Fibre types inLimulus telson muscles: morphology and histochemistry

Abstract

Using a variety of techniques, we have demonstrated the presence of at least two fibre types in Limulus median telson levator muscle. By light and electron microscopy, large (2,156 microns 2 mean cross-sectional area) fibres have A-bands of 4.1 microns, one-half I bands of 2.15 microns and Z lines less than or equal to 0.5 microns in width. Few mitochondria are found in these fibres, which comprise 54% of those present in a given microscope field and which occupy 82% of the total cross-sectional area. Small fibres (484 microns 2 mean cross-sectional area) have A bands of 6.3 microns, one-half I bands of 3.1 microns and Z lines between 0.5 and 1.0 microns in width and are rich in mitochondria. Although small fibres comprise nearly one-half (46%) of the fibres in a field, they occupy only 18% of the total cross-sectional area. Histochemical staining for alkaline-stable myofibrillar ATPase activity and mitochondrial reduced beta-nicotinamide adenine nucleotide (beta-NADH) tetrazolium reductase activity confirms the presence of two fibre types. The large fibres react positively for the myofibrillar ATPase activity and negatively for the mitochondrial enzyme activity. The reverse is seen with the small fibres. Some fibres of intermediate size, having intermediate staining characteristics, were also observed. Native gel electrophoresis of both myofibrillar and purified myosin preparations supports the observed differences in myofibrillar ATPase activity in that two myosin isozymes are resolved on pyrophosphate gels. Although the thick filaments isolated from unstimulated small fibres are longer (greater than 6.0 microns) than those isolated from unstimulated large fibres (4.26 microns), all have a similar appearance with respect to the arrangement of myosin heads on their surfaces, and similar diameters. The implications of the observed heterogeneity of fibre types is discussed with reference to previously reported phenomena in Limulus telson muscle, including changes in length of thick filaments on fibre stimulation and the shape of the length-tension curve obtained from fibre bundles.

Effects of phosphorylation by myosin light chain kinase on the structure of Limulus thick filaments.

“…For the telson to function as a lever to right the crab when it is upside down and also as a rudder in swimming, requires its full, >180 ° , range of movement in the vertical plane as well as equally large rotational movements (Eagles, 1973;Silvey, 1973). As we have earlier shown, the sarcomeres of largediameter fibers (Levine et al, 1989) undergo at least a twofold change in length from >9.0 to <4.0/zm, as do those of small diameter fibers (Levine et al, 1989), from >12.0 to <6.0 #m, as they shorten to produce full elevation of the telson from a fully depressed position. In each of these fiber types, the length of the "relaxed" thick filaments is longer than that of the fully shortened sarcomeres.…”

The effect of calcium activation of skinned fiber bundles on the structure of Limulus thick filaments.

“…Each longitudinally sectioned bundle contains between seven and twenty fibers. Both largediameter, long sarcomere and small-diameter, very long sarcomere fibers are present (Levine et al, 1989). In light micrographs of toluidine blue-stained, 1-#m sections of stretched fiber bundles, an unstained gap separates the A-and I-bands in both control and experimental (activated) bundles, indicating that the sarcomeres were stretched beyond overlap of thick and thin filaments ( Fig.…”

“…16% of this population includes thick filaments of 4.5/zm or longer. Presumably, these are separated from the small diameter fibers having very long sarcomeres and A-bands and comprising ~18 % of the cross-sectional area (Levine et al, 1989). A few fragments are present on control grids: the majority of these are derived from thick filaments that had broken at the bare zone region, probably during homogenization of the fiber bundles.…”

Spiny-Cheek Crayfish, Faxonius limosus (Rafinesque, 1817), as an Alternative Food Source

“…The size of muscle fibres in F. limosus muscles, characterised by the CSA, girth, V and H diameters, is smaller compared with that of other aquatic and terrestrial animals. For example, the CSA of muscle fibres in F. limosus (85.04 µm 2 ) is four times smaller than that in horseshoe crab, Limulus polyphemus-484 µm 2 [114]. Even greater differences in fibre size were observed when comparing crayfish meat to that of various fish species-sea bass [115], common carp [100], tench [116], as well as livestock-pork [103,104], beef [117], chicken [118], and duck [119].…”