Morphological Fiber Type Correlates of Physiological and Biochemical Properties in Crustacean Muscle

Abstract

SYNOPSIS. Morphological fiber typing is an attempt to group functionally related skeletal muscle fibers based on similarity of structure. Such grouping should bring a functional order or classification, based on structure, to a diverse set of skeletal muscle fibers which are each specialized so that the organism can produce a wide range of forces and movements. Fiber typing using morphological criteria is an attempt to identify a group of similarly performing fibers by simple visual inspection without the need for physiological measurement on each and every cell. If the typing system is unambiguous the system can then be used to study plasticity, maintenance, and development of particular functional/ biochemical modifications. In such a case, the method may predict both function and perhaps even alterations occurring in fiber type properties. If on the other hand, the typing methods are somewhat more ambiguous, such that intermediate characteristics are difficult to interpret with relation to functional or biochemical alteration the typing loses its usefulness as a predictive tool for the study of regulatory events controlling maintenance and plasticity of the fibers. In the vertebrate case, particularly for mammalian and avian systems, morphological fiber typing is a potent tool for studying regulation of muscle fiber properties. Classical histochemical fiber typing based on the combination of myofibrillar ATPase, oxidative enzyme activities, and glycogen content predict physiological function fairly well. These methods have been improved and substantiated in large measure by immunohistochemical procedures based on differences in biochemical isozymes of both contractile and sarcoplasmic reticulum proteins between fast and slow contracting fibers. Additionally, electron microscopic evaluation of organelle content also separates fibers in accord with their physiological performance. Thus the methodology for typing muscle fibers morphologically is well-developed and correlates well with biochemical and physiological performance, and therefore is used extensively to study muscle fiber development and plasticity. Crustacean skeletal muscle also shows a wide range for single muscle fiber performance, and this performance can be matched to morphological type generally. Morphological typing in crustacean systems rests on two major differences between the extremes on the performance continuum. Slow contracting fibers have long sarcomere lengths and low myosin ATPase activities, while very fast contracting fibers have short sarcomere lengths and high myosin ATPase activities. These methods, while useful in the majority of cases, unfortunately do not always accurately predict physiological performance of every fiber typed into a particular group. This makes the methodology particularly difficult to use in studies attempting to predict changes from an individual fiber adapted to one type of performance into a different type of fiber. The ambiguity therefore has not allowed morphological typing by itself to be as useful a tool in studying the properties which maintain fiber type in crustacean systems. Thus in these cases morphological typing must be confirmed using physiological measurement for each muscle fiber. Recent work indicates that crustacean fibers may be more accurately typed using immunohistochemical methods based on specific biochemical differences found in muscle fibers. Such methodology is commonly in use for defining mammalian muscle fiber types, and now appears to be a useful tool for typing of crustacean fibers. Typing of protein isoforms should not be as ambiguous as some of the classic methods for crustacean muscle fiber typing, and may be the tool which allows further understanding of fiber development, plasticity, and maintenance in crustacean systems.