PROLIFERATING CELLS IN THE PRIMARY SPONGIOSA EXPRESS OSTEOBLASTIC PHENOTYPE IN-VITRO

We have shown that intermittent parathyroid hormone (PTH) treatment ta rgets proliferating cells in the primary spongiosa of trabecular bone of young rats, resulting in an increased number of osteoblasts, To fur ther characterize these proliferating osteoprogenitor cells, bromodeox yuridine (BrdUrd) incorporated in vivo, was used as a marker to identi fy and isolate cells for in vitro studies, Proliferating cells were la beled in vivo in young rats with BrdUrd and 24 h later were isolated b y trypsinization of sections of the primary spongiosa of the distal fe mur metaphysis. Within 12 h of isolation, BrdUrd+ cells formed distinc t foci containing 20-500 cells with fibroblast morphology, Stimulation of proliferation as determined by [H-3]-thymidine incorporation was o bserved for these cells in response to fetal bovine serum, platelet de rived growth factor, and transforming growth factor beta-1. Neither in sulin-like growth factor-1 (IGF-1) nor insulin stimulated proliferatio n PTH (1-34) and dexamethasone inhibited proliferation, The effects of PTH and dexamethasone were additive, Cells expressed the osteoblast p henotype as evidenced by synthesis of type I collagen, expression of h igh alkaline phosphatase activity, and production of increased intrace llular cAMP in response to PTH (1-34), Confluent cell aggregates spont aneously formed mineralized nodules within 4-7 days, in the absence of inducers, These observations suggest that the primary spongiosa cells recapitulates the differentiation process in vitro in an accelerated fashion and may serve as a useful model to study osteoblast differenti ation. (C) 1997 by Elsevier Science Inc.