Studying cell-cell communication in co-culture.

The formation, maintenance, and repair of biological tissues rely on the interaction of cells with other cell types and with their extracellular matrix [2, 3]. These cellular communications can be bi-directional or multi-dimensional and may occur at both macroand micro-scales. Depending on the hypothesis of interest, co-culture models are used to discern the individual and collective effects of physical contact and soluble factors via paracrine signaling [4]. The simplest co-culture system that permits physical contact between cells consists of a mixed monolayer of the cell types of interest [5, 6]. This is achieved by combining the different cell suspensions at the desired co-culture ratio prior to seeding [7] (Fig. 1). The mixed monolayer model maximizes local heterotypic interactions, and can be used to control the relative levels of heterotypic and homotypic communication by altering the seeding densities of each cell type. Interpretation of mixed monolayer co-culture results must take into account a possible dilution effect due to mixed culture, as well as any metabolic differences between cell types. Moreover, the relative contribution of each cell population to any observed effects is not readily discernible in this model. Cell-cell contact can also be controlled by establishing physical barriers, which are used to regulate spatial and temporal cell seeding patterns in co-culture (Fig. 1). The divider may later be removed to permit cell migration and controlled cell-cell contact [6]. This model is advantageous because it exercises greater control over the extent of heterotypic and homotypic interactions, while permitting both physical contact and soluble factor interactions. The temporary divider system is, however, experimentally more challenging, as a complete seal between the individual cell compartments is required. Moreover, cell response and soluble factor transport in this model depends on the properties of the divider. In studies where paracrine interaction is of greater interest, a segregated co-culture system may be esHeterotypic and homotypic cellular interactions are essential for biological function, and co-culture models are versatile tools for investigating these cellular interactions in vitro. Physiologically relevant co-culture models have been used to elucidate the effects of cell-cell physical contact and/or secreted factors, as well as the influence of substrate geometry and interaction scale on cell response. Identifying the relative contribution of each cell population to co-culture is often experimentally challenging for these cellular interactions studies. In this issue of Biotechnology Journal, Hamilton et al. [1] report on a hydrogel-based co-culture system, that enables paracrine interactions. A simple and elegant method for enzymatic separation of cell populations post co-culture is introduced, thereby enhancing the ease for post-culture analysis of the effects of co-culture on individual cell populations.