An electron microscope study of quantitative relationships between axon and Schwann cell sheath in myelinated fibres of peripheral nerves

Abstract

The quantitative relationships between the cross-sectional area of the Schwann cell sheath (myelin included) and that of its related axon were studied by electron microscopy in the nerve fibres of the spinal roots of lizard (Lacerta muralis). In both ventral and dorsal roots the cross-sectional area of the Schwann cell sheath (myelin included) was found to be directly proportional to that of its related axon (correlation coefficients between 0.88 and 0.92). The ratio between the cross-sectional area of the Schwann cell sheath (myelin included) and that of its related axon tends to diminish as the cross-sectional area of the latter increases. Thus, under normal conditions, in myelinated fibres of the spinal roots of the lizard a quantitative balance exists between the nerve tissue and its associated glial tissue. This result agrees with those previously obtained in the spinal ganglia of the lizard, gecko, cat and rabbit. Some of the mechanisms probably involved in the control of the quantitative balance between nerve tissue and its associated glial tissue in peripheral nerves are presented and discussed.

Neuregulin‐1, a key axonal signal that drives Schwann cell growth and differentiation

“…More than 100 years ago, it was noticed that myelination is related to axon size (Donaldson and Hoke, 1905), although the morphological relationship between axons, myelin sheaths, and Schwann cells was unknown at that time. In the peripheral nervous system, axons greater than 1 lm in diameter are almost invariably myelinated, whereas those smaller than 1 lm are not (Fraher et al, 1990;Friede and Bischhausen, 1982;Pannese et al, 1987;Williams and Wendell-Smith, 1971). Myelinating Schwann cells sort axons into a oneto-one ratio and wrap one axon multiple times, while in rodents, the majority of nonmyelinating Schwann cells engulf several axons (typically sensory C-fiber axons) within a Remak bundle.…”

“…More than 100 years ago, it was noticed that myelination is related to axon size (Donaldson and Hoke,1905), although the morphological relationship between axons, myelin sheaths, and Schwann cells was unknown at that time. In the peripheral nervous system, axons greater than 1 μm in diameter are almost invariably myelinated, whereas those smaller than 1 μm are not (Fraher et al,1990; Friede and Bischhausen,1982; Pannese et al,1987; Williams and Wendell‐Smith,1971). Myelinating Schwann cells sort axons into a one‐to‐one ratio and wrap one axon multiple times, while in rodents, the majority of nonmyelinating Schwann cells engulf several axons (typically sensory C‐fiber axons) within a Remak bundle.…”

Biogenesis and maintenance of cytoplasmic domains in myelin of the central nervous system

“…In general, myelin volume is strongly correlated to both the axonal diameter (caliber) and the length of the myelinated internodal axonal segment, with only minor species-and nervespecific differences (Friede & Bischhausen, 1982;Pannese et al, 1987). In particular, the axonal caliber appears to be a critical parameter for myelination of small axons (Voyvodic, 1989).…”

Schwann cell differentiation in Charcot-Marie-Tooth disease 1 A (CMT1A)

“…In turn, each Schwann cell will produce a single myelin segment, termed internode, along an individual axon and leaving unmyelinated small regions in between which are called nodes of Ranvier. On the other hand smaller axons (<1 µm) bundles surrounded by Schwann cell cytoplasm instead of getting myelinated they become segregated from each other through ending up in separate pockets in the same Schwann cell forming a unique PNS myelin structure called Remak Bundle [10][11][12][13][14] (Fig. 1).…”