Biomedical Engineering Reference
In-Depth Information
It is generally established that the normal functioning of stem cells explicitly
requires a particular microenvironment within the tissue -
the stem cell niche
- as
an indispensable element (Spradling et al.
2001
; Morrison and Spradling
2008
).
The integrated view for the stem cell niche system was established substantially in
the
Drosophila
germline stem cells (GSCs) in both the testis and ovary (Spradling
et al.
2001
; Fuller and Spradling
2007
), as well as a number of other systems,
including mammalian hematopoiesis (Morrison and Spradling
2008
). Although it
is clear that the mammalian spermatogenic stem cell system also involves the niche
microenvironment in the testis, its cellular and molecular nature has not been char-
acterized to the extent that has been performed in the abovementioned systems.
However, in recent years, mammalian spermatogenic stem cell research has expe-
rienced several breakthroughs, leading to a greater understanding of the nature of
stem cells and their niche. The concept of niche may not be limited solely for the
self-renewing stem cells, but can be extended to broader populations such as so-
called progenitors. This idea is acknowledged because it is becoming apparent that
progenitors are also involved in the stem cell system as an indispensable component.
This chapter summarizes the anatomical and historical backgrounds as well as
ongoing research regarding the mammalian (mostly mouse) spermatogenic stem
cell system, with an emphasis on the stem cell niche. In addition, a comparison with
the characteristics of the
Drosophila
GSC system will be helpful.
8.2
Drosophila
Germline Stem Cell Niche System
The germline of the fruit fly,
Drosophila melanogaster
, represents a typical stem
cell niche system (Spradling et al.
2001
; Fuller and Spradling
2007
). In this organ-
ism, gamete production in both sexes is based on typical stem cell systems. The
ovary and testis demonstrate a clear polarity, with one end opening to outside of the
body while the other is a blind end, which provides the niche region that not only
tethers the stem cells but also controls their growth and differentiation (Fig.
8.1a, b
)
(Morrison and Spradling
2008
; Fuller and Spradling
2007
). Figure
8.1c
is a sche-
matic representation of the testis tip, where highly specialized somatic cells,
namely the hub cells, create the niche for GSCs. Hub cells form a tight contact with
GSCs and control the orientation of GSC division to occur perpendicularly relative
to hub cells. This results in one daughter that remains in contact with the hub and
the other that loses the contact: The former persists as GSC, while the latter enters
the process of differentiation, thus representing a typical asymmetric “stem cell
division.” Along with differentiation, the differentiating cells gradually leave behind
the niche and finally exit as the mature sperm. As a result, the stem cells and the
differentiating cells are arranged so that they recapitulate the chronological order of
differentiation from the distal niche region toward the proximal opening. In the
ovary, similarly, highly specialized somatic cells (cap and terminal filament cells)
comprise the niche that supports the female GSC system in an essentially identical,
but somewhat more complex, manner (Fuller and Spradling
2007
). The
Drosophila
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