Vessels of rejuvenation

The identification of specialized endothelial-cell populations in the blood vessels of bones, and their signalling pathways, reveals how the vasculature contributes to bone formation. See Article p.323 & Letter p.376 There is evidence to suggest that blood vessels, particularly their endothelial cells, control the growth, homeostasis and regeneration of organs. In two papers published in this issue of Nature, Ralf Adams and colleagues demonstrate that the bone vasculature contains endothelial cells specialized to support bone maturation and regeneration. Anjali Kusumbe et al. identify a capillary subtype in the mouse skeletal system that has a key role in mediating bone growth. These vessels contain so-called type H endothelial cells that preferentially associate with osteoprogenitors and are reduced during ageing. Hypoxia-inducible factor 1α (HIF-1α) is shown to be crucial in maintaining the type H cells, and the fact that these cells are lost in aged animals suggests that loss of HIF-1α signalling may be involved in age-related bone changes. In the second paper, Saravana Ramasamy et al. show that blood vessel growth in bone requires Notch signalling and involves a specialized form of angiogenesis that does not involve endothelial sprouts.