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STUDIES OF LIVING NERVES

1935; Marine Biological Laboratory (MBL); Volume: 68; Issue: 1 Linguagem: Inglês

10.2307/1537291

1939-8697

Carl Caskey Speidel,

Silk-based biomaterials and applications

In living salamanders (Triturus viridescens) individual nerve fibers have been directly observed for prolonged periods (several months) during the processes of growth and myelination, degeneration and regeneration, irritation and recovery. The behavior of nerves in the young salamander is fundamentally like that in the frog tadpole.In newly regenerating zones a few days old the earliest nerve sprouts are visible slowly probing their way through the mesenchymal spaces toward the skin. At the tip of each fiber is an enlargement, or growth cone, which advances by ameboid movement. In action the growth cone is provided with delicate pointed pseudopods; while at rest it is smoothly rounded; during retraction it is characterized by knob-like excrescences.Growth of the sprouts is not necessarily continuous, but is often sporadic in nature. That local conditions are not wholly responsible for growth cone progress is suggested by the differences in behavior exhibited by closely grouped growth cones subject to approximately the same local environmental influences.Barriers to growth cone progress are afforded by the tissues of the tail, such as the processes of connective tissue cells. Such barriers often induce retraction followed by some change of direction of growth cone extension. A varicosity may be left at a site of temporary obstruction.Cell mitosis, per se, either of sheath cells or of fibroblasts, probably has a stimulating effect on nerve sprouts. An active growth cone on touching a small nerve may also induce neuroplasmic movements at the point of contact.The second, third, and later growth cones usually follow more or less closely the pathway laid down by the first growth cone. Thus, the early unmyelinated nerves are formed. These are soon provided with sheath cells of Schwann, which migrate outward from the central regions and multiply by mitosis. These cells are necessary for the formation of both neurilemma (sheath of Schwann) and the myelin sheath.The myelin sheath is formed only through the coöperative activity of the nerve fiber and the sheath cell. Not all nerve fibers, however, are equally ripe for myelination. Those which emerge from a myelin sheath (myelin-emergent) are especially ripe for the production of myelin. The differential factor, therefore, responsible for myelin is inherent, not in the sheath cell, but in the nerve fiber.The myelin is laid down in segments, one segment genetically corresponding to the zone of influence of one sheath cell. The earliest myelin of a segment usually appears near the sheath cell nucleus. It grows by continuous extension in both directions away from the nucleus.The first segments appear proximally near the nerve roots. Progress of the sheath is in the distal direction, each new segment being added at the end of the myelin line.Myelination in salamanders usually involves myelin-emergent nerve fibers which are intimately intermingled with non-myelin-emergent fibers. These are often enclosed at first by a single neurilemma. There is a progressive sorting out of such fibers so that the composite nerve containing both myelin-emergent and non-myelin-emergent fibers within a single neurilemma is transformed into a mixed nerve in which some of the constituent fibers are provided with individual sheaths.Myelin segments, though relatively stable, may undergo various adjustments. Thus, end-to-end anastomosis of two segments may occur; a portion of a segment may be appropriated by the next one and a new node of Ranvier established; bare lengths of nerve fiber between two segments may become ensheathed by the process of intercalation of a whole segment.Varieties of nerve regeneration and phenomena associated with nerve irritation and recovery in the salamander are quite like those already recorded in the frog tadpole.Polariscopic observations of all stages of myelinogenesis indicate that the development of anisotropy closely parallels the development of the myelin sheath. The pre-myelin substance of prospective myelin segments is isotropic; the youngest segments are weakly anisotropic; the older and thicker segments are strongly anisotropic.By means of ciné-photomicrography, permanent records have been obtained which illustrate the essential changes exhibited by individual nerve fibers in salamanders during growth, regeneration, and myelination.