New and old thoughts on the segmental organization of the forebrain in lampreys.

Ten years ago, we published the first detailed prosomeric map of the forebrain in lampreys. In the interim, the prosomeric model has been modified and simplified to better explain numerous data on the expression patterns of regulatory genes, as well as data from chemical, hodological and neuroembryological experiments, mostly in amniote vertebrates. In this report we first review the main historical concepts of lamprey forebrain organization, relating them to either columnar- or segmental-influenced models and explicit or implicit axial references. Next, our previous prosomeric model of the lamprey forebrain is updated, postulating some new hypotheses on the organization of the secondary prosencephalon.