3‐Dimensional configuration of perimysial collagen fibres in rat cardiac muscle at resting and extended sarcomere lengths

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Abstract
1 We have used fluorescence confocal laser scanning microscopy to attain the three-dimensional (3-D) microstructure of perimysial collagen fibres over the range of sarcomere lengths (1.9–2.3 μm) in which passive force of cardiac muscle increases steeply. 2 A uniaxial muscle preparation (right ventricular trabecula of rat) was used so that the 3-D collagen configuration could be readily related to sarcomere length. Transmission electron microscopy showed that these preparations were structurally homologous to ventricular wall muscle. 3 Trabeculae were mounted on the stage of an inverted microscope and fixed at various sarcomere lengths. After a trabecula was stained with the fluorophore Sirius Red F3BA and embedded in resin, sequential optical sectioning enabled 3-D reconstruction of its perimysial collagen fibres. The area fraction of these fibres, determined from the cross-sections of seven trabeculae, was 10.5 ± 3.9 % (means ± s.d.). 4 The reconstructed 3-D images show that perimysial collagen fibres are wavy (as distinct from coiled) cords which straighten considerably as the sarcomere length is increased from 1.85 ± 0.06 μm (near-resting length) to 2.3 ± 0.04 μm (means ± s.d., n= 4). These observations are consistent with the notion that the straightening of these fibres is responsible for limiting extension of the cardiac sarcomere to a length of ≈2.3 μm.