Chicken breast muscle is usually considered to be a relatively homogeneous white muscle and has therefore been widely used for studies of muscle proteins. In a previous study, however, we have found different M-region structures in different fibres from this muscle. Because of this result, we have now carried out a combined histochemical and ultrastructural survey of this muscle. In particular, we have made use of large transverse cryo-sections that include most of the muscle cross-section. Although the white region is fairly homogeneous in fibre content according to normal histochemical criteria (mATPase), we have found that there is a gradation of fibre structure across the muscle. The bulk of the muscle stains conventionally for Type-II fibres according to mATPase tests (the "white" part) but, in the small "red" part of the muscle, there are also Type-I fibres together with the Type-II fibres. Superimposed on this division into Type-I and Type-II fibres are variations in fibre size, oxidative and glycolytic staining properties, and variations of Z-band width and M-band structure; there is no strict correlation among any of these parameters. The apparently uniform staining across most of the muscle when tested for myofibrillar ATPase may be a misleading indicator of fibre properties.
SUMMARY
Ultrathin sections of rapidly frozen, briefly pre‐treated muscle tissue are cut and thereafter are thawed and contrasted using a negative staining technique. The method has provided micrographs in which the in‐vivo order in the muscle fibres has been preserved well enough to enable both a more complete interpretation of X‐ray diffraction evidence from muscle, and also a gain of new ultrastructural information on aspects of myofibril and myofilament architecture in different types of fibre. Examples here are taken from chicken, rabbit and fish muscles and show both the M‐band and the bridge region of the A‐band in great detail. To enhance the detail in the original images, one‐dimensional (1‐D) and 2‐D averaging techniques (lateral smearing and step averaging, respectively) are used. Although there is major shrinkage in section thickness to about one‐third of its original value, demonstrated here for the first time is the fact that the characteristic A‐band lattice planes are preserved in these sections in 3‐D. This confirms the usefulness of cryosections not just for 1‐D and 2‐D image processing, but also for 3‐D reconstruction. Thus, in combination with techniques of image processing, cryoultramicrotomy can give the muscle morphologist the detailed data that are needed to match the molecular biologists, biochemists and immunologists in the interpretation of their data about physiological and pathophysiological events in muscle fibres at the macromolecular level.
SUMMARY
Cryomethods were used in order to investigate the ultrastructure of native elastin fibres from beef ligamentum nuchae. Filaments of diameter 5 nm, running almost in parallel in purified, negatively‐stained elastin preparations, were also seen running along the elastin fibre both in freeze‐fractured and etched elastin, that had been stretched up to 200%, and in cryo‐sectioned elastin that had been stretched and chemically fixed before freezing. Interconnections between elastin filaments were revealed by the freeze‐etching technique. Glycerol treatment, which probably leads to hydration of specimens, resulted, however, in disorganization of filaments and swelling of the elastin fibre. In conclusion, by the use of cryotechniques, it was convincingly demonstrated that elastin molecules are arranged in long interconnecting filaments of about 4–5 nm width.
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