Structure and function of connective tissue in cardiac muscle: collagen types I and III in endomysial struts and pericellular fibers.

Heart myocytes and capillaries are enmeshed in a complex array of connective tissue structures arranged in several levels of organization: epimysium, the sheath of connective tissue that surrounds muscles; perimysium, which is associated with groups of cells; and endomysium, which surrounds and interconnects individual cells. The present paper is a review of work in this field with an emphasis on new, unpublished findings, including composition of endomysial fibers and disposition of newly described perimysial fibers. The role of scanning electron microscopy in the development of current understanding is also outlined. Biaxially arranged epimysial fibers form a sheath around papillary muscles and trabeculae that becomes increasingly well-oriented with the muscle axis during stretch. Perimysial structures are associated with groups of cells, and include weaves and septa of collagen, tendon-like fibers between weaves, ribbon-like fibers perpendicular to myocytes, and the newly described coiled perimysial fibers, which form an array in parallel with the myocytes and the epimysial net. The endomysium includes struts that bridge cells and pericellular fibers; both contain collagen types I and III. The evidence for the latter is presented in this paper and depends upon the use of antibody localization with fluorescent markers in light microscopy and colloidal gold for scanning electron microscopy. The implications of the composition of collagen fibers for myocardial function are discussed in relation to intra-cellular and other extra-cellular structures.