ephrinB2 and EphB4 are Expressed on the Chondrocyte during Fracture Healing

Vascular formation is an essential process for bone regeneration, and several molecules important for its attainment have been discovered. ephrinB2, a trans-membrane protein, is one of the ligands of EphB4, a receptor of the ephrin family. These molecules have tyrosine kinase activities and regulate endothelial proliferation and differentiation in vascular formation in addition to axon guidance in nerve development. Recently, ephrinB2 mRNA expression was identified in the articular cartilage of chick embryos. These data let us hypothesize that ephrinB2-EphB4 interaction may be involved in mammalian cartilage and bone formation. We investigated these expressions of mRNA of these molecules during fracture healing, and the result of in situ hybridization showed that both ephrinB2 and EphB4 were co-localized in the cartilage, especially in the proliferating zone during the reparative phase. The results of immuno-histochemistry indicated that proteins of ephrinB2 and EphB4 were seen in the chondrocyte. On the other hand, in vitro, we used ATDC5 to investigate the expression of ephrinB2 and EphB4 in the chondrification, and those expressions reached their peak during the proliferating phase. This suggests that ephrinB2 expresses in the arterial endothelial cell, EphB4 expresses in the venous endothelial cell, and together form the boundary in the capillaries. These results indicated that ephrinB2 and EphB4 are involved in proliferation of the chondrocyte and that they have other functions such as the boundary formation between vascular and non vascular areas, for example, between bone and cartilage.