Peripheral glia diversity

Abstract

Recent years have seen an evolving appreciation for the role of glial cells in the nervous system. As we move away from the typical neurocentric view of neuroscience, the complexity and variability of central nervous system glia is emerging, far beyond the three main subtypes: astrocytes, oligodendrocytes, and microglia. Yet the diversity of the glia found in the peripheral nervous system remains rarely discussed. In this review, we discuss the developmental origin, morphology, and function of the different populations of glia found in the peripheral nervous system, including: myelinating Schwann cells, Remak Schwann cells, repair Schwann cells, satellite glia, boundary cap-derived glia, perineurial glia, terminal Schwann cells, glia found in the skin, olfactory ensheathing cells, and enteric glia. The morphological and functional heterogeneity of glia found in the periphery reflects the diverse roles the nervous system performs throughout the body. Further, it highlights a complexity that should be appreciated and considered when it comes to a complete understanding of the peripheral nervous system in health and disease.

Keywords: Schwann cells; boundary cap-derived; cutaneous; ensheathing; enteric; olfactory; perineurial; perisynaptic; satellite; terminal.

FIGURE 1

Glia found in the peripheral nervous system are anatomically, morphologically, and functionally diverse. Depiction of the diverse types of glial cells found within the peripheral nervous system. (a) Myelinating Schwann cells wrap large diameter axons with concentric layers of myelin sheath, generating myelinating internodes with gaps in the myelin sheath (nodes of Ranvier) which allow for saltatory conduction. Each myelinating Schwann cell associates with one axon. (b) In contrast, Remak Schwann cells associate with numerous smaller axons. (c) Following peripheral nerve injury, myelinating and Remak Schwann cells are capable of transdifferentiation into repair Schwann cells, which, among other functions, elongate to form bands of Büngner in order to guide axonal regrowth. (d) Satellite glia are found in sensory and autonomic ganglia, are flat, and wrap around the neuronal cell body. (e) Boundary cap cells are a transient population found close to the PNS/CNS boundary. (f) Perineurial glia cells compartmentalize axons and their associated Schwann cells into nerve fascicles. (g) Terminal Schwann cells associate with nerve terminals at the neuromuscular junction. (h) Numerous Schwann cells are found in the skin, including cutaneous Schwann cells which extend into the epidermal layer. (i) Olfactory ensheathing cells ensheath axons bundled together. (j) Enteric glia functionally and morphologically vary depending on their location within the gut wall. Peripheral glia cells which are largely considered to be derived from the neural crest are colored blue. Perineurial glia cells, which are at least in part of CNS origin, are shaded green. Neuronal cells are depicted in yellow