Figure 6.3 Endochondral ossification of a long bone. a. A cartilaginous model develops during fetal development. b. A periosteum develops. c. A primary ossification center contains spongy bone surrounded by compact bone. d. The medullary cavity forms in the diaphysis, and secondary ossification centers develop in the epiphyses. e. After birth, growth is still possible as long as cartilage remains at the epiphyseal plates. f. When the bone is fully formed, the remnants of the epiphyseal plates become a thin line.

Bone Growth and Repair

Bones are composed of living tissues, as exemplified by their ability to grow and undergo repair. Several different types of cells are involved in bone growth and repair:
     Osteoprogenitor cells are unspecialized cells present in the inner portion of the periosteum, in the endosteum, and in the central canal of compact bone.
     Osteoblasts are bone-forming cells derived from osteoprogenitor cells. They are responsible for secreting the matrix characteristic of bone.
     Osteocytes are mature bone cells derived from osteoblasts. Once the osteoblasts are surrounded by matrix, they become the osteocytes in bone.
     Osteoclasts are thought to be derived from monocytes, a type of white blood cell present in red bone marrow. Osteoclasts perform bone resorption; that is, they break down bone and assist in depositing calcium and phosphate in the blood. The work of osteoclasts is important to the growth and repair of bone.

Bone Development and Growth
The term ossification refers to the formation of bone. The bones of the skeleton form during embryonic development in two distinctive ways-intramembranous ossification and endochondral ossification. In intramembranous ossification, bone develops between sheets of fibrous connective tissue. Cells derived from connective tissue become osteoblasts that form a matrix resembling the trabeculae of spongy bone. Other osteoblasts associated with a periosteum lay down compact bone over the surface of the spongy bone. The osteoblasts become osteocytes when they are surrounded by a mineralized matrix.

The bones of the skull develop in this manner. Most of the bones of the human skeleton form by endochondral ossification. Hyaline cartilage models, which appear during fetal development, are replaced by bone as development continues. During endochondral ossification of a long bone, the cartilage begins to break down in the center of the diaphysis, which is now covered by a periosteum (Fig. 6.3). Osteoblasts invade the region and begin to lay down spongy bone in what is called a primary ossification center. Other osteoblasts lay down compact bone beneath the periosteum. As the compact bone thickens, the spongy bone of the diaphysis is broken down by osteoclasts, and the cavity created becomes the medullary cavity. After birth, the epiphyses of a long bone continue to grow, but soon secondary ossification centers appear in these regions. Here spongy bone forms and does not break down. A band of cartilage called an epiphyseal plate remains between the primary ossification center and each secondary center. The limbs keep increasing in length and width as long as epiphyseal plates are still present. The rate of growth is controlled by hormones, such as growth hormones and the sex hormones. Eventually, the epiphyseal plates become ossified, and the bone stops growing.

Endochondral ossification of a long bone

Remodeling of Bones
In the adult, bone is continually being broken down and built up again. Osteoclasts derived from monocytes in red bone marrow break down bone, remove worn cells, and assist in depositing calcium in the blood. After a period of about three weeks, the osteoclasts disappear, and the bone is repaired by the work of osteoblasts. As they form new bone, osteoblasts take calcium from the blood. Eventually some of these cells get caught in the mineralized matrix they secrete and are converted to osteocytes, the cells found within the lacunae of osteons. Strange as it may seem, adults apparently require more calcium in the diet (about 1,000 to 1,500 mg daily) than do children in order to promote the work of osteoblasts. Otherwise, osteoporosis, a condition in which weak and thin bones easily fracture, may develop.

Bone Repair
Repair of a bone is required after it breaks, or fractures. Bone repair occurs in a series of four steps:
1. Hematoma. Within six to eight hours after a fracture, blood escapes from ruptured blood vessels and forms a hematoma (mass of clotted blood) in the space between the broken bones.
2. Fibrocartilaginous callus. Tissue repair begins, and fibrocartilage fills the space between the ends of the broken bone for about three weeks.
3. Bony callus. Osteoblasts produce trabeculae of spongy bone and convert the fibrocartilaginous callus to a bony callus that joins the broken bones together and lasts about three to four months.
4. Remodeling. Osteoblasts build new compact bone at the periphery, and osteoclasts reabsorb the spongy bone, creating a new medullary cavity.
In some ways, bone repair parallels the development of a bone except that the first step, hematoma, indicates that injury has occurred, and then fibrocartilage instead of hyaline cartilage precedes the production of compact bone. The naming of fractures describes what kind of break occurred. A fracture is complete if the bone is broken clear through and incomplete if the bone is not separated into two parts. A fracture is simple if it does not pierce the skin and compound if it does pierce the skin. Impacted means that the broken ends are wedged into each other, and a spiral fracture occurs when the break is ragged due to twisting of a bone.

Surface Features of Bones
The various bones of the skeleton, refer to Table 6.1, which lists and explains the surface features of bones.