Microscopic spheres that allow adherent cells to be suspended in culture media. Adherent cells do not grow unless they are attached to a solid object, usually on flat plastic plates in laboratory settings. To culture these cells in large bioreactors, the cells must first attach to a surface that is large enough to allow them to grow but small enough to stay in a liquid media suspension. Microcarriers are typically made of polystyrene (plastic) or hydroxyapatite (a complex calcium phosphate and the main component of bone) and can be coated with a variety of substances to promote the attachment of specific cell types.