The morphologic development of the sinoatrial node in the mouse

A plastic embedding technique was employed on mouse embryos after fixation with a gluteraldehyde-paraformaldehyde mixture and preparation of large, 1 to 2 μ sections cut on a rotary microtome with steel knives. It was then possible to trace the sinoatrial node back to mouse embryos of 4 sol1 2 mm crown-rump length ( 10 1 2 days gestational age, corresponding developmentally to 6 mm human embryos). In the mouse, the sinoatrial node lies anteromedially just above the cavoatrial junction. The nodal artery, which is located more or less centrally, is a branch of the internal mammary artery, not of the coronary arterial system. Even in the youngest embryos examined thus far, the node is strictly unilateral in structure.