During embryonic and early post-natal development, many cells migrate over substantial distances, changing their spatial relationship to other cells of the same or distinct differentiation state and engaging in cross-talk that helps establish the tissue and organ structure of the adult. However, once this body plan has been established, such active migration and positional exchange is rare in the undiseased state. Even in epithelial tissues that undergo constant renewal, there is an absence of free cell movement; rather, underlying cells replicate and push the older cells to the surface (skin) or apex (villus) where they are sloughed, having followed a fixed path of movement from birth to death.