Ultrastructural quantitative characterisation of sinus and atrioventricular nodal cells in the developing caprine heart compared with ordinary atrial and ventricular myocardial cells.

Cellular growth and changes in subcellular components as well as in intercellular junction structure were investigated quantitatively in sinus and atrioventricular nodes in comparison with right atrial and left ventricular myocardial cells of the goat heart in fetuses aged 6, 8, 10, 12, 14, 16 and 18 gestational weeks, young animals aged 1 and 3 postnatal weeks, and adults. At the earliest stage examined (sixth gestational week) no clear difference was recognised, in the patterns of star diagrams representing cellular characteristics, between nodal cells and ordinary cardiac muscle cells, except for a larger number of myofibrils in the ordinary cells. Atrial specific granules also appeared in the atrial cells at this stage. In general, initial signs of differentiation in ordinary cardiac muscle cells were detected at the sixteenth gestational week when transverse tubule formation occurred in the ventricular muscle cells. Throughout the developmental period, no appreciable alterations in the ultrastructural features were found in the sinus nodal cells whereas the atrioventricular nodal cells revealed slight changes at the later fetal and adult stages.