Stem Cell Niche System in Mouse Spermatogenesis - Male Germline Stem Cells Developmental and Regenerative Potential - page 172

proliferation, and self-renewal within the niche, expansion of the transient amplifying
spermatogonia and differentiation into sperm); it would be interesting to deter-
mine which step actually requires b1-integrin.

8.8 Transcriptional Regulator in Somatic Cells

8.8.1 Etv5 (ets Variant Gene 5/ERM or ets-Related Molecule)

( Etv5 )-homozygous testes have apparently normal spermatogenesis in the initial
cycles after puberty; however, it does not persist in the following cycles, result-
ing in the loss of germ cells in older animals (Chen et al. 2005 ). The Etv5 gene
encodes a transcriptional regulator that is detected, according to this initial
report, only in the Sertoli cell nuclei in the adult testis. Therefore, it can be pos-
tulated that Etv5 can regulate Sertoli cell functions that mediate the regulation
of germ cells. Etv5 expression in Sertoli cells appears to be uniform and does
not show any apparent position-specificity (Chen et al. 2005 ). However, it has
been recently reported that Etv5 is also expressed in some germ cells in younger
stages, and that Etv5 mutant germ cells either do not form colonies after trans-
plantation or exhibit an impaired response to GDNF (Tyagi et al. 2009 ), raising
the possibility that the stem cell maintenance phenotype could be germ cell-
autonomous. Clarifying the site of action of this gene in terms of cell type and
developmental stages would be essential in order to explain such an intriguing
phenotype and to achieve a greater understanding of the stem cell niche
system.

8.9 Conclusion

This chapter has the current knowledge regarding the mammalian spermatogenic
stem cell niche system. Readers should recognize that we have a number of pieces
of knowledge in our hands. Several further investigations will be required before a
comprehensive view of this system can be obtained, where anatomical, functional,
and molecular aspects of stem cell niche somatic cell interactions will be integrated
into a single scheme. This is a challenge and includes a number of essential issues
such as the precise identification of stem cells and localization in the seminiferous
tubules. Once this is achieved, however, the mammalian spermatogenic stem cell
niche system will definitely provide invaluable insights for stem cell research in
general.

Acknowledgments I thank Prof. Yo-ichi Nabeshima at the Department of Pathology and Tumor
Biology, Graduate School of Medicine, Kyoto University, for his continuous support. Most of the