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| On the left is normal lamellar bone. Adjacent to this, separated by artifact, is fatty marrow with aggregations of cells around vessels. These cells act as the reserve that responds in situations of need by proliferation of cells to produce a blastema, the fibrovascular engine for the rapid production of bone. Endothelial cells proliferate and bud, forming new capillary arrays. Perivascular stem cells are stimulated to become cells of the osteoblast line, ultimately making bone. Situations of need include fracture, infection, rapid growth of tumors. | ||||