Multipotential (osteogenic, adipogenic, chondrogenic, and myelosupportive) cells associated with the bone marrow stroma are revealed by in vitro or in vivo differentiation assays. If considered in the context of development, growth, and adaptive changes of bone as an organ, the hierarchical organization, histophysiology, and biological significance of the so-called" stromal system" appear distinct from those predicted from the commonly used analogy with the hematopoietic system, with which the stromal system and its putative" stem" cell are usually compared. The plasticity of differentiated phenotypes and the emergence of individual lineages in a defined temporal succession throughout development and postnatal life reflect the role of the multipotential cells in the stromal system in tissue adaptation and growth, rather than in cell consumption and replacement. This makes the stromal system and its progenitors an interesting paradigm of the biology of an individual cell's flexibility in complex organisms.