Structure and ultrastructure of the external glial layer in the hypothalamus of the hamster.

The hypothalamic subpial limitans has been studied by light and electron microscopy. It consists of a palisade of astrocytic cell bodies or their processes. Beneath the optic chiasma (white matter) the glial layer consists of flattened astrocytic cell bodies arranged in parallel to the myelinated fibres or located between the fibres, but sending their processes towards the basement membrane. Beneath the ventral hypothalamus nuclei, the astrocytic cell bodies can appear in two places: a) in contact with the basement membrane with their processes arranged in parallel to it, or b) located a certain distance from the basement membrane, sending long processes that, near the limitant zone, enlarge to form end-feet (supraoptic nuclei) or sending processes located in parallel to the basement membrane (mamillary nuclei). The astrocytic processes forming the glia limitans show gap junctions, and desmosomes. The different arrangement of the astrocytic processes and the different types of junctions suggest that the marginal glia might represent a regulatory mechanism for the diffusion between the different compartments of the brain.