Post mortem breakdown of the myotendinous junction in fish

The myotendinous junction of fish muscle was investigated in detail using both scanning and transmission electron microscopy (SEM, TEM respectively). The properties of the main connective tissue structure, type I collagen, were characterised and the purified collagen was used to prepare antibodies to examine structural aspects by immunogold procedures. The SEM studies showed that a network of fibrous connective tissue surrounds each muscle fibre, linking it into a socket-like indentation in the myocomma. These connective tissue fibres were degraded in chill storage, leading to the detachment of muscle fibres from the myocommata and subsequent loss of tissue integrity. Detailed study by TEM demonstrated grooves and invaginations in the terminal ends of the muscle fibres that were filled with fine collagen fibres from the myocomma. These collagen fibres were linked by fine connections to the basal lamina which in turn was linked to the sarcolemma. During chill storage, the basal lamina, the fine connections and the fine collagen fibres progressively degraded and significant deterioration occurred in the myotendinous junction. This deterioration preceded any obvious changes within the muscle fibre structure. The major structural collagen of the skin and muscle and other organs in the fish blue grenadier is an heterotrimer of type I collagen. This collagen was highly soluble in dilute acid and the proportion of insoluble collagen increased with the age of the fish. The collagen possessed three alpha chains in its molecular structure and the amino acid composition of the a-3 chain indicated its derivation from the a-1 chain. The melting and shrinkage temperatures obtained for this collagen were consistent with the imino acid levels and the environment of the fish. Immunogold labelling procedures were developed which confirmed the presence of type I collagen fibres in the myocomma adjacent to the muscle fibre cell but which were inadequate to define individual collagen fibre types. These studies illustrate the complex and intricate nature of the myotendinous junction in commercial fish species. They show that post mortem degradation occurs external to the muscle fibre cell in the extracellular matrix and, in particular, in the fine collagen fibres that form the muscle cell envelope and fill the interstitial muscle space. The study has thus shown that the initial problems of post mortem softening and gaping have their origin in the myotendinous junction not within the muscle fibre itself. The TEM work confirms the SEM work that the breakdown occurs at the interface between the muscle fibres and the connective tissue of the myocomma. It has highlighted the need for further work on the nature and properties of the interstinal collagens and on the nature, activity, specificity and location of the enzymes responsible for the degradation. It has also pointed out the need to establish whether the marine equivalents of minor components of mammalian muscle and its extracellular matrix occur in fish.