A type of MPCs, known as multipotent adult progenitor cells (MAPCs), was first identified over a decade ago with characteristics similar to most adult somatic stem cells; they proliferate without senescence and have a broad differentiation ability [ 86, 87].
Haematopoietic stem cell (HSCs) are among the most well characterised of somatic stem cells and have served as a paradigm for stem cells in other tissues, with the notion that maintenance of quiescence is critical for somatic stem cell function being largely derived from studies of HSCs.
Telomere length is maintained by a cellular enzyme, telomerase, in germ cells and stem cells, while most somatic cells have very low telomerase activity, thus leading to telomere length shortening with cellular division [ 1, 2].
Telomerase expression is low or absent in most somatic tissues, but not in germ cells, stem cells, and tumors (Meyne et al., 1989).
For example, most somatic cells exhibit a limited division potential, whereas the stem cells that generate them can divide indefinitely.
In humans, telomerase activity is detectable in germline cells and in certain stem cells, but not detectable in most somatic cell lineages (Harley et al, 1997).
Among these, one important point is that the somatic stem cells that have been identified in most organs and tissues including skin, liver, brain, bone marrow, blood vessels, muscles, are always present in poor numbers.
While ESC can differentiate into any type of tissue, the potential of somatic stem cells is usually restricted to certain types, most prominently those derived from the same germ layer from which they are derived.
By the late L2 stage, these 12 cells, which until then are intermingled with germline stem cells, rearrange to form the somatic gonad primordium: most somatic cells coalesce in the proximal region, with the distal tip cells (DTCs) remaining at the distal ends to nurture the germline and lead the extension of the gonad arms.
Among the different stem cells, embryonic stem (ES) cells are considered the most pluripotent as they have the potential to form every cell type, including other somatic stem cells.
