Almost unipotent (single cell type), e.g. germline stem

Almost all adult tissues require periodic replenishments and damage repairs. A group of slow
dividing cells, called stem cells, nestled within each tissue carries out the
job throughout the lifespan of an organism. Stem cells have a unique capability
of undergoing regulated asymmetric divisions resulting in self-renewal and generation
of a progeny cell with predetermined cell-fates (Friedenstein et al., 1976, 1974; Gilbert, 2000). Often, the stem cell progeny
undergoes a few rounds of mitoses before differentiation. The capacity to
divide and differentiate varies with the type of stem cell (reviewed in Weissman, 2000). They are broadly classified into two categories on the basis of the developmental
state of the derived tissue. Stem cells derived from an early embryo
(inner cell mass of the blastocyst), known as embryonic stem cells,
possess potential to generate almost all cell types (except extra-embryonic
tissue) and hence termed as pluripotent (Smith, 2001).

Stem cells
maintaining adult tissues, known as adult stem cells, have reduced potency and
can give rise to limited cell types. Specific
adult stem cells have been described as multipotent (multiple cell types),
e.g., hematopoietic stem cells which generate a large variety of blood and
associated tissue (Hoang,
2004);
oligopotent (only few cell types), e.g. neuronal stem cells which generate
different types of neurons and glia (Reynolds
et al., 1992),
and unipotent (single cell type), e.g. germline stem cells (GSCs) which produce
either egg or a sperm (Yuan
and Yamashita, 2010). Unraveling the molecular mechanisms regulating (or
governing) the stem cell self-renewal has been of great interest in
developmental biology (Guo
et al., 2011; Hirai et al., 2011; Simons and Clevers, 2011; Staal et al., 2011;
Yun et al., 2010; Zhao et al., 2011). Some of the critical
unresolved issues are how the cell fates are defined and what regulates the
extent of divisions of the stem cell progeny. Deciphering the underlying
signaling molecules involved in these processes are expected to play essential roles in developing regenerative
medicine and therapies.