Fig. 4. Schematic representation of colonisation of the embryonic liver with HSC/RUs. The number of HSC/RUs in the liver are as determined in vivo. The number of HSC/RUs in the AGM region and the YS are the numbers that these tissues are able to generate in vitro. In vivo, the high cumulative activity of the AGM region and the YS may provide the liver with a high proportion of the `ready-to-use' pool of definitive HSC/RUs. The data suggest consecutive colonisation of the embryonic liver with HSC/RUs from the AGM region and the YS.