Cortico-cerebral development in the gray short-tailed opossum Monodelphis domestica

The marsupial South-American short-tailed opossum, Monodelphis domestica, is an appealing animal model for developmental studies on cortico-cerebral development, since the opossum cortex mainly develops after birth and newborns are particularly suitable for early ex-utero micro-surgical manipulations of this structure and the entire CNS. Opossum have been also largely employed as an ideal substrate for regenerative studies, since the pup is able to regenerate connections between neurons of the cerebral cortex and spinal cord upon experimental trauma. Moreover, branching from the common mammal ancestor about 180 My ago, Marsupials might provide a valuable tool for tracing evolutionary origins of key traits peculiar to the eutherian central nervous system (CNS). Until recently, the cortico-cerebral marsupial development has been prevalently investigated by methods of classical histology, but several features of Monodelphis corticogenesis were still unknown. By taking advantage of molecular tools set up for developmental studies in Placentals and availability of Monodelphis domestica genomic sequence, we tried to fill gaps in our knowledge of opossum corticogenesis, studying in particular: origin of cortical neurons, their laminar differentiation and their migration profiles, from their birthplaces to their final layer positions. We found many similarities between marsupial and placental corticogenesis, as for neuron generation, their laminar diversification and “inside-out” migration. This allowed us to establish a comparative time-table of mouse and opossum corticogenesis. One major difference emerged from our study. In the opossum, projection neurons are mainly born from apical progenitors and a basal proliferative compartment is hardly detectable.