Modularity in vertebrate brain development and evolution

Embryonic modularity and functional modularity are two principles of brain organization. Embryonic modules are histogenetic fields that are specified by position-dependent expression of patterning genes. Within each embryonic module, secondary and higher-level pattern formation takes places during d evelopment, finally giving rise to brain nuclei and cortical layers. Define d subsets of these structures become connected by fiber tracts to form the information-processing neural circuits, which represent the functional modu les of the brain. We review evidence that a group of cell adhesion molecule s, the cadherins, provides an adhesive code for both types of modularity, b ased on a preferentially homotypic binding mechanism. Embryonic modularity is transformed into functional modularity, in part by translating early-gen erated positional information into an array of adhesive cues, which regulat e the binding of functional neural structures distributed across the embryo nic modules. Brain modularity may provide a basis for adaptability in evolu tion. (C) 2001 John Wiley & Sons, Inc.