How are the billions of cells in our bodies made? In plants and animals, there are cells capable of producing every type of cell the organism will need from birth to death. Once an organism begins to mature, other cells are required to produce the range of specialized cells needed for a functioning organ or tissue. Such cells are known as stem cells. As technology has developed and the micro-dissection of tissue became possible, it was evident that most, if not all, organs and tissues in the body have their own stem cells which are capable of dividing and differentiating into mature functional cells. In some ways, stem cells can be imagined as blank canvases with many hidden cellular pictures already imprinted on them. Different combinations of proteins (growth factors) or other stimuli such as fats or sugars that touch the cell can stimulate division and allow the daughter cells to take on changed characteristics. In this chapter we look briefly at the types of stem cell and where they come from.
In simple terms, there are two major types of stem cells—embryonic and adult ( Figure 16 ). Embryonic stem cells have different biological properties to the adult stem cells that are found close to and after birth, hatching, or germination. The archetype of stem cells in development is the zygote, produced by the fusion of egg and sperm, which has the complete ‘potency’ to generate every tissue and cell type in the body (a property termed‘totipotent’). As the embryo grows, ‘pluripotent’ stem cells appear, which are limited in their potency. These cells divide and differentiate into the main classes of ES cells (known as the germ layers in mammals) and then, in turn, develop into organs and tissues. From embryo to adult, our cellular growth relies on: ‘multipotent’ cells that give rise, after division and differentiation, to cell types belonging to a related cell family; ‘oligopotent’ stem cells that are more limited still, producing just a handful of closely related cells, for example myeloid blood cells; and ‘unipotent’ stem cells that will give rise to only one cell type, for example muscle cells. The term progenitor or progenitor stem cell is often used to describe those cells still rapidly dividing but not as yet fully differentiated. Adult progenitor stem cells repair tissues by producing specific cell types needed to maintain normal turnover of regenerative organs.