A startling image of the clinical sparseness of the skeleton is often created when we think of bones. The importance of bones providing the framework structure in the body is clear, but their complexity and exact nature is not always fully appreciated.

Bone is in fact living tissue that is constantly being renewed and removed through the bone turnover cycle, which is carried out by the bone cells.  As a consequence, they are as vulnerable to disorders and disease as other body tissues, the most prevalent disease being osteoporosis.  We will look at how osteoporosis develops, but first of all a closer look at the bones themselves is required.

Bone composition and growth

Bone is composed primarily of the minerals calcium and phosphorus and the protein collagen.  It is the calcium and phosphorus component of bone that makes the bones hard and rigid and the arrangement of the collagen fibres that make it strong.

Throughout childhood, cartilage is converted to bone, a process which is stimulated by the growth hormone which causes them to lengthen.  Bones stop growing in length by the age of 16-18, but continue to grow in strength (density) and thickness into the mid 20s.

When new bone is formed, it is coated with the minerals, namely calcium and phosphorus which form the mineral salt hydroxyapatite, and with lining cells and the protein collagen.

Bone structure

The bone lattice consists of two layers.  The outer corticol bone and the inner trabecular bone, which is covered with a thin membrane, the periosteum, where pain can generate.

The corticol or compact bone is a hard, dense shell, comprised of columns of cells, each with a central hollow, known as a Haversion canal.  These canals are important for the nutrition, growth and the repair of bone.

The trabecular or spongy bone is a mesh-like structure filled with collagen (protein), calcium salts and other minerals.  The spaces of the honeycomb structure contain blood vessels and bone marrow, the fatty tissue in which blood cells are formed. 

Corticol bone represents 75% of the bone in the body, trabecular the remaining 25%.

Bone turnover

The bone lattice is continuously being removed and replaced.  10% of the bone mass is restored each year through this essential cycle and adults renew their entire skeleton every 7-10 years.

There are two different types of bone cells responsible for this remodelling of bones; the osteoblasts which are responsible for bone formation and the osteoclasts which are responsible for bone resorption (removal).

The osteoclasts remove the mineral compound and protein matrix from the bone, in a one stage process, assisting in the breakdown.  The resulting cavity on the surface is then refilled by the action of the osteoblasts which assist bone formation.

This occurs in two stages.  Firstly the osteoblasts produce collagen fibres and other components of the matrix, which becomes the osteoid, and secondly they harden the matrix by encouraging the deposition of the minerals on to the protein framework to form new bone.

The actions of these cells are under hormonal control.  This has great significance when considering the risk factors and treatments for the disease.  The hormones involved include growth hormone secreted by the pituitary gland, the sex hormones oestrogen and testosterone, the adrenal hormone, parathyroid hormone and the thyroid hormone, thyrocalcitonin.

20/05/2009