Em. Hauge et al., Cancellous bone remodeling occurs in specialized compartments lined by cells expressing osteoblastic markers, J BONE MIN, 16(9), 2001, pp. 1575-1582
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.