The ultrastructure of sheath cells in developing rat vomeronasal nerve.

Maturation of the vomeronasal nerve was studied in fetal, newborn and 3 months old rats. Early in development, each nerve consists of large numbers of bare axons with clusters of polygonal sheath cells lying at the periphery. The latter insinuate themselves between the axons which they segregate into bundles. The sheath cells and their processes which first delineate axon bundles from one another form a network in the interstices of which lie the emergent axon bundles. Each sheath cell is not confined to the sleeve around a single bundle. Its perikaryon and processes commonly contribute to the septa between several adjacent bundles. Eventually, each bundle comes to be surrounded by its own proper sheath which consists of processes of more than one Schwann cell. These developmental trends, of a progressive increase in the number both of axons per bundle and of Schwann cells associated with each bundle, are the reverse of those found in the PNS generally, where bundle size decreases and axon size increases with maturation. As individual bundles separate from one another, interfascicular collagen appears between them and each comes to be surrounded by a basal lamina. Separation is rarely complete, however; even at the mature stage, processes are exchanged between adjacent sheaths at one or more points on their circumferences. Schwann cell processes surrounding individual bundles become increasingly complex with maturation. Where adjoining processes meet, they commonly branch profusely and interdigitate intricately, forming stacks of closely apposed layered processes. In other areas, the branches are covered by basal lamina and bound intricate labyrinths which commonly extend deeply into the bundle and contain collagen fibrils.