Figure 7.

Proposed model of GCP differentiation in the EGL. The current model of GCP morphological transitions postulates that mitotically active, polyhedral or round cells, with a possible basal attachment to the pial surface proliferate in the outer layer of the EGL. Only after the cell becomes postmitotic, it extends small horizontal processes and begins tangential migration. Those processes continuously extend medio-laterally until the cell makes a switch to radial migration, at which point the horizontal processes are considered nascent parallel fibres. We suggest that this model is incomplete. The proposed model retains and confirms the morphological features of GCs in the EGL, but suggest that cells previously considered postmitotic, can in fact be proliferative. Divisions of GCPs occur in all layers of the EGL and cells with long horizontal processes are able to undergo mitosis by retracting their processes and rounding up before division. In the proposed model, white shading represents a highly proliferative precursor, and black shading denotes a postmitotic cell. As the cell migrates through the EGL and is exposed to diverse signalling, its proliferative potential declines. Consequently, the number of times the cell divides decreases as the cell transitions through EGL layers, with many mitoses in the outer EGL and very occasional mitoses in the inner EGL.