The transitional node of Ranvier at the junction of the central and peripheral nervous systems: an ultrastructural study of its development and mature form.

When the central-peripheral transitional node first appears it lies immediately distal to the astrocyte processes delineating the cord surface. Its initial location may be influenced indirectly by the astrocytes, since they determine the position of the transitional Schwann cell by preventing it from invading the central nervous system. The central end of the Schwann cell becomes specialised to form a narrow cytoplasmic collar which closely envelopes, and so may influence, the developing nodal axon segment. The earliest nodal specializations include subaxolemmal undercoating which first appears as discrete plaques. These soon fuse to form a complete layer. The transitional node is closely related to the Schwann cell collar throughout its maturation. However, the presumptive oligodendrocyte paranode lies a considerable distance central to the node for some time. The intervening axon segment is enveloped by astrocyte processes. Terminal pockets of the oligodendrocyte gradually extend distally along this segment until they reach the node. Here the distal end of the oligodendrocyte is at first apposed to the Schwann cell collar. With maturation, astrocyte processes extend into the node gap, intervening between the two. The Schwann collar gradually becomes retracted distally as it gives rise to microvilli which project into the node gap space. With maturation, the astrocyte processes form a progressively more complete barrier between the oligodendrocyte and the node gap space. As the myelin sheaths on either side of the transitional node become thicker, the angles through which their turns incline inwards towards the axon progressively increase. The node gap thus tends to become deeper and to be bounded by steeper walls. However, in small fibres and in a proportion of large fibres, this angle remains relatively small and the node gap is therefore relatively open. Axonal protrusions commonly arise from the nodal and the paranodal segments of the axon. With maturation they become more frequent at the latter. Small recurrent collateral axon branches arise at transitional nodes of large fibres in increasing numbers with maturation. They possess thin myelin sheaths. Most run centrally in the intramedullary bundle towards the anterior horn grey matter.