Since birth, child caries fixed amount of stem cells of various lineages. Starting from hematopoetic stem cells, going to stem cells persisting in different tissue involved in its regeneration. During ontogenesis number of pluripotent cells is decreasing. When person reaches his 60-70 years, this number of pluripotent cells is near the critical level. Because of that old people are keen to get ill of different chronic origin illnesses. It is well known that stem cells undergo self-renewing divisions, whereby the daughter cells retain the exact same characteristics as the parent but can also give rise to more committed progenitor cells that differentiate into specific cell types (Fig. 1).

Fig. 1 Stem cells, progenitor cells and differentiated cells. Pluripotent stem cells proliferate into self-renewing stem cells. Stem cells can also divide asymmetrically to produce more committed progenitors, which in turn are destined to give rise to differentiated cells.

Nevertheless during aging stem cell number decreases and they are not so potent and "clean" as they were during our birth. Tissue-specific stem cells are found in adult tissues including blood, skin, central nervous system, liver, gastrointestinal tract, fat and skeletal muscle. Stem cells are responsible for regenerating damaged tissue and maintaining tissue homeostasis. In contrast to the pluripotent embryonic stem (ES) cells, the differentiation potential of adult stem cells has been thought to be restricted to the tissues in which they reside. Adult stem cells, however, can differentiate into a variety of tissues (Table 1). Bone marrow-derived cells not only replenish blood but can also contribute to heart, brain, liver, muscle, skin, and vascular endothelium. Neural stem cells can give rise to blood or skeletal muscle; hepatic stem cells can turn into pancreatic islet cells or intestine; skin stem cells can produce smooth muscle, neurons, glia or adipocytes; adipose tissue-derived stem cells can differentiate into neuronal, chondrogenic, myogenic or osteogenic cells. Thus, at least in some situations, adult stem cells appear to show sufficient plasticity to differentiate into cell types outside their predicted developmental lineage and even cross germ layer boundaries.

Table 1 Differentiation potential of adult stem cells

As is already assumed, the therapeutic potential of stem cells is great, but there is another side of this coin too. As we age our bodies are exposed to different impacts of environment. As the pollution grows, the possibility to get mutation into any gene enhances. Because of this, during aging our organisms alter the cells do either. It doesn't matter if they are stem cells or somatic ones. Mutations are cruel to all cells indifferently. So beside these therapies would be applied in medicine, scientists should solve the problem of bad stem cells, to avoid making already damaged organ or tissue of our own (Fig. 2).

Fig. 2 Therapy, using "not clean" cells. In case "not clean" cells are used the consequences of therapy are not predictable.