Vasculature-Associated Cells Expressing Nestin in Developing Bones Encompass Early Cells in the Osteoblast and Endothelial Lineage

Nestin-positive (Nes(+)) cells are important hematopoiesis-supporting constituents in adult bone marrow. However, how these cells originate during endochondral bone development is unknown. Studies using mice expressing GFP under the direction of nestin promoter/enhancer (Nes-GFP) revealed distinct endothelial and nonendothelial Nes(+) cells in the embryonic perichondrium; the latterwere early cells of the osteoblast lineage immediately descended from their progenitors upon Indian hedgehog action and Runx2 expression. During vascular invasion and formation of ossification centers, these Nes(+) cells were closely associated with each other and increased in number progressively. Interestingly, cells targeted by tamoxifen-inducible cre recombinase driven by nestin enhancer (Nes-creER) in developing bone marrow were predominantly endothelial cells. Furthermore, Nes(+) cells in postnatal bones were heterogeneous populations, including a range of cells in the osteoblast and endothelial lineage. These findings reveal an emerging complexity of stromal populations, accommodating Nes(+) cells as vasculature-associated early cells in the osteoblast and endothelial lineage.