proliferation is a phenomenon seen during fetal heart growth and early post natal stages after which diminishes over time on day 20 of cardiac differentiation higher percentages of proliferating cms were detected in 2d monolayers as compared to 3d cardiac tissues while both containing a similar percentage of ctnt positive cells other cellular properties including cardiac gene expression and calcium handling were similar between 2d and 3d at this time point therefore we believe that 3d culture conditions and a more physiological microenvironment might be the cause for lower degrees of cm proliferation and not due to cm maturation during early stages of differentiation day 14 sc cms exhibit mechanical and electrical features that are not well developed although 2d and 3d cultured sc cms respond to outside pacing frequencies up to 1 5 hz higher maximum capture rates at later time points are desired increased beating rates of spontaneously contracting 3d cultured cms in response to isoproterenol suggests that u03b2 adrenergic signaling is operational in early stage cms human adult cms are difficult to obtain for experimentation and are desired for myocardial repair in the adult patient for better mechanical and electrical integration the process of human cm maturation is not studied yet but will provide important insights about adolescence and how the human heart remodels after birth as well as provide more physiologically relevant features for toxicology screening and disease modeling of the adult human myocardium here 3d differentiated and cultured sc cms present sarcomere spacing of 1 9 u00b1 0 1 u03bcm similar to mature cms furthermore ultrastructural features of mature cms were identified on day 124.