Structure and protein composition of sites of papillary muscle attachment to chordae tendineae in avian hearts.

Most cardiac myocytes transmit force across fasciae adherentes, specialized sites of cell-cell adhesion. However, some cardiac myocytes in papillary muscle terminate on collagenous connective tissue in the chordae tendineae. These papillary myotendinous junctions (MTJs) are specialized for force transmission from myocytes to extracellular matrix. In the present study, we compared structural molecules at papillary MTJs to those at fasciae adherentes and skeletal MTJs. By using indirect immunofluorescence, we found that papillary MTJs more closely resemble skeletal MTJs in their molecular composition in that they are enriched in talin, vinculin, integrin, and fibronectin. Zeugmatin and alpha-actinin, both components of fasciae adherentes, are absent from papillary MTJs. Although papillary MTJs and skeletal MTJs display strong similarities in structural protein composition, ultrastructural organization of the two junctions is different. Papillary MTJs display little folding of the junctional membrane and, according to morphological criteria, more closely resemble sites of thin filament-membrane association in smooth muscle than skeletal MTJs. Thus, papillary MTJs display a combination of structural characteristics described previously in skeletal and smooth muscles but exhibit few structural features observed previously in cardiac fasciae adherentes.