Internal vascularity of rodent bone, its connection with the marrow cavity, and its association with osteogenic cells. (A) Contact microradiograph of a 50-μ section of an 8-week-old rat tibia demonstrating a rich network of intracortical vessels opening in the marrow at the endosteal surface (white arrows). Also note individual vascular spaces looping within bone and reentering the BM (black arrows). (B) Low-temperature processed, glycol methacrylate-embedded, 4-μ thick undecalcified sections of an 8-week-old wild-type mouse tibia stained for ALP activity (blue). Note that intracortical vessels are coated with an adventitial layer of ALP-positive cells.