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 population at 11 days, an ultrastructural differentiation occurs from day 12. So, one could see: a) irregular-shaped cells with dark nucleus and medium-sized contractile apparatus which we have identified as ordinary ("working") atrial myocardiocytes and, b) pale cells with a clear spheroidal nucleus and cytoplasm containing few organelles and fine myofibrils which we have classified as nodal cells. Numerous undifferentiated cells of intermediate morphology appear intermingled with nodal and ordinary cells. Throughout development, nodal and ordinary cells progressively enhanced their mutual differences. Ordinary myocardiocytes become increasingly rich in myofibrils and mitochondria, and nodal cells contain scanty organelles and fine myofibrils, whereas undifferentiated cells are few at every stage. At the end of prenatal life, the sinuatrial node shows numerous unmyelinated axons of immature aspect but direct contacts between nodal cells and 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.