A therapeutic serum suitable for inclusion in a cosmetic preparation may be produced by stressing a co-culture including proliferative cells. The co-culture of cells may be obtained by growing first culture to less than one-hundred percent confluence on a surface. After a monolayer of first culture is established, a second culture may be seeded onto at least one cell free area on the surface, the resulting co-culture grown to less than one-hundred percent confluence. Additional cultures may then be seeded onto cell free areas of the surface and established until a monolayer having the desired population of cells is obtained. The monolayer is then stressed to obtain a serum by conditioning a collection medium. The obtained serum may be combined with a suitable cosmetic base to provide a cosmetic preparation.