A cell in vivo lives in a community, and understands its fate by interacting with other cells. It learns its position, characteristics and roles by cellular signals such as extracellular matrix, cytokine, hormone and other secretions. Even though cell-to-cell interaction is considered most critical in cellular events, its complexity has been the main obstacle for us to study details. In this respect, we need to simplify the study of cellular interaction by limiting the number of associating cells. Also a long-time observation is required to provide affluent information, because...