Biology Reference
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leading to the inheritance of the midbody ring by the cell with the mother centrosome
(Gromley et al.
2005
; Kuo et al.
2011
). The cell containing the mother centrosome was
found to have multiple midbody derivatives through numerous successive cell divi-
sions (Kuo et al.
2011
). Multiple pathways have been discovered to alleviate the over-
accumulation of midbody derivatives: they could be either digested by autophagy or
released from cells (Kuo et al.
2011
; Pohl and Jentsch
2009
).
6.2 Stem Cells and Asymmetric Stem Cell Division
The balance between stem cell self-renewal and differentiation is critical for
sustaining tissue growth during early development, repairing damaged tissue after
injury, and maintaining tissue homeostasis during adulthood (Morrison and
Kimble
2006
; Morrison et al.
1997
; Watt and Hogan
2000
). Failure to regulate the
number and function of stem cells has been speculated to lead to tumorigenesis
due to the over-proliferation of stem cells or tissue aging/degeneration due to a
decline in stem cell functions (Brunet and Rando
2007
; Clarke and Fuller
2006
;
Clevers
2005
; Kirkwood
2005
; Rando
2006
). Asymmetric stem cell division
creates one daughter cell that retains a stem cell identify and a daughter cell that
undergoes differentiation, thereby precisely balancing self-renewal and differen-
tiation. There are two major regulatory mechanisms that ensure asymmetric stem
cell division: extrinsic and intrinsic fate determinants.
6.2.1 Asymmetric Stem Cell Division Regulated
by Extrinsic Fate Determinants
Many stem cells reside in a microenvironment, or niche, that provides extrinsic fate
determinants that specify stem cell identity (Morrison and Spradling
2008
). These
extrinsic fate determinants, locally secreted within the stem cell niche, promote the
ability of stem cells to self-renew and/or repress their differentiation. The Drosophila
male germline stem cell (GSC) niche is among the best studied of these systems,
where stem cells undergo asymmetric stem cell divisions in the context of the stem
cell niche. The male GSC niche is composed of hub cells and cyst stem cells (CySCs,
also known as cyst progenitor cells). The hub cells are located at the apical tip of the
testis, to which the GSCs and the CySCs attach via adherens junctions (Hardy et al.
1979
; Raymond et al.
2009
). Hub cells secret the ligand Unpaired (Upd), which
specifies the stem cell identity through activating JAK-STAT signaling pathways
within GSCs and CySCs (Kiger et al.
2001
; Leatherman and Dinardo
2008
,
2010
;
Tulina and Matunis
2001
). GSCs divide asymmetrically, producing one GSC and one
gonialblast, and CySCs also divide asymmetrically, producing one CySC and one
cyst cell (Cheng et al.
2011
; Yamashita et al.
2003
) (Fig.
6.2
a).
