Extracellular matrix-sarcolemmal surface interconnections: a quick-freeze deep-etch study.

Using ultrarapid freezing and deep-etch fracture techniques, the extracellular matrix in unfixed rabbit papillary muscles is seen to consist of an extensive network of collagen, microfibrils, and microthreads. The microfibril-microthread lattice appears to weave around collagen fibers connecting them to each other and to the external lamina of the sarcolemma. The external lamina appears to insert into the bilayer via trabeculae. With 10 min exposure to zero-Ca solution, the external lamina of the myocytes detaches from the membrane surface but is held from complete removal by some remaining trabecular attachments. This detachment of external lamina affords a view for the first time of the surface of the myocardial sarcolemma. Particles of varying sizes (6-13 nm) may represent the external portions of some integral proteins or protein molecules associated with the membrane surface. They can also represent attachment sites of the external lamina. The serious risks for the fibrous network structure representing an artifact caused by precipitation of matrix proteins during deep etching are discussed.