Temporal and spatial patterns of phosphotyrosine immunolocalization during cardiac myofibrillogenesis of the chicken embryo.

To investigate the possible role of phosphorylation of protein tyrosine during myofibrillogenesis (6- to 13-somite stages) of the chicken embryonic heart tube, immunolocalization of phosphotyrosine (P-Tyr) and the relationship between P-Tyr and developing myofibrils were studied by means of confocal scanning laser microscopy and immuno-electron microscopy. The staining pattern of P-Tyr varied in different sites of myocytes at different stages of embryonic development: At the cell-cell boundaries, P-Tyr was localized at the adhesion belt of outer myocardial layer cells (6- to 13-somite stages), non-junctional cell-cell contacts (6- to 13-somite stages) and early intercalated disks of both the outer and inner myocardial layer cells (8- to 13-somite stages). At the cell-extracellular matrix boundaries of inner layer cells, the first stages of myofibril formation appeared as serially aligned areas of P-Tyr localization closely associated with circumferentially aligned thick actin bundles (8- to 9-somite stages). This P-Tyr immunostaining decreased when the thick actin bundles developed into mature striated myofibrils at the 10- to 13-somite stages. These findings suggest that the phosphorylation of protein tyrosine residues is primarily concentrated at the modulating cell-cell and cell-matrix adhesion sites of developing myocytes and myofibrils.