Bone

Bone is the calcified component of the skeleton which in the human comprises 206 individual bones. The matrix of bone, as a rigid connective tissue, consists of collagen embedded in a ground substance on which hydroxyapatite (a complex inorganic material) is deposited. Types of bone include:

• Long bones eg the humerus
• Short bones eg the carpals
• Flat bones eg frontal bone
• Irregular bones eg vertebrae
• Sesamoid bones eg patella

At a microanatomic level there are two types of bone: cortical and cancellous. Bone can also be woven or lamellar though, and are distinguished by the oritenation of their collagen fibres. In woven bone the fibres are randomly arranged whereas in lamellar bone the collagen is parallel and arranged into sheets (lamellae), making it stronger.

Cortical bone lies on the outer portion of long bones and vertebrae. It is composed of long parallel columns (osteons) which are made up of concentric rings of bone (lamellae) surrounding a central Haversian canal containing blood and lymphatic vessels.

Buried within bone are osteocytes, which lie within lacunae. Osteocytes are terminally differentiated osteoblasts, and have a cell body and cell processes are in canaliculi. Cortical bone is covered by periosteum, which consists of fibrous tissue. The medullary cavity (centre of bones) is composed of cancellous bone, with intervening marrow.

Cortical bone is composed of functional untis called osteons. These consist of long parallel columns, each made up of concentric rings of bone (lamellae) around a central channel containing blood and lymphatic vessels. These channels are called the Haversian canals.

Cancellous bone is composed of a network of bony plates or struts called trabeulae. These connect with each other and to the endosteum of cortical bone. In adults, the spaces between the trabeculae are filled with either haemopoetic bone marrow or adipose tissue.

Bone development and growth

In the foetus, hyaline cartilage forms a provisional skeleton, which is replaced by bone during endochondral bone formation. Soon after birth and up to adolescence, hyaline cartilage is an integral component of epiphyseal growth plates, which control the growth and shape of long bones.

Endochondral ossification is the process whereby long bones elongate during development. Initially the mineralised cartilaginous matrix formed at the growth plate is replaced by woven bone, which is then remodelled into lamellar bone.

Intramembranous bone formation is the process whereby modelling and remodelling occurs at trabecular and cortical surfaces in the developing and adult skeleton. This differs from endochondral ossification in that no cartilaginous template is formed before bone deposition. Therefore, intramembranous bone can only form by appositional growth. The formation follows a temporal sequence whereby stromal and bone lining cells differentiate to form mature osteoblasts, which secrete an unmineralised organic matrix (osteoid) onto the bone surface. Following osteoid formation, the matrix undergoes and period of maturation creating an environment permissive for mineralisation.