Final answer:
During the bony callus phase of bone fracture healing, ossification replaces the fibrocartilaginous callus with spongy bone, primarily through the action of osteoblasts over two months.
Step-by-step explanation:
During the bony callus stage of healing a bone fracture, ossification occurs, which is the process of converting the previously formed fibrocartilage into bone tissue. This involves the formation of a bony callus of spongy bone, which takes about two months to join the broken bone ends together. Osteoblasts, osteoclasts, and bone matrix are present during this stage. Osteoblasts play a crucial role in this stage by depositing new bone matrix, resulting in the formation of spongy bone that eventually replaces the fibrocartilaginous callus. This transformation mirrors the endochondral ossification that takes place during the normal development of bone, and it generally takes about two months for the broken bone ends to become firmly rejoined.