PTH and hyperparathyroidism: In cancellous bone, the BMU is separated from the marrow by a canopy of flat cells in close association with blood vessels lying just outside the canopy.(1,2) Hypoparathroidism and hyperparathyroidism lead to a decrease or increase in the number of BMUs, respectively, consistent with a major role of parathyroid hormone (PTH) in the rate of bone remodeling.(3,4) A remarkable feature of PTH-regulated remodeling in cancellous bone is that the number of osteoblasts recruited to the BMU is sufficient to completely refill the resorption cavity.