Glial-Schwann cell specialisations at the central-peripheral nervous system transition of a cyclostome: an ultrastructural study.

This study describes for the first time the central (CNS)-peripheral (PNS) nervous system transitional zone (TZ) of a member of the vertebrate class Agnatha, namely, the sea lamprey. It is concerned in particular with the glial tissue contribution to the TZ and the Schwann cell-CNS interface. Plastic-embedded specimens of spinal cord and nerve roots were examined using light and electron microscopy. At the TZ of each root a glial barrier, continuous with and similar in form to the surrounding glia limitans, stretches across the nerve bundle. In possessing such a barrier the lamprey TZ follows the general vertebrate pattern and differs from the Cephalochordata and invertebrates where a corresponding barrier is absent. The glial barrier is of similar thickness to the glia limitans generally, unlike in the mammal where it is thicker at the TZ than elsewhere. All lamprey axons are unmyelinated peripherally and most of them traverse the glial TZ barrier singly in individual tunnels. In this they resemble mammalian axons of similar calibre (which are, however, myelinated), but differ from unmyelinated mammalian axons, which generally traverse it in bundles. The lamprey TZ has specialisations not found in mammals. These include prominent, multiple interconnected strata of microfilament bundles which are continuous deeply with glial filament bundles and which lie under the surface plasmalemma of the glia limitans, to which they are connected by hemidesmosomes. Features of the mature lamprey TZ resemble those of developing mammalian TZs. For example, slender Schwann cell processes extend below the cord surface and become closely apposed to glial processes. In addition, together with the axon, the TZ glial processes and the basal lamina, they bound an extensive periaxonal network of spaces at the TZ, into which fine Schwann cell processes project. Accordingly, the networks could represent a primitive form of node gap.