A 5 year old boy falls off his bike and fractures his humerus. He is taken to the emergency room , and the bone is set by one of the emergency room physicians. Which of the following is responsible for producing the majority of the new bone that will reunite the two fragments?

When the periosteum is torn during a fracture, it supplies cells that develop into osteoblasts and are the major producers of the new bone that reunites the two ends. Heterotopic ossification (bone formed outside the regular bone) can occur as a complication of fracture if some of the osteoblastic cells are misdirected into adjacent tissues. Preexisting cancellous bone and compact bone are not the major source of osteoblasts that form the new bone. Cailage and marrow do not contribute to new bone formation after fracture. Ref: Mescher A.L. (2013). Chapter 8. Bone. In A.L. Mescher (Ed), Junqueira's Basic Histology, 13e.