Cancellous bone remodeling occurs in specialized compartments lined by cells expressing osteoblastic markers

We describe a sinus, referred to as a bone remodeling compartment (BRC), wh ich is intimately associated with cancellous bone remodeling. The compartme nt is lined on its marrow side by flattened cells and on its osseous side b y the remodeling bone surface, resembling a roof of flattened cells coverin g the bone surface. The flat marrow lining cells are in continuity with the bone lining cells at the margins of the BRC. We examined a large number of diagnostic bone biopsy specimens received during recent years in the depar tment. Furthermore, 10 patients (8 women and 2 men, median age 56 [40-69] y ears) with the high turnover disease of primary hyperparathyroidism who wer e treated with parathyroidectomy and followed for 3 years were included in the histomorphometric study. Bone samples for the immuno-enzyme staining we re obtained from an amputated extremity of child. The total cancellous bone surface covered by BRC decreases by 50% (p < 0.05) following normalization of turnover and is paralleled by a similar 50% decrease in remodeling surf ace (p < 0.05). The entire eroded surface and two-thirds of the osteoid sur face are covered by a BRC. BRC-covered uncompleted walls are 30% (p < 0.05) thinner than those without a BRC. This indicates that the BRC is invariabl y associated with the early phases of bone remodeling, that is, bone resorp tion, whereas it closes during the late part of bone formation. Immuno-enzy me staining shows that the flat marrow lining cells are positive for alkali ne phosphatase, osteocalcin, and osteonectin, suggesting that they are bone cells. The first step in cancellous bone remodeling is thought to be the l ining cells digesting the unmineralized matrix membrane followed by their d isappearance and the arrival of the bone multicellular unit (BMU). We sugge st that the lining cell barrier persists during bone remodeling; that the o ld lining cells become the marrow lining cells, allowing bone resorption an d bone formation to proceed under a common roof of lining cells; that, at t he end of bone formation, new bone lining cells derived from the flattened osteoblasts replace the marrow lining cells thereby closing the BRC; and th at the two layers of lining cells eventually becomes a single layer. The in tegrity of the osteocyte-lining cell system is reestablished by the new gen eration of lining cells. The BRC most likely serves multiple purposes, incl uding efficient exchange of matrix constituents and minerals, routing, moni toring, or modulating bone cell recruitment, and possibly the anatomical ba sis for the coupling of bone remodeling.