A cell's surroundings can be critical in determining its fate. Adult stem cells, for example, which populate some mammalian tissues, are influenced by their microenvironment to either stay quiescent, divide or differentiate toward a particular lineage. Despite the interest and possible therapeutic potential of these cells, knowledge about their in vivo niche remains limited. One way to overcome this is to model the niche in the dish—as recent work has shown for spermatogonial stem cells (SSCs).

SSCs are responsible for producing spermatozoids and reside in the seminiferous tubules of the testis, where they are thought to interact with the basement membrane and with somatic cells known as Sertoli cells. Detailed study of SSCs and their niche has proven hard, partly because of their rarity and the difficulty in isolating them.

For years, SSCs have been identified retroactively, on the basis of their ability to re-initiate and maintain sperm production when transplanted into an infertile mouse host. But Mito Kanatsu-Shinohara and her colleagues, of Kyoto University in Japan, wanted to study the relationship between SSCs and their niche in greater detail, so they worked to develop a culture method in the likes of such an environment.

Kanatsu-Shinohara and her collaborators were inspired by the bone marrow—the best-studied stem cell niche in mammals. In this system, partial in vitro reconstruction of the in vivo niche is possible by culturing hematopoietic stem cells (HSCs) together with a stromal cell layer. HSCs form colonies of characteristic cobblestone morphology in which the cells maintain a steady state of self-renewal and differentiation resembling the in vivo situation. The number of cobblestone colonies also serves as a good estimate of the number of HSCs or progenitors that exist in vivo.

To mimic the basement membrane and cellular composition of the in vivo SSC niche, the researchers first cultured somatic cells from testis of infertile mice on laminin. They then used these cultures as feeder layers onto which they added cells isolated from normal mouse testis enriched for SSCs. Two types of colonies arose from these cultures: one type seemed to gather in clumps; the other looked strikingly similar to cobblestone colonies in bone marrow cultures.

Kanatsu-Shinohara was excited when she saw cobblestone clusters in the SSC cultures—indeed, she knew these characteristic assemblies well from her years as a graduate student studying embryonic development of HSCs.

The group demonstrated that the number of cobblestone clusters served as a good estimate of SSC activity by doing parallel transplantation experiments. They then used this in vitro model to identify molecular players involved in stem cell homing and found two chemokines, GDNF and CXCL12, that played a role in this process—a finding that they also confirmed in vivo.

Though it is a big step forward, this culture setup can still be improved, explains Kanatsu-Shinohara. Currently, only primary testis cell feeders (containing a mixture of different testis somatic cells, including Sertoli cells) give rise to these cobblestone clusters. But the group wants to develop a Sertoli cell line that could be used instead, thereby making the entire process less labor intensive and more reproducible. They are also trying to identify additional surface markers expressed by SSCs that could be used to improve the isolation of these cells. Also on their 'to-do' list is the addition of a third dimension to the culture setup to better mimic the in vivo situation.

Ultimately, the team is interested in using SSCs as targets for germline modification of animals. A human-based version of this culture could also be useful for infertility treatments or for establishing disease models. Looking at how useful the cobblestone assay has been for understanding the bone marrow stem cell niche, one can't help but be excited for the bright prospects awaiting the spermatogonial field.