Fine structure of nerves in the regenerating limb of the newt Triturus.

Abstract

AbstractThe structural integrity of some tissues and the regeneration of extremities in some vertebrates depends upon the nervous system. To investigate the structure of nerves exhibiting trophic function, nerves in regenerating forelimbs of adult newts, Triturus viridescens, were studied with the electron microscope. Nerve fibers sprout from the transected axons, 2–3 days after amputation of the limb, and invade all portions of the blastema and epidermis in large numbers. Regenerating nerve fibers contain a greatly increased amount of smooth‐surfaced channels of endoplasmic reticulum containing moderately dense material, an increased number of microtubules, and large (1000 Å) membrane‐bounded dense granules. The latter were not observed normally and could be distinguished from synaptic vesicles and dense‐core vesicles thought to contain catechol amines. In larger nerves, the organelles are distributed in the peripheral axoplasm around a central zone containing neurofilaments.The relationship of the fine structural changes occurring during regeneration to the trophic action of nerves and the regeneration of nerve fibers is discussed. The tortuous membranous tubules of endoplasmic reticulum could serve as channels for the transport of substances, either trophic material or materials necessary for growth of the nerve, down the axon. Microtubules may play a role in the regulation of form of the growing axon and also be related to axoplasmic flow or migration of particles (e.g., granules) along their length. The large membrane‐bounded dense granules appearing during regeneration resemble neurosecretory granules, which have been associated with regeneration in some invertebrates. These structures could, therefore, contain a trophic substance or hormone that is transported down the axon, released into the intercellular spaces, and controls subsequent regeneration of the limb.