Future application of hematopoietic stem and progenitor cells (HSPCs) in clinical therapies largely depends on their Successful expansion in vitro. Fetal liver (FL) is a unique hematopoietic organ in which hematopoietic cells markedly expand in number, but the mechanisms involved remain unclear. Stromal cells (StroCs) have been suggested to provide a suitable cellular environment for in vitro expansion of HSPCs. In this study, murine StroCs derived from FL at E14.5, with a high level of Sonic hedgehog (Shh) and Writ expression, were found to have an increased ability to support the proliferation of HSPCs. This effect was inhibited by blocking Shh signaling. Supplementation with soluble Shh-N promoted the proliferation of hematopoietic cells by activating Writ signaling. Our findings suggest that FL-derived StroCs Support proliferation of HSPCs via Shh inducing an autocrine Wnt signaling loop. The use of FL-derived StroCs and regulation of the Shh pathway might further enhance HPSC expansion. (C) 2009 Elsevier Inc. All rights reserved.