When discussing mechanisms of ageing it is important to distinguish between the different components which make up living tissue, as the predominant ageing mechanism for each is going to be different. It is unlikely that the ageing process is governed by one universal mechanism. Human tissue is composed of a mixture of long-lived structural proteins (such as crystallin and collagen), post-mitotic cellular elements (such as mature neurones and muscle fibre cells) and mitotic cells (such as T-cells, endothelial cells and fibroblasts). The gradual biological alterations which drive the ageing process cause changes to occur in all the components of this complex mixture. However, the mechanisms by which each component degenerates are often distinct and since all these components interact with each other, a change in one will have a direct impact on another.