Bone lining cells (BLC's) cover inactive (nonremodeling) bone surfaces, particularly evident in the adult skeleton. BLC's are thinly extended over bone surfaces, have flat or slightly ovoid nuclei, connect to other BLC's via gap junctions, and send cell processes into surface canaliculi. BLC's can be induced to proliferate and differentiate into osteogen ic cells and may represent a source of "determined" osteogenic precursors . BLC's and other cells of the endosteal tissues may be an integral part of the marrow stromal system and have important functions in hematopoiesis, perhaps by controlling the inductive microenvironment. Because activation of bone remodeling occurs on inactive bone surfaces, BLC's may be involved in the propagation of the activation signal that initiates bone resorption and bone remodeling. Evidence also suggests that BLC's are important in the maintenance of the bone fluids and the fluxes of ions between the bone fluid and interstitial fluid compartments for mineral homeostasis.