1051. BMP2 Induced Chondrogenesis: Can Monocytic Progenitors Differentiate into Chondrocytes?

Abstract

A major reason for bone graft failure is inadequate vascularization. To address this deficiency, we studied de novo formation of blood vessels and cartilage in a murine model of bone formation, in which newly formed cartilage is apparent within 4 |[ndash]| 5 days after intramuscular injection of cells expressing BMP2. Within 24 hours after BMP2 induction, endothelial cells (ECs) began proliferating, as did vascular smooth muscle cells (VSMCs). This correlated witha rapid, transient increase in the expression of several VEGFs within the lesional area. Within 72 hours, ECs and some of the VSMCs had co-localized in muscle tissue, indicative of the formation of new vessels. However, most of the VSMCs remained as conglomerate of |[ldquo]|mesenchymal-like|[rdquo]| cells expressing smooth muscle alpha actin (SMA), smooth muscle myosin heavy chain (SMMHC) and CD68, a monocytic marker. This expression was lost by days 4-5 coincident with upregulation of Sox9, and appearance of chondrocytes. Studies using a Cre/Lox system in which a myeloid specific promoter driving Cre recombinase can irreversibly unblock expression of |[beta]|- galactosidase only in cells of myeloid origin, showed specific activity in the newly formed chondrocytes. These results suggest that vascular expansion is a very early event in bone formation, preceding chondrogensis and involves a myelo-monocytic-mesenchymal progenitor which contributes to neovascularization and chondrogenesis. These factors may be critical to engineering vascularized bone for grafting.