Single Schwann Cell Research Articles

Autonomic nervous system of the cat middle ear mucosa.

The autonomic nerves of the cat middle ear mucosa were studied by the methods of surface preparation, histochemistry and electron microscopy. The tympanic nerve was found to contain large myelinated fibers probably of a sensory nature. Other sympathetic bundles consisted primarily of unmyelinated fibers with a sparse distribution of myelinated fibers. The inferior tympanic artery was accompanied by many vasomotor nerves and its branches were also innervated predominantly by adrenergic fibers. A dense neural network which showed acetylcholinesterase activity was found in the mucosa around the Eustachian tube orifice. These nerves consisted of several unmyelinated fibers encapsulated by a single Schwann cell just below the epithelium. The fine structure of the small ganglions of the tympanic plexus revealed that the cholinergic nerve endings synapse with the cell body of the ganglion. This finding suggests that these ganglions are autonomic.

An untrastructural analysis of remyelination following segmental demyelination.

Electron microscopic observations were made of early remyelination after segmental demyelination in experimental allergic neuritis (EAN) and experimental diphtheritic neuropathy. In EAN 71% of axons which had been demyelinated were invaginated each within a single Schwann cell; remyelination associated with this cellular relationship was in accordance with the spiral myelin concept, 29% of demyelinated axons in EAN were initially enveloped each by several Schwann cell processes and the associated mechanism of remyelination was by “tunication” resulting in a transient irregular distribution of myelin lamellae around the axon circumference. In diphtheritic neuropathy regeneration more closely paralleled development in ontogeny. 88% of demyelinated axons were invaginated each within one Schwann cell, only 12% of axons were each enveloped by more than one Schwann cell process. Early remyelination by tunication was not observed in diphtheritic neuropathy. Additional loosely associated Schwann cell processes lying within the “old” Schwann cell basement membrane occurred frequently in both experimental conditions. Schmidt-Lanterman clefts, “redundant” myelin, and “desmosomes” were observed in the sheaths of incipient remyelination.

Schwann cell dysfunction in acromegaly.

ABSTRACT The pathologic features observed in a sural nerve biopsy from a subject with acromegalic peripheral neuropathy are presented. Examination of individually teased nerve fibers revealed diffuse segmental demyelination. Since each internodal segment represents the territory occupied by a single Schwann cell, the presence of the segmental loss of myelin indicates dysfunction throughout the Schwann cell system. It is concluded that the metabolic derangements occurring in association with acromegaly result in Schwann cell abnormalities. Repeated episodes of segmental demyelination may then predispose to eventual connective tissue hypertrophy.

A study of chemoreceptor and baroreceptor A and C-fibres in the cat carotid nerve.

1. 149 A-fibres and 52 C-fibres from the cat carotid nerve were studied in vivo with single-unit recording techniques. These units subserved chemoreceptor and baroreceptor modalities. In addition, half of the C-fibres were determined to be efferent in origin. The estimated fibre diameter spectrum for chemoreceptor and baroreceptor A-fibres is described.2. The discharge pattern of chemoreceptor A and C-fibres was characteristically irregular both at rest and during activation. However, about 5% of the chemoreceptor A-fibre population exhibited a very regular discharge pattern, even at low rates of firing.3. In comparing A and C-fibres, it was found that chemoreceptor and baroreceptor A-fibres had lower thresholds, shorter response latencies, more rapid acceleration of discharge and higher discharge frequencies than their C-fibre counterparts.4. During strong chemoreceptor or baroreceptor stimulation, interaction of the ;spontaneous' whole nerve activity with the evoked A and C-fibre compound action potentials provided a method of estimating the relative proportions of chemoreceptors and baroreceptors in the A and C-fibre populations of the carotid nerve. The A-fibre population was found to be comprised of approximately 2/3 chemoreceptors, 1/3 baroreceptors. The reverse was true for the C-fibre population, i.e. 2/3 baroreceptors, 1/3 chemoreceptors.5. A stepwise C-fibre response is described which may arise from the several C-fibres within a single Schwann cell.

The cytology of encapsulated nerve endings in the rat penis

The specialized encapsulated nerve endings found in the superficial dermis of the rat penis have been studied by light and electron microscopy. The nerve fiber in these endings lose their myelin sheath and become associated with specialized encapsulating cells termed laminar cells. This cell provides the cellular investment of the unmyelinated neurite and the successive thin cytoplasmic sheets which envelope the neurite. These lamellae are separated by reduplicated basement membranes. The separation between successive lamellae is remarkably constant and remains so when a process of a capsular cell intervenes between successive lamellae. Capsular cells lack basement membranes and appear similar to fibroblasts. Small unmyelinated neurites in a single Schwann cell investment are also found in these endings. These endings are similar to those described by Dogiel and Krause as genital end bulbs.

AN ELECTRON MICROSCOPE STUDY OF THE PERIPHERAL NERVES OF THE HAGFISH (MYXINE GLUTINOSA, L.)

The peripheral nerves of the hagfish, Myxine glutinosa, L. have been studied by electron microscopy. These nerves are unmyelinated and each axon is separately enclosed by a sheath composed of single layer of Schwann cells, whose nuclei lie parallel to the long axis of the nerve. Usually each axon has its own individual sheath, but sometimes two axons, though separated, are ensheathed by the same Schwann cell. In transverse sections, the number of cells forming the sheath varies with the diameter of the axon; fibres of diameter 3 µ and less, have sheaths composed of a single Schwann cell, but a number of cells are present in the sheaths of the larger axons. Where adjacent Schwann cells come together, they are most commonly separated by a distance of about 120 Å, and this is the distance that separates the irregular inner surface of the Schwann cell sheath from the axolemma. The outer surface of each sheath is smooth and is separated from the collagen and reticulin of the endoneurium by a basement membrane.

Morphogenesis of the Schwann channels in the squid nerve

In the unmyelinated giant fiber of the squid, ∼60 A wide slit channels crossing the Schwann layer were previously described. The present study deals with the morphogenesis of these channels. It was carried out on the nerve fiber of the squid Doryteuthis plei. The fibers were fixed in either osmium tetroxide or potassium permanganate and were embedded in either methacrylate or Araldite. The observation of the whole contour of cross sections of the nerve fibers have shown the existence of as many as eight Schwann cell nuclei. The nonsyncytial nature of the Schwann cell is demonstrated. The Schwann cells appear imbricated and form a layer one cell thick. The existence of cytoplasmic islets surrounded by unit membranes in the Schwann layer are considered to be sections of fingerlike processes of the Schwann cell surface. The digitations might belong to a single Schwann cell or to two or more cells whose processes interdigitate. The apposition of neighboring Schwann cells or of the islets give origin to the channels. The findings here summarized support the extracellular nature of the Schwann channel lumina whose function has been thoroughly explored.

Multiple myelin sheaths in single Schwann cells.

DURING an electron microscopic investigation of the normal structure of the trigeminal ganglion in the rat, particular attention was given to various stages of myelin sheath formation1. Frequently it was observed that two or more myelinated axons were enclosed within the cytoplasm of a single Schwann cell. Such a finding was difficult to correlate with the widely accepted theories of myelin formation from the Schwann cell surface2, and the possibility was considered that this variation in myelin contour was an indication of axon branching.

The structure of the peripheral nerves of the lamprey ( Lampetra fluviatilis)

In the lamprey, the peripheral nerves are unmyelinated, but the axons are surrounded by Schwann cells. The larger axons have individual sheaths in which the number of Schwann cells visible in cross-sections varies with the size of the axons, so that while a fiber 5 μ in diameter has a sheath composed of between 4 and 6 Schwann cells, smaller fibers, about 0.3 μ in diameter, are enclosed by a single Schwann cell. Where adjacent Schwann cells come together in these sheaths, they are separated by a distance of 120–150 A. A similar distance separates the axolemma from the inner surfaces of the Schwann cells, which have irregular outlines and frequently bulge into the axon, especially where they contain nuclei and mitochondria. The outer surface of the Schwann cells is more regular and is covered by a basement membrane. The structure of the sheaths is considered in relation to those of other animals.

Electron microscope studies of the formation of nodes of Ranvier in mouse sciatic nerves.

Observations with the electron microscope of longitudinal sections of the sciatic nerves of infant mice during the period of early myelin formation are described. These observations are interpreted in relation to previous studies of transverse sections, and a general picture of the formation of an internodal length of the myelin sheath in three dimensions is formulated. In general, an internodal length of myelin sheath is attained by the spiral wrapping of the infolded Schwann cell surface; the increase in length of the internode during maturation is at least partially explained by the increased length of axon covered by the overlapping of successive layers during the wrapping of the infolded Schwann cell surface; and the nodes of Ranvier refer to the structure complex at the junctions of adjacent non-syncytial Schwann cells. The fact that the mode of formation of myelin brings each of its layers into intimate contact with the axon surface at the nodes is emphasized because of the possible functional significance of this arrangement. The manner of origin of Schmidt-Lantermann clefts remains obscure. Certain isolated observations provide evidence for the possibility that occasional internodes of myelin may form from several small segments of myelin within a single Schwann cell.

The fine structure and morphological organization of non-myelinated nerve fibres.

The fine structure and morphological organization of non-myelinated nerve fibres were studied by ultra-thin sectioning and electron microscopy in peripheral nerves, autonomic nerves and dorsal roots. Several non-myelinated fibres share the cytoplasm of a Schwann cell. The Schwann cells of non-myelinated fibres form a syncytium. The fibres are incompletely sur­rounded by Schwann cell cytoplasm and are suspended in the cytoplasm by mesaxons formed by the plasma membranes of the Schwann cell. The various relationships of mesaxon and nerve fibre are described. Non-myelinated fibres which do not share a Schwann cell are seen very frequently in the sciatic nerve of a new-born mouse but become less common as myelination proceeds and are rare in adults. It is therefore suggested that in developing peripheral nerves, the non­ myelinated fibres that are destined to myelinate are not organized into groups within a single Schwann cell, even before their myelin sheath has appeared; they are, at least for the ages examined here, individuals in relation to a surrounding individual Schwann cell. It is also suggested that the non-myelinated fibres that will never acquire a myelin sheath are organized in a developing peripheral nerve in the same manner as in the adult nerve—several fibres sharing a single Schwann cell that is part of a syncytial system of Schwann cells. Thus, in a developing peripheral nerve, it appears that two types of non-myelinated fibres are present—one destined to myelinate and lying alone in its own Schwann cell and the other, destined to remain unmyelinated and sharing, along with other non-myelinated fibres of the same type, a Schwann cell. The significance of these observations is discussed in relation to the development of nerve fibres and possible physiological importance.