Synchronization of heterogeneous systems that consist of oscillatory and passive elements are studied in cardiac myocytes/fibroblasts co-cultures. It is found that beating clusters of cardiac myocytes surrounded by fibroblasts will be formed. The beatings of the cardiac myocyte clusters are not correlated at early times, but get synchronized as the cultures mature. This synchronization can be understood by a Kuramoto model with a time-increasing coupling strength. Our findings show that the growth of the coupling strength between clusters is linear, while the overall wave dynamics of the system is controlled by the passive fibroblast in the system which presumably is growing exponentially.