Analysis of cerebrocortical neuronal migration in three-dimensional fetal mouse cerebral explants: comparison with in...

Organotypic cultures of cerebral mantle offer a useful method for in vitro investigation of mechanisms involved in the migration of neuronal precursors to the neocortex. Studies reported to-date have indicated that, while neurons continue to migrate in cerebral explant cultures for a few days, they eventually stop migrating and fail to reach their pre-programmed destinations and thus distinct layers fail to develop within the cortical plate. Hypothesizing that this migrational arrest stemmed, in part, from severe distortion and flattening of explants in the roller-tube culture method used in previous studies by others, we studied neocortical neuronal migration using a recently developed three-dimensional explant culture system. Explants 300-350 microm thick were derived from E16 fetal mouse cerebrum 24 h following administration of 5-bromodeoxyuridine (BrdU), encased in collagen, placed on porous membranes, and alternately exposed to medium and air on a rocking platform. Cultures were maintained for 7, 11 and 14 days, then analyzed for the location of BrdU-labeled cells. Newborn mice labeled at E15 and analyzed at P1 and P9 served as controls. In the explants, labeled cells continued to migrate into the cortical plate until 11 days in culture, at which point the ventricular zone had disappeared and two distinct cortical cell layers sometimes developed. BrdU-labeled cells were present throughout the cortical plate, particularly the mid portion. In P9 controls, five distinct cortical layers were present with labeled cells mostly in the outer half (layers II, III). Thus, preservation of three-dimensional tissue integrity in culture was accompanied by a modest improvement in migration of neurons to the neocortex in comparison with previous reports. The fact that normal layer formation failed to develop in thick explants suggests that factors other than tissue distortion contribute to this outcome.