HINDLIMB UNLOADING IN RAT DECREASES PREOSTEOBLAST PROLIFERATION ASSESSED IN-VIVO WITH BRDU INCORPORATION

Immobilization affects hone formation. However, the mechanisms regulat ing the decrease in osteoblast recruitment remain unclear. The aim of our study was to determine in vivo osteoblastic proliferation after sh ort-term immobilization among the different bone compartments. Twelve Wistar 5-wk-old rats were assigned to two groups: six tail-suspended a nimals for 6 days and their six age-related controls. Osmotic minipump s, each containing 40 mg of bromodeoxyuridine (BrdU), were implanted i ntraperitoneally at day 4 until euthanasia. Histomorphometric measurem ents found a significantly lower bone volume in primary (ISP, -22%) an d secondary spongiosa (IISP, -37%) in unloaded rats compared with thei r age-related controls. BrdU immunohistochemistry showed that the prol iferation capacity of osteogenic precursors in ISP (-29%) and preosteo blasts in IISP (-80%) and in periosteum as well as bone marrow cells ( -40%) was lowered by unloading. We demonstrated in vivo for the first time that B-day tail suspension induced a significant decrease in prol iferation of periosteal and trabecular preosteoblasts in ISP and IISP as well as in bone marrow cells.