[Ultrastructure of the embryonic mouse sinus node at the early stages of development (author's transl)].

At early stages of development in the mouse embryo (12-13 days post-coitum) the cells of the sino-atrial node are homogeneous but are different to the atrial myocardic cells at this stage of development or a younger stage (9-10 days post-coitum). Sino-atrial node cells are small; they have many ribosomes, the myofibrillar material and the Golgi apparatus are poorly developed. Membrane contacts are represented by simple appositions and some fascia adherentes. These cells show many pinocytic invaginations and subsarcolemmic vesicles. From the 14th day post-coitum the sino-atrial node becomes heterogeneous. 'Clear cells' are present in the middle part of the node; they are characterized by their irregular shape and the poor development of their myofibrillar material 'Dark cells' are observed at the periphery of the node and seem related to myocardial type cells described in the sino-atrial node of the late mouse foetuses. Our observations do not support the existence of a 3rd cellular population, called by some authors: 'transitional cells'. The problem of the segregation between sino-atrial cells and myocardial cells is discussed.