Stem Cell Niche System in Mouse Spermatogenesis - Male Germline Stem Cells Developmental and Regenerative Potential - page 162

the stem cell niche intercellular signaling mediated mainly by Upd (male) and Dpp
(female) ligands, and the intracellular molecular mechanisms in stem and niche
cells [for detail, see (Fuller and Spradling 2007 )].
The findings obtained from the Drosophila GSC research will provide much
information for the future investigation of the mammalian spermatogenic stem cell
system. However, it is not believed that the mammalian system will be a copy of
that in Drosophila because the biological contexts of these organisms look different.
What is important will be to understand the common basis among species and to
clarify the characteristic features for the mouse spermatogenic stem cell niche
system.

8.3 Architecture of the Mouse Testis

The anatomy of the testis of the mouse and other mammals makes a good contrast to
that of the fruit fly. Spermatogenesis proceeds inside the seminiferous tubules - long,
convoluted tubules that have a diameter of up to 200 mm (Russell et al. 1990 ). This
structure is common in amniotes (i.e., mammalians, birds, and so-called reptiles).
Seminiferous tubules form loops that open into the rete testes at both the ends, with a
total length of up to 2 m per testis in the case of mice (Fig. 8.2a ). Blood vessels, which
nourish the tubules, never penetrate them but run in the interstitial spaces between the
tubules to form a network (Fig. 8.2c, d ). The interstitial space also includes Leydig
cells (the major producers of androgens), macrophages, lymphoid epithelial cells, and
connective tissue cells (Russell et al. 1990 ; Hinton and Turner 1993 ).
Spermatogenesis progresses uniformly all over the inner surface of the tubules
or the seminiferous epithelium (Russell et al. 1990 ). Therefore, one can hardly
recognize the overall polarity that governs the entire gonads or particular segments
within the tubules where stem cells are accumulated (Fig. 8.2b , compare with
Fig. 8.1b ). In addition to a far bigger size than the Drosophila gonads, such a uni-
form architecture of the seminiferous tubules makes it difficult to demonstrate stem
cell localization based on the presumptive niche structure. Indeed, based on trans-
plantation and regeneration experiments, it has been suggested that stem cells are
scattered all through the tubules (de Rooij and Russell 2000 ).
As shown in Fig. 8.2e , the anatomical framework of seminiferous epithelium is
composed of the basement membrane and two types of somatic cells, Sertoli and
peritubular myoid cells, which line the inside and outside of the basement membrane,
respectively. Sertoli cells form a beautiful epithelium with tight junctions between
them, which is the anatomical basis of the blood-testis-barrier. Sertoli-cell tight
junctions separate the tubules into basal and adluminal compartments (Fig. 8.2e ).
The basal compartment between the tight junction and the basement membrane is
occupied with all the stages of spermatogonia, defined as mitotic germ cells in the
mature testis (Fig. 8.2f ). Note that there is a discrepancy in terminology between
mice and fruit fly. In Drosophila , the single, differentiation-oriented cells
(a differentiating daughter of GSC) and the subsequent interconnected cysts are