Development of the dorsal thalamus in a reptile: identification of subdivisions and their associated nuclei.

How the dorsal thalamus of amniotes (reptiles, birds, and mammals) is organized remains an important but incompletely answered question. Identification of meaningful subdivisions would greatly aid in its understanding. Because the dorsal thalamus is more simply organized during development, studies have examined this structure during embryogenesis. Most reports using this approach have examined the developing dorsal thalamus in mammals and birds. Only rarely has the development of the dorsal thalamus been investigated in reptiles. Regardless, any approach to identify subdivisions, the presumed building blocks of the dorsal thalamus, should include representatives of all three classes of vertebrates. To fill this gap in knowledge, the development of the dorsal thalamus was investigated in Alligator mississippiensis, a member of the reptilian group most closely related to birds. As the first detailed study of its kind, cytoarchitecture and calretinin expression were used to examine dorsal thalamus development. Three subdivisions, termed tiers, and the individual nuclei originating from each tier, were identified. These three tiers were similar to the subdivisions found in birds and, to a limited extent, in mammals. Taken together, these early subdivisions may represent the common building blocks of the dorsal thalamus and provide clues to understand how evolution has sculpted this structure in amniotes.