Relationships between glial and neuronal elements in the development of long term cultures of the spinal cord of the fetal mouse

The development of glial and synaptic relations has been studied in long term cultures of spinal cord by light and electron microscopy. During the first three days in vitro there is extensive cell death in the superficial parts of the explants, where one sees many phagocytes and much cellular debris. The ependyma of the explants early forms a closed sac in the deep parts of the culture next to the collagen substrate. Cells migrate from this sac to form a continuous basal layer upon the substrate. Some of these cells turn around the edges of the explant and migrate onto its surface, where they contribute to an ependyma‐like epithelial covering. As the explants mature most of the cellular debris is cleared; the epithelial covering separates the neuronal elements from the feeding solution and from phagocytes, most of which migrate to lie freely on the surface of the epithelium. Two types of outgrowth are formed. The first consists of bundles of nerve fibers which are accompanied by individual undifferentiated glial cells. The second forms as a broad sheet of undifferentiated glial cells and astrocytes and this is continuous, at the edge of the explant, with the glia of the basal layer. Nerve fibers that grow into this sheet appear to survive better than nerve fibers that grow out in bundles. Oligodendroglial cells become recognizable by electron microscopical criteria when the axons start to myelinate. Dendrites can be identified in the sheet‐like outgrowth but not in the bundles. Synapses are seen only where dendrites are identifiable. Serial axo‐axonal junctions have not been found and axosomatic synapses are relatively rare.