A co-culture can be defined as the growth of more than one distinct cell type in a combined culture. Such in vitro models can provide a more physiologically relevant way of demonstrating in vivo-like tissue morphology and function. Co-cultures are useful for modelling and studying the interaction and signalling between different cell types. Co-cultures can be employed to monitor intercellular communication, cell migration dynamics, and stimulation and maintenance of cell function and differentiation. For example, in previous studies: rat primary hepatocytes have demonstrated enhanced functionality and maintained their viability for longer when co-cultured with 3T3 cells; the co-culture of carcinoma and intra-tumoural stromal cells simulated a breast cancer micro-environment, allowing cell-to-cell communication to be monitored for the initiation and progression of the cancer; a study carried out using endothelial and smooth muscle cells concluded that a co-culture enabled the structure of a vessel wall to be mimicked, creating an improved model of the in vivo environment.