Stem cell autotomy and niche interaction in different systems

Figure 4 Short range signalling between the germline stem cells and their niche, which consists of the hub cells and the cyst stem cells. The hub cells secrete the ligand Upd, which activates JAK/STAT signalling in GSCs and CySCs. Whereas JAK/STAT activation is sufficient for CySC maintenance and self-renewal, GSC self-renewal requires additional signals. Hub cells and CySCs secrete the ligands Gbb and Dpp, which activate BMP in GSCs. BMP suppresses the transcription of Bam that inhibits the differentiation of GSCs to gonialblasts. BMP activation in GSCs is also supports by Gbb and Dpp produced by the CySCs. Gbb and Dpp in CySCs are conceivably activated by STAT and/or Zfh1 and Chinmo. Hub cells also secrete the ligand Hh (hedge hog), which supports CySC self-renewal in addition to, but independently of, JAK/STAT activity. Hh binds to the transmembrane receptor PTC (patched) of CySC, which releases Smo (smoothened) from repression. This leads to the activation of the transcription factor Ci (cubitus interruptus), which activates the transcription of target genes that support maintenance and self-renewal of CySCs besides JAK/STAT activity. The Hh signalling pathway in CySCs may also affect Zfh1 and, via Gbb/Dpp, influence BMP signalling in GSCs. Thus, Hh signalling in the testis niche apparently has a dual role. CC: Cyst cell; GSCs: Germline stem cells; CySCs: Cyst stem cells.