Cellular Mechanisms of Bone Remodeling Evaluated at the Intermediary Level of Organization of Bone

It is now well-known that the remodeling system which operates in adult lamellar bone is a quantal phenomenon due to the coordinated activity of two continually renewing cell populations that are opposite in action: osteoclasts and osteoblasts.They are tightly coupled in space and time, as shown by H.Frostl who proposed the concept of basic multicellular unit (BMU),It is less known that the original morphological observations of H.Frost which have directly demonstrated the cellular coupling between osteoclastic resorption and osteoblastic formation were made at the intermediary organization of bone level, i.e. on the cortical basic structure units (BSU), or osteons. H.Frost made the observation that the cement line limiting the outer circumference of the osteon was always scalloped, thus representing always the limits of a preceding resorption phase. This was the first observation made at the BSU level which considerably enlightened the cellular mechanisms of bone remodeling. Despite H.Frost 2 described next year the possibility to measure the mean wall thickness (MWT) of cortical osteons and to derive from this parameter — and from the osteoblastic appositional rate — the activation frequency and the lifespan of the active osteoblastic cells, this intermediary level of organization of bone was temporarly forgotten in many laboratories. During about 15 years most studies of bone histomorphometry were concentrated on the percentage of bone surfaces occupied by formation or resorption sites, and most studies at the individual cellular level having used tissue cultures or electronmicroscopy. These studies provided informations which unfortunately cannot be extrapolated to the tissue- and organ-levels, without taking into account the intermediary level of organization of bone, i.e. the basic stucture units of cortical and trabecular bone. They represent the end-products of the osteoblastic activity and are bound on one side by the bone-marrow interface or the haversian canal, and on the other side by the scalloped cement line which represents the limits of the resorption phase (Figure 1). The average distance between the cement line and the surface of the completed packet is the mean wall thickness and is measured under polarized light. This parameter provides a direct indication of the amount of bone formed by the BMU. These basic structure units cannot be studied by any other tool than bone histomorphometry and have been compared by H. Frost3 to the nephrons of the kidney.