A developmental approach to forebrain organization in elasmobranchs: new perspectives on the regionalization of the...

It is essential to consider chondrichthyans (cartilaginous fishes) in analyzing ancestral brain organization because this radiation represents the out-group to all other living gnathostomes (jawed vertebrates). It is particularly crucial to understand the evolution of the telencephalon in chondrichthyans, as this structure develops by evagination (as in most other vertebrates), whereas in most osteichthyans (bony fishes), it develops by eversion, a markedly different process. Among chondrichthyans, the Lesser Spotted Dogfish Scyliorhinus canicula (Elasmobranchii) appears to offer the most potential as a model species for study. Developmental studies of Scyliorhinus have revealed a segmentary pattern in the developing forebrain, similar to that described in other vertebrates, as well as the occurrence of tangential cell migration within the telencephalon, especially in relation to the pallial-subpallial boundary. These observations indicate that major morphogenetic processes thought to be a hallmark of mammalian brains actually existed much earlier in vertebrate phylogeny. In addition, analysis of telencephalic development in Scyliorhinus indicates the existence of telencephalic structures that are probably related to the ganglionic eminences of mammals.