sentence: Epicardial cells also express the cytokine , cxcl12a , after injury [ 21,23 ] and treatment with inhibitors of its receptor , CXCR4 , impede proper regeneration of the myocardium [ 23 ] , suggesting a role for the epicardium in guiding cardiomyocytes during cardiac regeneration.Epicardial cells covering the myocardium derive from the PE , a cluster of cells located close to the venous pole of the embryonic heart tube .