PRENATAL DEVELOPMENT OF THE RAT SINUATRIAL NODE

In order to study the origin and mode of differentiation of the cells which make up the sinuatrial node, samples of the sinuatrial junction of rat embryos of different ages were studied by transmission electron microscopy. From a seemingly morphologically homogeneous cell populat ion at Il days, an ultrastructural differentiation occurs from day 12. So, one could see: a) irregular-shaped cells with dark nucleus and me dium-sized contractile apparatus which we have identified as ordinary (''working'') atrial myocardiocytes and, b) pale cells with a clear sp heroidal nucleus and cytoplasm containing few organelles and fine myof ibrils which we have classified as nodal cells. Numerous undifferentia ted cells of intermediate morphology appear intermingled with nodal an d ordinary cells. Throughout development, nodal and ordinary cells pro gressively enhance their mutual differences. Ordinary myocardiocytes b ecome increasingly rich in myofibrils and mitochondria, and nodal cell s contain scanty organelles and fine myofibrils, whereas undifferentia ted cells are few at every stage. At the end of prenatal life, the sin uatrial node shows numerous unmyelinated axons of immature aspect but direct contacts between nodal cells End nerve fibers are not seen. The images we obtained suggest that the sinuatrial node must not be taken as an embryological remnant. Nodal cells are recognized from the 12th day of embryonic life as a particular form of differentiation of the cells which make up the sinuatrial region at the preceeding stages.