How crocodiles lost a forebrain circuit.

Why do brain circuits differ among species of the same or different classes of vertebrates? Answers to this question are critical in determining brain organization and evolution. Investigations in mammals and in certain model systems have been the main focus of these studies. Among the areas that have commonly been investigated are cell death, synaptic pruning, and synapse organizing proteins. Whether these same or even other factors sculpt similar circuits in other vertebrates such as reptiles is unknown. The present study examined the development of a well defined circuit in crocodiles and compared it with a similar circuit in mammals. In mammals, cortical areas project directly back to the same dorsal thalamic nucleus that provides its input. However, in reptiles, pallial regions in receipt of dorsal thalamic input do not project directly back to dorsal thalamic nuclei. Instead, pallial areas in reptiles project to the basal nuclei, similar, in part, to the striatum of mammals. The basal nuclei then project back to the dorsal thalamus. This suggested the possibility that this circuit might have been lost in reptiles or perhaps added in mammals. To determine which of these two possibilities might be the case, the development of this circuit in Alligator embryos and juvenile crocodiles was investigated. The results indicate that cell death caused the loss of this circuit in crocodiles and is the likely explanation for this difference. However, a distinction between neuronal death and axonal re-modelling could not be determined.