The proepicardium (PE) is a cluster of mesothelial cells that develops at the venous pole of the heart. In many vertebrates including the chick, PE development is enhanced on the right side via a signalling pathway involving FGF8 and SNAI1 (Schlueter and Brand, 2009 PNAS 106:7485-7490), leading to the formation of a right-sided proepicardial tissue bridge via which PE cells colonize the heart and form the epicardium. In this study we have analysed the embryonic origin of proepicardial cells and uncovered a novel cellular contribution of pericardial mesothelium to the proepicardium. We performed labelling experiments of the lateral mesocardia and the pericardial mesoderm by DiI injection and electroporation of a GFP reporter gene and observed a cellular contribution of these tissues to the sinus venosus. Expression analysis of TWIST1, which represents a potential downstream target of the FGF8/SNAI1 pathway, revealed enhanced expression in the right somatopleura and subsequently in the right sinus horn and lateral mesocardium. Expression of TWIST1 terminated shortly before the onset of PE formation. We hypothesize that these proepicardial progenitor cells of somatopleural origin are asymmetrically mobilized by the expression of TWIST1. Indeed forced expression of TWIST1 on the left side lead to cell invasion of the sinus venosus and the heart by mesoderm cells of somatopleural origin. We are currently performing loss-of-function experiments to interfere with TWIST1 expression on the right side to study its effect on PE formation. These observations point to a similar origin of a subset of proepicardial cellsand the neighbouring pericardial mesoderm in the chick embryo.