Abstract
Unlike the cells of the working myocardium (WM), myocytes from the sinoatrial node (SAN) of the hearts of 16- to 18-day-old mouse embryos are small and 'pale'. They contain few poorly organized myofibrils and no specific granules. The frequency of myocytes labeled with 3H-thymidine or dividing mitotically is 3-4 times lower in the SAN compared to the WM. The ultrastructure of myofibrils and other cytoplasmic organelles in DNA-synthesizing myocytes of the SAN remains practically the same as in most of its muscle cells not labeled with 3H-thymidine. However in prometaphase Z-discs disappear and myofibrils break into free myofilament bundles. Intercalated discs and desmosomes persist in all mitotic phases. As a result of cytotomy, two daughter myocytes appear, while occasional acytokinetic mitoses give rise to binucleated cells of the SAN.
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