A scanning and transmission electron microscopic study of the embryonic mouse telencephalon.

The fibre systems of the developing mouse forebrain were examined by a combination of scanning and transmission electron microscopy. No clear pattern of ependymoglial fibre distribution emerged, due to the numerous intersecting fibre pathways. Scanning electron microscopy did show the presence of numerous fine fibres, particularly in regions rich in ependymoglial processes, such as the caudopallial angle; but definite identification of processes as ependymoglial or neuronal was not possible. Transmission electron microscopy confirmed the presence of numerous intersecting fibre bundles, particularly in the intermediate layer. In the neostriatum, scattered fibre bundles of the internal capsule frequently contained ependymoglial fibres which may act as a skeleton for guiding developing axons to their appropriate destinations. The difference in depth of the ventricular layer at different parts of the ventricle was clearly shown in the scanning electron microscope and, at the stages examined, a subventricular layer was apparent in the region of the ventricular elevations.