1 Bone

Chapter contents

Structure of bone

Ossification

Types of bones

‘Normal’ radiographic bone appearances

Terminology

Bone is a highly vascular connective tissue, in which bone cells are enclosed in a mineralised intercellular matrix interposed with a system of collagenous fibres.

The organic matrix forms one-third of the structure and gives resilience and a degree of flexibility; the mineral salts (mainly calcium and phosphorus) form the remaining two-thirds and provide the strength and weightbearing capabilities of the bone.

Structure of bone

There are two main types of bone: compact (dense) and cancellous (spongy). Compact bone forms the surface layers or cortex of mature bones and cancellous bone the interior aspect.

Compact bone

This type of bone is found mainly in the shafts of long bones where a strong, tubular structure is required. It consists of a number of cylindrical structures called haversian systems (Figs 1.1 and 1.2). Each system comprises:

Fig. 1.1 Transverse section of bone (microscopic).

A – Haversian canal

B – Canaliculi

C – Circumferential lamella

D – Interstitial lamella

E – Lacuna

F – Lamella.

Fig. 1.2 Longitudinal section of bone (microscopic).

1 – Haversian canal

2 – Volkmann’s canal

3 – Lacuna

4 – Canaliculi.

A central haversian canal –

which contains blood, lymphatic vessels and nerves.

Lamellae –

rings of bone round the haversian canal.

Lacunae –

spaces between the lamellae, which contain osteocytes (mature bone cells).

Canaliculi –

channels carrying nutrient fluid, which connect the lacunae and communicate with the haversian canal.

Interstitial lamellae –

fill the spaces between adjacent haversian systems.

Circumferential lamellae –

rings of bone round the edge of the bone.

Volkmann’s canals –

join the various haversian canals.

Periosteum –

a membrane which surrounds the bone, except at the articular surfaces, where articular hyaline cartilage allows friction-free movement. The inner layer is vascular and cellular providing nutrition, growth and repair. The outer layer is fibrous and blends with tendons and ligaments.

Cancellous bone

This type of bone is found in the parts of bones where lightness, strength and area are required. It is similar in structure to compact bone but bone marrow is found between the trabeculae which form the internal support structure of the bone.

Bone marrow

The medullary cavity of bone and the spaces between the trabeculae of cancellous bone are filled with bone marrow. At birth this is red bone marrow, which produces red and white blood cells.

In adults, active red bone marrow is found only in the:

upper femora vertebrae scapulae sternum

ribs clavicles diploë of skull bones hip bones.

Elsewhere the red bone marrow becomes inactive yellow marrow.

Development

Some bones develop from rods of cartilage, e.g. the bones which form the limbs, the trunk and the base of the skull. Some bones develop from membranes, e.g. bones of the vault of the skull, the face and the clavicle. Some bones develop in tendons, e.g. sesamoid bones – the patella and fabella.

Ossification

Ossification is the formation of bone from connective tissue and requires:

adequate calcium and phosphate in the blood

a supply of vitamins A, C and D.

Growth of the bone is influenced by the following hormones:

Parathormone (from the parathyroid glands) –

controls the level of calcium, and indirectly the level of phosphates in the blood.

Growth hormone (from the anterior lobe of the pituitary gland) –

influences growth and replacement of bone tissue.

Thyroxine (from the thyroid gland) –

influences normal physical development.

Testosterone (in the male) and oestrogen (in the female) –

influences normal skeletal growth especially at puberty.

Weightbearing and exercise also stimulate bone growth and general ill health inhibits it.

Intramembranous ossification

This takes place in a membrane, e.g. in the vault of the skull in the developing fetus. At the point of ossification, osteogenic fibres and bone cells appear in the connective tissue and calcium salts are deposited to form osteoid tissue. Ossification spreads from the centre outwards.

Intracartilaginous ossification

The process by which bone formation takes place, e.g. in a long bone (Fig. 1.3).

Fig. 1.3 Section through a developing long bone.

A – Diaphysis

B – Epiphysis

C – Epiphyseal plate

D – Metaphysis

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