Changes in motor end-plates resulting from muscle fibre necrosis and regeneration: A light and electron microscopic study of the effects of the depolarizing fraction (cardiotoxin) of Dendroaspis jamesoni venom

Abstract

The venom of Dendroaspis jamesoni (Jameson's mamba) was fractionated by column chromatography. One of the fractions (F8) caused rapid depolarization of muscle fibres when tested in vitro, an action similar to that of “cardiotoxins” of other snake venoms. A single injection of 20 μg of the depolarizing fraction was made directly into the muscles of one hindleg in mice. Light and electron microscopic studies were made of muscle fibres and motor end-plates of animals allowed to survive from 30 min to 6 months after the injection. The depolarizing fraction caused rapid and severe disorganization of the internal structure of muscle fibres affecting slow and fast types of fibre equally severely. Muscle fibres were affected in segments and not along their entire length. Early changes were the disappearance of Z-lines, the dissolution of myofilaments and degenerative changes in mitochondria, but the basement membranes of the muscle fibres remained intact. Regeneration of muscle took place rapidly and was usually complete within 2 to 3 weeks but centrally-placed nuclei, rows of nuclei and malorientation of myofibrils including the formation of striated annulets remained as permanent abnormalities and were used as indicators of where necrosis had occurred. Myelinated axons and motor nerve terminals were normal in appearance and contained abundant vesicles and mitochondria even when the muscle fibres they innervated were necrotic. The axonal terminals became enclosed within Schwann cell processes. After muscle regeneration was complete the end-plate morphology remained abnormal. With light microscopy the end-plates were seen as aggregations of sub-units, each aggregation being limited to a single muscle fibre and innervated by a single pre-terminal myelinated axon. The electron-microscopic appearance of the motor end-plates also remained abnormal and there was a striking absence of post-synaptic folds of the sarcolemmal membrane. The observations made in these experiments indicate that the cardiotoxin selectively damages muscle fibres leaving the motor innervation structurally intact and also show how morphological abnormalities may develop in motor end-plates as a result of a primary lesion of the muscle fibre. The appearances are similar to those occurring in the innervation of muscle in human myopathies. The absence of branching of preterminal axons and of sprouting from axon terminals is in contrast to the changes found in human and experimental conditions in which there is known to be a disorder of the lower motor neurone.