[Ultrastructure of the myocytes of the sinoatrial node of the mouse embryonic heart in mitosis].

The sinoatrial node (SAN) of 16-18-day old mouse embryos contains only 0.4% of myocytes in mitosis; this index is by ten times lower than that of the working ventricular myocardium. The SAN myocyte contractile apparatus is also less differentiated than that of the working ventricular and atrial myocardium. In interphase and early prophase thin and sparse myofibrils with poorly developed Z-disks occur. In prometaphase (perhaps in midprophase) myofibril disintegration takes place as a result of Z-band material resorption. In metaphase, anaphase, and telophase, Z-disks are absent; there are only dispersed thin myofilament bundles. In late telophase or in G1 phase, patches of Z-band material appear among myofilament bundles, thus indicating myofibrillar gradual restoration. Unlike Z-bands, desmosomes and fasciae adherentes of intercalated disks remain through all the stages of mitosis. In telophase, after nuclear envelope reconstruction, chromosomal spindle microtubules can enter the nucleus to retain the connection with chromatin. A cleavage furrow can be seen in early telophase, but cytokinesis occurs not in all the cells, which results in the appearance of single binucleate cells. Myofibrils of SAN and ventricular embryonal myocytes undergo the same reversible disintegration during mitosis. The cyclic changes of other organelles are also similar to those occurring in the ventricular embryonal myocytes.