This article is not about prison reform, death in police custody or design of medieval monasteries. Instead the cells that it concerns are the living cells that make up our bodies. Most readers will be aware that estimates of the number of human bodies on the planet reached 7 billion in 2011 and none of us has difficulty recognizing 7 billion as a simply enormous number. Therefore it may come as a surprise to discover that 7 billion cells would make up only the terminal joint of my index finger (Figure 1.1). The total number of cells in the human body is best estimated at 100 trillion, 1014. The inevitable conclusion from this is that cells are extremely small, with occasional conspicuous exceptions, like an ostrich egg, which begins as a single fertilized egg and is thus an enormous single cell.

This chapter starts by a simple introduction to the beauty and fascination of living cells. They are responsible for building all the tissues of the body, including blood, nerves, muscle, bone, yet they are all formed by progressive specialization from the cells generated by division of a single fertilized egg. This poses two extraordinary challenges. The first is the nature of the molecular mechanisms that allow cells to diverge and to specialize to fulfil particular functional niches, but the full details of these mechanisms lie outside the scope of this chapter. The second challenge is that of producing stable and balanced numbers of each type of cell within the body. How are the ratios of blood cells to nerve cells or cells that line our gut balanced and managed? This question is made all the more acute by the fact that different types of cell persist in the body for very different lengths of time. Thus the cells that line our intestines, or the cells that line the ducts that carry digestive juices from our pancreas into the gut, survive for only a few days before they are replaced by new counterparts. In contrast most of our nerve cells persist throughout our adult lifetime. Although some types of nerve cell can be formed during our lifetime, others cannot and most remain with us throughout adult life. This poses an extraordinary challenge of bookkeeping and management of cell production and replacement.