The haematopoietic system is not generally regarded as a tissue that undergoes patterning during development. However, on , Elaine Dzierzak and co-workers report that instructive signalling from ventral embryonic tissues affects the specification of haematopoietic stem cells(HSCs) in the aorta-gonad-mesonephros (AGM) region of the mouse embryo. The ventral position of haematopoietic clusters in the AGM suggests that signals from embryonic tissues ventral to the AGM (for example, the gut) might influence HSC specification. The authors tested this possibility by measuring the HSC content of AGMs cultured with dorsal or ventral tissues. Ventral tissues, they report, increase the number of HSCs in the AGM, whereas dorsal tissues decrease it. Using chimaeric explant cultures, the authors show that instructive signalling from ventral tissues, rather than the influx of ventrally derived HSCs or their precursors into the AGM, accounts for the effect of ventral tissue on AGM HSC activity. Finally, the researchers suggest that Hedgehog, although not ventrally restricted, is an HSC-inducing signal, a possibility that requires confirmation in future studies.
