Early development of a vertebrate telencephalon: Observations before areal differentiation

The telencephalon is a fundamental part of the brain in all amniotes (reptiles, birds, mammals). Although many similarities are shared by amniotes, telencephalon development and organization differ significantly between mammals and sauropsids (reptiles and birds). Two major differences are the development and structure of the pallium and the presence of the dorsal ventricular ridge in sauropsids but not in mammals. How these differences have occurred remains incompletely understood. While multiple studies have described telencephalon development in mammals, similar reports in reptiles are few. To fill this gap in knowledge, the present analysis was undertaken. This report details the development of the telencephalon in Alligator mississippiensis using a variety of cell stains. This study, which is the first of its kind in any reptile, begins shortly after egg laying and ends when differentiation of major brain areas occurs. The development of the subpallium from a unitary structure into its subsequent divisions and the changes in the septum and pallium are described. This analysis provides a roadmap and timetable for early telencephalon development. As such, these observations will serve as the basis for future studies to compare, for example, the expression of genes and transcription factors with other amniotes.