Modularity in vertebrate brain development and evolution

Abstract

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 development, finally giving rise to brain nuclei and cortical layers. Defined subsets of these structures become connected by fiber tracts to form the information‐processing neural circuits, which represent the functional modules of the brain. We review evidence that a group of cell adhesion molecules, the cadherins, provides an adhesive code for both types of modularity, based on a preferentially homotypic binding mechanism. Embryonic modularity is transformed into functional modularity, in part by translating early‐generated positional information into an array of adhesive cues, which regulate the binding of functional neural structures distributed across the embryonic modules. Brain modularity may provide a basis for adaptability in evolution. BioEssays 23:1100–1111, 2001. © 2001 John Wiley & Sons, Inc.