The cell body of a single-celled organism, such as an amoeba, is structured to perform all the functions necessary to survive. It can move, ingest and digest food particles and it can reproduce itself. If in larger organisms the cells continued to work in such an independent way, life as we know it would be impossible. The work needed to sustain a multicelled organism can be done much more efficiently if it is divided between cells that have specialized in only one or a few function. Cells dealing with movement, for example, can perform this function much better if they do not also have to be engaged in the complicated process of acquiring and digesting food. Of course, this implies that other cells need to specialize as well: to supply those ‘muscle’-cells with ready-to-use fuel. The origin of a human being is a single cell (the fertilized ovum) and every cell is ultimately derived from it. This means that no matter how diverse the functions of different cells, they all contain the same genetic material. At a very early stage during embryo-logical development, cells start to differentiate. In an embryo of a size smaller than 1 mm, for example, it is already predetermined which cells will form the central nervous system.

Cells that have specialized in performing the same function usually lie in groups. Such a collection of cells is called a tissue. Depending on the task they have, tissues are structured differently. The covering tissue, or epithelium, of the skin, for instance, is necessary to protect the body from potentially harmful substances. That is why the cells are closely attached to each other. In the supporting connective tissue, the cells are more isolated. By secreting fibrous substances into the intercellular spaces they give the tissue a firm structure.

The formation of tissue is only one step on the road to ultimate specialization. To perform more complex tasks, various tissues have to combine to form an organ. Combinations of different organs in their turn form organ systems, such as the cardiovascular system (heart and bloodvessels), the urinary system or the digestive system.

A human being is made up of organ systems, all doing their own work with extreme efficiency, under the direction of the central nervous system. Such a division of labour means that the total performance of an organism is much more than just the combined capabilities of its components.

There is a danger, however, that if some specialized structures are destroyed (for example, as a result of disease), the life of the individual as a whole is threatened. This is especially the case for what are known as permanent tissues. Nerve cells for example cannot divide and thus are not able to replace damaged nervous tissue. One might suppose that nearby cells of the supportive tissue could change into nerve cells to repair the damage. After all, every body cell is derived from only one cell and they all have the same genetic material. This is not the case however: differentiation apparently also implies that some innate properties get lost.