Proliferation and biosynthetic activities of myocytes from conductive system and working myocardium of the developing mouse heart. Light microscopic autoradiographic study.

Incorporation of 3H-thymidine, 3H-uridine and 3H-leucine into myocytes and their mitotic activity have been investigated in different compartments of the mouse heart conductive system (CS) and working myocardium (WM) at pre- and postnatal stages of development. On 15th and 18th d of embryogenesis, 3H-thymidine pulse and cumulative labelling indices (LI) of myocytes in the sinoatrial node (SAN), atrioventricular node (AVN) and His bundle (HB) were found to be 4 to 6 times lower than in WM (0.001 less than or equal to P less than or equal to 0.01). On 3rd to 9th d after birth LI remained decreased (0.001 less than or equal to P less than or equal to 0.05) in SAN while replicative activity of myocytes in AVN and HB approached that of both ventricular and atrial WM. Since 13th d after birth LI did not exceed 1% in all the compartments studied. In the prenatal period LI in SAN were higher than in AVN P less than or equal to 0.01), whereas in the postnatal one it was just the opposite (0.001 less than or equal to P less than or equal to 0.05). During cardiogenesis change of mitotic indices in WM and CS correlates with that of LI. Duration of cell cycle and its periods (G2, G2 + 1/2 M, S) estimated by curves of labeled mitoses, double-labelling technique with 14C-thymidine and 3H-thymidine and label "dilution" method differ slightly in myocytes from WM and CS. The latter incorporate 3H-uridine much less intensively at most stages of heart development compared with WM. At perinatal stages of cardiogenesis 3H-leucine labels CS myocytes somewhat weaker than WM ones until 8th d after birth when the labelling intensities in all cell types become similar. It can be concluded that on the whole CS myocytes from the developing mouse heart appear to be less active than those of WM concerning replication, transcription, and translation processes.