Content area
Full Text
REVIEWS
THE ORIGIN AND DEVELOPMENT OF GLIAL CELLS IN PERIPHERAL NERVES
Kristjan R. Jessen and Rhona Mirsky
Abstract | During the development of peripheral nerves, neural crest cells generate myelinating and non-myelinating glial cells in a process that parallels gliogenesis from the germinal layers of the CNS. Unlike central gliogenesis, neural crest development involves a protracted embryonic phase devoted to the generation of, first, the Schwann cell precursor and then the immature Schwann cell, a cell whose fate as a myelinating or non-myelinating cell has yet to be determined. Embryonic nerves therefore offer a particular opportunity to analyse the early steps of gliogenesis from transient multipotent stem cells, and to understand how this process is integrated with organogenesis of peripheral nerves.
Before the onset of gliogenesis in the spinal cord, neural crest cells BOX 1 have already given rise to the early glial cells that are found among the axons of nascent nerves as they work their way through body tissues to reach distal targets and establish functional links between the CNS and the rest of the body1,2. These early embryonic nerves are compact columns built exclusively from axons and tightly associated Schwann cell precursors (SCPs). Notably, they have no reinforcing connective tissue or protective covering, and do not even have their own blood supply. These features arise later, at about the time that nerves reach their targets (REFS 3,4; A. Kumar, R.M. and K.R.J., unpublished observations).
A surprising finding is that although SCPs are intimately associated with the axon bundles of these nerves, they are not actually required for the nerve to grow and reach its final target fields57. Rather, SCPs have four main functions. Their most obvious role is, of course, as an intermediary precursor stage between neural crest stem cells and Schwann cells and, therefore, as the immediate source of the Schwann cells present in perinatal nerves6,8,9. Another major function of these cells is likely to be the provision of essential trophic support for sensory and motor neurons at limb levels of the spinal cord most of these neurons
die in mouse mutants in which SCPs are absent10. In addition, SCPs are essential for normal nerve fasciculation10. Finally, SCPs might be the source of not only Schwann cells, but also the...