8 Development and types of bone

Learning objectives

After reading this chapter you should understand:

• the formation of bone

• the role of vitamin D in bone formation

• the structure of bone

• the different types of bone

• the growth of a long bone

The skeletal system is made up of just over 200 bones, joined together to provide a strong, movable, living framework for the body.

The skeletal system has four main functions:

1. It supports and protects the surrounding soft tissues and vital organs.

2. It assists in body movement by giving attachment to muscles and providing leverage at joints.

3. It manufactures blood cells in the red bone marrow.

4. It provides storage for mineral salts, particularly phosphorus and calcium.

This chapter looks at the developmental processes undergone by bone in the production and maintenance of the human skeleton. The ways different types of bone can be described anatomically are also outlined.

Cartilage provides the environment where bone develops. Development is from spindle-shaped cells, called osteoblasts, which are found underneath the tough sheath of fibrous tissue that covers the bone, called the periosteum, and in cavities within the bone. These cells are able to convert the soluble salts of calcium and magnesium in the blood, such as calcium chloride, into insoluble calcium salts, chiefly calcium phosphate. A second set of cells is also present; these can change the insoluble calcium phosphate into soluble calcium salts, which are carried away by the blood. These cells are capable of causing the absorption of any unwanted bone and are known as osteoclasts. Both types of cell are active during the growth period: osteoblasts produce bone and osteoclasts remove it to maintain the form and proportion of the bone. For example, osteoblasts build up bone on the surface of a hollow bone, while osteoclasts absorb bone on the inner side to enlarge the cavity and prevent the bone from becoming too heavy.

A loss of bone density due to excessive absorption of bone material is called osteoporosis; it leads to a weakening of bones, making them more susceptible to fracture. Loss of bone density is a feature of ageing, especially in postmenopausal women. Mild exercise in older people can help to reduce the effects of osteoporosis.

Ossification

There are two types of ossification: intramembranous and intracartilaginous.

Intramembranous ossification

• This is a process where dense connective tissue is replaced by deposits of calcium salts, forming bone.

• The bones of the skull form by this process.

Intracartilaginous ossification

• In this process cartilaginous structures are replaced with bone.

• Most bones form by this process.

Bone growth and repair

Plentiful supplies of calcium and phosphorus are essential in the diet of the pregnant and nursing mother, the growing child, and where bone repair is taking place after injury or disease. Calcium is present in milk and eggs and in green vegetables, and phosphorus in meat, fish and the yolks of eggs. Vitamin D is necessary so that calcium and phosphorus may be absorbed from the intestine and fully used within the body; lack of vitamin D causes rickets in children and osteomalacia in adults. Both these conditions cause soft bones, which bend under the weight of the body, and various deformities of the weight-bearing bones may result. Vitamin D is found in animal fat and fish oils and in margarine to which it has been added.

The human body can also manufacture vitamin D – the action of ultraviolet rays from the sun converts ergosterol in the skin into vitamin D.

Vitamin C is important in bone growth as it plays a part in laying down collagen, which is the main constituent of connective tissue. Vitamin C is found in fresh fruit (particularly citrus types), blackcurrants, green vegetables, tomatoes and potatoes.

Growth and development of bone are also affected by exercise and rest. Exercise causes an increased blood supply to the muscles and therefore to the underlying bones. Because the blood carries necessary building materials, exercise results in increased growth, and recognition of this fact is responsible for attention to physical exercise in schools. The pull of developing muscles is an important factor in determining the shape of the bones, and posture may also affect shape by altering stresses on bones. The importance of rest is interesting. The bones of a child are comparatively elastic so that at the end of a day of standing and running about, there is an appreciable loss of height. During rest the bones recover their full length, therefore, a midday rest and a long night will help growth.

A further factor controlling growth is the secretion of certain ductless glands. This was mentioned in Chapter 7.

Types of bone tissue

Bone tissue is of two types: compact and spongy.

Compact bone appears to be solid but when it is examined microscopically it is found to consist of haversian systems (Fig. 8.1), which have the following components:

• a central canal, called a haversian canal, which contains blood vessels, nerves and lymphatics

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