Histogenesis of ferret somatosensory cortex.

The ferret has emerged as an important animal model for the study of neocortical development. Although detailed studies of the birthdates of neurons populating the ferret visual cortex are available, the birthdates of neurons that reside in somatosensory cortex have not been determined. The current study used bromodeoxyuridine to establish when neurons inhabiting the somatosensory cortex are generated in the ferret; some animals also received injections of [3H]thymidine. In contrast to reports of neurogenesis in ferret visual cortex, most neurons populating the somatosensory cortex have been generated by birth. Although components of all somatosensory cortical layers have been produced at postnatal day 0, the layers are not distinctly formed but develop over a period of several weeks. A small number of neurons continue to be produced for a few days postnatally. The majority of cells belonging to a given layer are born over a period of approximately 3 days, although the subplate and last (layer 2) generated layer take somewhat longer. Although neurogenesis of the neocortex begins along a similar time line for visual and somatosensory cortex, the neurons populating the visual cortex lag substantially during the generation of layer 4, which takes more than 1 week for ferret visual cortex. Layer formation in ferret somatosensory cortex follows many established principles of cortical neurogenesis, such as the well-known inside-out development of cortical layers and the rostro-to-caudal progression of cell birth. In comparison with the development of ferret visual cortex, however, the generation of the somatosensory cortex occurs remarkably early and may reflect distinct differences in mechanisms of development between the two sensory areas.