Brain segmentation and forebrain development in amniotes

This essay contains a general introduction to the segmental paradigm postul ated for interpreting morphologically cellular and molecular data on the de veloping forebrain of vertebrates. The introduction examines the nature of the problem, indicating the role of topological analysis in conjunction wit h analysis of various developmental cell processes in the developing brain. Another section explains how morphological analysis in essence depends on assumptions (paradigms), which should be reasonable and well founded in oth er research, but must remain tentative until time reveals their necessary s tatus as facts for evolving theories (or leads to their substitution by alt ernative assumptions). The chosen paradigm affects many aspects of the anal ysis, including the sectioning planes one wants to use and the meaning of w hat one sees in brain sections. Dorsoventral patterning is presented as the fundament for defining what is longitudinal, whereas less well-understood anteroposterior patterning results from transversal regionalization. The co ncept of neural segmentation is covered, first historically, and then step by step, explaining the prosomeric model in basic detail, stopping at the d iencephalon, the extratelencephalic secondary prosencephalon, and the telen cephalon. A new pallial model for telencephalic development and evolution i s presented as well, updating the proposed homologies between the sauropsid ian and mammalian telencephalon. (C) 2001 Elsevier Science Inc.