Embryonic development of connections in turtle pallium

We are interested in similarities and conserved mechanisms in early develop ment of the reptilian and mammalian thalamocortical connections. We set out to analyse connectivity in embryonic turtle brains (Pseudemys scripta eleg ans, between stages 17 and 25), by using carbocyanine dye tracing. From. th e earliest stages studied, labelling from dorsal and ventral thalamus revea led backlabelled cells among developing thalamic fibres within the lateral forebrain bundle and striatum, which had similar morphology to backlabelled internal capsule cells in embryonic rat (Molnar and Cordery, 1999). Howeve r thalamic crystal placements did not label cells in the dorsal ventricular ridge (DVR) at any stage examined. Crystal placements into both dorsal and lateral cortex labelled cells in the DVR and, reciprocally, DVR crystal pl acements labelled cells in the dorsal and lateral cortices. Retrograde labe lling revealed that;thalamic fibres arrive in the DVR and dorsal cortex by stage 19. The DVR received projections from the nucleus rotundus and the do rsal cortex exclusively from the perirotundal complex (including lateral ge niculate nucleus). Thalamic fibres show this remarkable degree of specifici ty from the earliest stage we could examine with selective retrograde label ling (stage 19), Our study demonstrates that axons of similar cells are amo ng the first to reach dorsal and ventral thalamus in mammals and reptiles. Our connectional analysis in turtle suggests that some cells of the mammali an primitive internal capsule are homologous to a cell group within the rep tilian lateral forebrain bundle and striatum and that diverse vertebrate br ains might use a highly conserved pattern of early thalamocortical developm ent. J. Comp. Neurol. 413:26-54, 1999. (C) 1999 Wiley-Liss, Inc.