THE MORPHOLOGY AND MECHANICAL-PROPERTIES OF ENDOMYSIUM IN SERIES-FIBERED MUSCLES - VARIATIONS WITH MUSCLE LENGTH

In the series-fibred muscle architecture commonly found in large muscl es of mammals and birds, the intrafasciculary-terminating muscle fibre s have no direct tendinous attachments. Contractile force produced in these fibres must be transmitted between adjacent muscle fibres via th e endomysial connective tissue which separates them. The endomysium is thus an essential mechanical component in such muscles. Studies of mo tor end-plate banding patterns and the frequent occurrence of tapering ends of fibres within the fascicles of the bovine sternomandibularis muscle show it to be a series-fibred muscle. Sodium hydroxide digestio n of fixed samples of this muscle to remove the myofibrillar apparatus revealed the endomysium to be a disordered planar network of mainly c urvilinear collagen fibrils. The orientation distribution of the colla gen fibrils in the endomysial network was measured by image analysis o f scanning electron micrographs. Analysis of endomysial preparations f rom muscle fixed at sarcomere lengths between 1-4 mu m showed that the orientation distribution of collagen fibrils is quantitatively relate d to muscle length. At rest sarcomere length the collagen fibril netwo rk is not completely random, but has a slight circumferential bias. Th e orientation distribution shows a progressive shift towards the circu mferential direction at short sarcomere lengths and towards the longit udinal direction at long sarcomere lengths. The relationship between t he number-weighted mean collagen orientation and sarcomere length was compared to two geometric models of network behaviour, the isoareal an d constant shape models. Both fitted the data reasonably, although the constant shape model described the rate of change of mean orientation more closely. From fibrous composites theory, the reinforcement effic iency factor, eta, was calculated from the measured collagen fibril or ientation distributions. These calculations predict a non-linearly inc reasing longitudinal tensile modulus for the endomysium with increasin g sarcomere length, in agreement with its known non-linear properties, but confirm that the tensile properties of the endomysium are unsuita ble for transmission of tensile force from muscle fibres contracting n ear rest length. This reinforces a previous interpretation that contra ctile force is transmitted between neighbouring muscle fibres by trans -laminar shear through the endomysium rather than by in-plane tension.