It is well known that, in addition to providing nutrients to growing tissues, blood vessels in the developing embryo can play more active roles in directing morphogenesis, patterning and differentiation – primarily through the secretion of signalling molecules. In bone, invasion of blood vessels precedes osteogenesis, and endothelial-derived signalling factors have been shown to regulate ossification. On p. 3933 Elazar Zelzer and co-workers now identify another role for the vasculature in controlling bone morphogenesis. They find that collagen I, the main extracellular matrix component that serves as a template for mineralisation, is deposited by osteoblasts onto endothelial cells within the bone. This is possible because, unlike most blood vessels, vessels within the developing bone are devoid of basement membrane. The collagen-coated vessels then serve as a template for mineral deposition, such that ossification spatially and temporally follows vascular patterning. Notably, disrupting vascularisation of the bone also disrupts bone deposition. This work establishes a previously unrecognised mechanism by which the vasculature regulates bone morphogenesis, and also raises a number of intriguing questions as to the mechanisms underlying the regulation of endothelial basement membrane deposition and the fate of mineralised vessels.
