part of atrial cardiac muscle cell differentiation cardiac muscle cell development atrial cardiac muscle cell development atrial cardiomyocyte development GO:0055014 atrial heart muscle development The process whose specific outcome is the progression of an atrial cardiac muscle cell over time, from its formation to the mature state. Cardiac muscle cells are striated muscle cells that are responsible for heart contraction. The atrium is the part of the heart that receives blood into the organ. biological_process