interface of action and perception important for both verbal and nonverbal communication. In this review, we focus on structural features of Broca's area in adult and developing human brain. We emphasize that: (a) in terms of architectonic classification, Broca's area displays a dual nature by virtue of its intermediate position between agranular motor-premotor cortex and granular prefrontal associative cortex; and (b) numerous studies of morphological asymmetries are of limited value for understanding neurobiological basis of functions implemented by Broca's area. Finally, we summarize findings from our ongoing study on postnatal development of two defining cytoarchitectonic features of Broca's area in the adult brain, magnopyramidality (meaning that pyramidal neurons in layer III are larger than those in layer V) and dysgranularity (meaning that cortical layer IV is present, but poorly developed). We conclude that areal specification of areas 44 and 45 requires at least 2 (and probably 3) years of postnatal life for its gradual completion.