Forebrain Development: Prosomere Model

The prosomeric model is a segmental structural model of the brain of vertebrates that explicitly holds that the brain is formed by an uninterrupted series of transverse subunits of the neural tube, generally called neuromeres. Among such subunits is a large rostral forebrain unit – the secondary prosencephalon – that encompasses hypothalamus, eyes, and telencephalon, followed by three caudal forebrain or diencephalic neuromeres (i.e., prosomeres), which are regarded as being serially homologous with more-caudal neuromeres, namely, a single midbrain mesomere, 11 hindbrain rhombomeres, and the spinal myelomeres. It is important to note that for descriptive purposes, the model postulates in all vertebrates a morphogenetic bending of the longitudinal axis of the tubular neural primordium, most marked at the cephalic flexure, whose incurvation causes wedge-shaped deformation of the topologically transverse cylindrical neuromeric sectors of the neural tube. These units share a set of fundamental longitudinal zones (due to common dorsoventral (DV) patterning processes) and therefore represent segments, that is, metameric developmental units (anteroposterior (AP) patterning). The common causal background of the longitudinal zones establishes the property of metamery (i.e., serial homology) across all neuromeres, irrespective of their differential molecular identities and individual prospective adult fates and of the variable border properties of the cells found at the interneuromeric boundaries. Therefore, the prosomeric model visualizes all vertebrate brains as segmented structures constructed along the same Bauplan (same set of DVand AP developmental units). Orthogonal intersection of DV and AP boundaries in the neural tube wall defines a checkerboard pattern of domains (histogenetic areas) inwhich specific properties and finer regionalization phenomena appear (shared or not among vertebrates). This makes the model useful for systematic descriptive neuroembryology, comparative neuroanatomy, and causal analysis of conserved or variant brain morphogenesis.