Bone is formed by the process known as ossification. Ossification refers either to the process of bone formation, or to the conversion of fibrous tissue or cartilage into bone. It starts as early as the second month of development in the womb.
Some bones, including the flat bones of the skull, form directly in membranes. This process is known as direct or intramembranous ossification.
The short and long bones are formed by the gradual replacement of hyaline cartilage, from the foetal stage of development up until full maturation in the late teens. This process is known as indirect or endochondrial ossification.
Below is outlined the process of endochondrial ossification in a long bone:
- Bone forming cells called osteoblasts first appear in the centre of the shaft (diaphysis) and surround themselves with calcium and phosphate ions supplied by the blood. Blood vessels then invade the calcified cartilage and a cavity begins to form once the cartilage is replaced by bone. When the osteoblast becomes rooted in the lacuna of the bone matrix, it becomes what is called an osteocyte (bone cell).
- At birth most of the diaphysis consists of bone, and bone has started to appear in the epiphyses. On the outside of the bone, periosteal ossification continues – bone becomes wider as a result of the osteoblasts laying down new layers of bone tissue in the deeper layers of the periosteum.
- A disc of bone separates the bone in the diaphysis from the bone in the epiphysis, and it is called the epiphyseal disc. As the child grows, it is only at this place where an increase in the length of the bone can take place. On the outside edge of the bone, periosteal ossification continues. At this stage and until the epiphyseal plate seals in early adulthood, the activity of bone cells is controlled by growth hormone, produced by the pituitary gland. From the teenage years, the sex hormones oestrogen and testosterone also stimulate bone growth.
- When growth stops, the bony diaphysis is amalgamated with the epiphysies, and where they join is a layer of bone called the epiphyseal plate. The age at which they unify varies, but long bones normally stop growing in late adolescence. Throughout the development of the long bone, parts of the bone are reabsorbed so that unnecessary calcium phosphate is removed and structures such as the medullary canal are created. Specialist cells called osteoclasts perform this job.
Remodelling is the ongoing replacement of old bone tissue by new bone tissue, and redistribution of bone tissue along lines of mechanical stress. Damage or injury to the epiphyseal plate may result in abnormalities in bone growth. If the entire width of the plate is affected, bone growth in terms of length may be retarded, or will stop. If part of the plate is affected, bone growth will be unequal across the plate, resulting in abnormal shape or deformity. This is why heavy resistance training or high impact activities are not recommended for children or teenagers – their epiphyseal plates have not yet fused and their skeletons are considered immature, which may therefore be vulnerable to damage.
The process of ossification in a short bone takes place from the centre of the bone and radiates outwards.
Flat and irregular bones are developed in one stage directly from connective tissue. The sutures of the cranium gradually ossify during the development of a young child into adulthood, as do the clavicle, scapula and pelvis. At ossification centres within the membrane, osteoblasts produce bone tissue along the membrane fibres to form cancellous bone. Beneath the periosteum, osteoblasts lay down compact bone to form the outer surface of the bone.