Internodal microvillus-like Schwann cell fingers in myelinated fibres in mouse spinal roots

Internodal microvilli of Schwann cells of myelinated fibres in lizard spinal roots project onto unmyelinated axons

Tufts of microvilli originating from the internodal cytoplasm of Schwann cells associated with myelinated axons in apparently normal lizard spinal roots have been studied under the electron microscope by means of both single and serial sections. More than one tuft of internodal microvilli may arise from a single Schwann cell. Sometimes mitochondria and more frequently an organelle resembling a multivesicular body with a clear matrix can be found in the Schwann cell cytoplasm underlying a tuft of internodal microvilli. The dimensions (length: 0.4-1.0 microns; diameter: 40-70 nm) and structure of internodal microvilli of the Schwann cell are very similar to those of nodal microvilli of the same cell. Each tuft of internodal microvilli projects towards an adjacent unmyelinated axon which at this site is partly devoid of its own Schwann cell sheath. Thus a single Schwann cell may be related to a myelinated axon and an unmyelinated axon at the same time. Patches of a dense axolemmal undercoating (which could be portions of the cytoskeleton) are present in the unmyelinated axon in close spatial correlation with internodal microvilli. The factors which could induce the formation of internodal microvilli as well as the possible role (or roles) of these microvilli are briefly discussed.

Attempt to classify glial cells by means of their process specialization using the rabbit retinal Müller cell as an example of cytotopographic specialization of glial cells

The rabbit retinal Müller cell is one of the most widely studied glial cell types, and it has all forms of contacts that a glial cell can express, viz. 1) to a (ventricular) fluid space, 2) to a mesenchymal borderline (basal lamina), and 3) to neuronal compartments. This cell demonstrates the local adaptation of cell processes to the microenvironment with which they are in contact. Summarizing available data on Müller cells and other glial cell types, it is concluded that the structure with which the process is in contact determines the type of glial cell process that develops. The type I process has microvilli, desmosome-like junctions, and high Na+,K+-ATPase activity; this type of process is in direct contact with a fluid such as cerebrospinal fluid. The type II endfoot-bearing process contains gliofilaments and has a high K+ conductivity; this type of process is covered by a basal lamina and is in contact with mesenchyme. The type III sheath-bearing process insulates neuronal compartments and expresses suitable membrane properties for glia-neuronal communication. Since structurally similar processes have been shown to have similar physiological properties, a new systematic classification of glial cells is proposed, based on the presence or absence of defined types of cell processes. This approach is believed to provide new insights into the function of neuroglia in both the central and peripheral nervous systems, in vertebrates and invertebrates, and even during ontogenetic development.

The pathology of early leprous neuropathy

A qualitative and quantitative study was made of early changes in nerves from 10 patients with tuberculoid or lepromatous type of leprosy. Five nerve biopsies, taken from sites remote from skin lesions, were considered to be unaffected when examined by paraffin histology but showed abnormalities in semi-thin resin sections and by electron microscopy; 5 showed mild to moderate involvement by paraffin histology. Changes in 'unaffected' nerves in both types of leprosy included the presence of subperineurial oedema; occasional evidence of fibre regeneration, sometimes with atypical features; increased numbers of small myelinated fibres, possibly a consequence of axonal atrophy; a few thinly remyelinated fibres, probably due to secondary demyelination, and some loss of unmyelinated axons. In more affected nerves there was variable loss of axons, both myelinated and unmyelinated. Demyelination was not a conspicuous feature; there was evidence of axonal atrophy in some fibres. Similarities in some of the changes observed in tuberculoid and lepromatous types of leprosy suggest a common mechanism of nerve damage, at least in the early stages. The presence of abnormalities in nerves at a distance from skin lesions implies a more diffuse nerve involvement than might have been expected in both types of leprosy.

Internodal Schwann cell fingers in the ventral spinal roots in mice: incidence and relationship to the diameter of myelinated fibers

Internodal Schwann cell fingers were present in the lumbar, cervical, and thoracic spinal roots of adult mice at the age of 3 months, but they were not recognized in five mice examined at the age of 3 weeks. In the L4 ventral roots of ICR mice aged 3 months, the incidence of internodal Schwann cell fingers was 0.79% at the central-peripheral transitional zone and 2.43% at the distal regions, respectively. The ratio of axon diameter to total fiber diameter of myelinated fibers with internodal Schwann cell fingers was lower than that of fibers without them. Therefore, we conclude that internodal Schwann cell fingers are probably related to the developmental increase in the thickness of the myelin sheaths in the ventral spinal roots at all levels of the spinal cord.