Model. In the E10.5 AGM region, Hes1-GFP is expressed in all functional pre-HSCs type I, demonstrating that the Notch pathway is activated in these cells. Notch1 is the main receptor at this stage; later on, Notch2 is upregulated during the pre-HSC type I to pre-HSC type II transition. Although both Notch1 and Notch2 are expressed in maturing pre-HSCs and dHSCs, Notch activity decreases because some pre-HSCs and dHSCs at E11.5 and E12.5, respectively, become Hes1-GFP⁻. Functional analysis showed that Notch activity is essential during the first steps of pre-HSC development. However, the decrease of Notch activity is accompanied by a progressive loss of Notch dependency, as some E11.5 pre-HSCs can complete development in the absence of Notch. In the fetal liver, HSCs either fully lack Notch activity or exhibit it at a low level, despite the presence of both Notch1^61,76  and Notch2 at their surface.