2D cell culture, also known as monolayer cell culture or adherent cell cultures, involves the culture of cells in a single layer, submerged in culture medium. Cells are only exposed to culture medium, and can be either maintained in an “undifferentiated” specialized phenotype, or induced to further differentiate. In contrast 3D culture involves the culture of cells to form multi-layered, more differentiated cell structures. 3D culture may be used to maintain certain kinds of lower differentiated cells (for example grown as spheres), or may also be used to create fully differentiated cell sheets, for example of the epidermis. 3D epidermal cultures allow the full spectrum of cell differentiation, extending from terminally differentiated non-proliferative cells on top, down to proliferating progenitors in the basal layer. In this way, the 3D structures can closely mimic the structures see in vivo.