results in acquisition of features of regular atrial cardiac myocyte cell differentiation cardiac muscle cell differentiation atrial cardiac muscle cell differentiation The process in which a relatively unspecialized cell acquires specialized features of a cardiac muscle cell in the atrium. 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 atrial cardiomyocyte differentiation atrial heart muscle cell differentiation GO:0055011