DEVELOPMENT OF THALAMOCORTICAL PROJECTIONS IN THE SOUTH-AMERICAN GRAYSHORT-TAILED OPOSSUM (MONODELPHIS-DOMESTICA)

We determined the time-course and general pattern of thalamocortical d evelopment of Monodelphis domestica by tracing projections with carboc yanine dye in fixed postnatal brains between postnatal day 2 (P2) and P30. By P2, the first neurons have migrated to form the preplate of th e lateral cortex and have sent out axons into the intermediate zone. B y P3, fibers from the preplate of more dorsal cortex have entered the intermediate zone, and, by P5, they reach the primitive internal capsu le. Crystal placements in the dorsal thalamus at P2-P3 reveal thalamic axons extending down through the diencephalon and growing out through the internal capsule among groups of back-labelled cells that already project into the thalamus. Thalamic axons arrive at the cortex after the arrival of cells of the true cortical plate has split the preplate into marginal zone and subplate. Axons fi om the ventral part of the dorsal thalamus reach the lateral cortex by P5: Dorsal thalamic fibers arrive at the extreme dorsal cortex by P9. The deeper layers of the c ortex appear to mature relatively earlier in Monodelphis than in euthe rian mammals, and the subplate becomes less distinct. Thalamic fibers and their side branches proceed into the cortex without an obvious per iod of waiting in the subplate, but they do not penetrate the dense co rtical plate itself. Monodelphis could provide an excellent model spec ies, because the development of its thalamocortical connections is ent irely an extrauterine process: The period P0-P15 corresponds to that o f E12-P0 in the rat. J. Comp. Neurol. 398:491-514, 1998. (C) 1998 Wile y-Liss, Inc.