Hox genes and stem cells

Stem cells are cells that undergo self-renewal as well as differentiation into progenitor cells. They are abundantly present, although ill defined, during development and it is believed that most, if not all, adult tissues harbor small populations of stem cells. Adult stem cells have been described for intestine, skin, muscle, blood and nervous system and may provide a tissue specific resource for tissue damage repair. A balance between stem cell self-renewal and differentiation maintains homeostasis in adult tissue. Thus normal tissue stem cells are defined by three common properties. (1) extensive self-renewal capacity, (2) strict control of stem cell numbers and (3) ability to undergo extensive differentiation to reconstitute all the functional elements in a given tissue. In this chapter we review the evidence that Hox genes may be involved in stem cell maintenance and control of self-renewal in different cellular and developmental contexts. Strong evidence exists for their role in controlling Drosophila neuroblast numbers and fate decisions and for their role in controlling expansion, self-renewal and lineage specification in the mammalian hematopoietic stem cells. There are tantalizing clues for their involvement in stem cell expansion during embryonic development and later in the process of oncogenesis