Final answer:
The fibrocartilaginous callus is the tissue involved in bridging the broken ends of a bone after a fracture, consisting of a mix of collagen fibers and cartilage. It forms within 48 hours post-fracture and takes about two months to convert into a bony callus made of spongy bone, with osteoblast and osteoclast activity similar to endochondral bone formation.
Step-by-step explanation:
The mass of repair tissue mentioned in the student’s question refers to the fibrocartilaginous callus, which is a key part of the bone healing process after a fracture. This callus is a mixture of collagen fibers and cartilage that serves as a bridge between the broken ends of a bone. Within approximately 48 hours following a fracture, chondrocytes from the endosteum contribute to forming this callus internally by secreting a fibrocartilaginous matrix. Concurrently, on the exterior, periosteal chondrocytes and osteoblasts form an external callus comprising hyaline cartilage and bone respectively, which stabilizes the fracture site. Over time, typically about two months, the fibrocartilaginous callus is converted into a bony callus of spongy bone, connecting the broken bone ends firmly as they are ossified by osteoblasts. The process mirrors endochondral bone formation, involving the crucial participation of osteoclasts and the bone matrix.
Following the initial stabilizing phase, the bone undergoes further healing which includes the remodeling of the bony callus by osteoclasts and osteoblasts, changing it into a more robust structure of compact bone that matches the unbroken bone's original architecture. This part of the healing process can extend over several months and, on occasion, the bone may still show signs of unevenness for years after the fracture has occurred.