Model of IL-1–related interactions in the E11 mouse AGM region. The IL-1RI is expressed on endothelial, mesenchymal, and at lower levels on some HSCs. Receptor expression appears to be stable. The expression of IL-1 is variable, with high levels being produced by HSCs and other hematopoietic cells. Expression of IL-1 is low or negligible in the endothelial and mesenchymal compartment. IL-1 (from HSCs or other hematopoietic cells) is thought to interact with IL-1RI–expressing endothelial and/or mesenchymal cells on the ventral side of the E11 dorsal aorta. IL-1RI signaling results in the induction of unknown factor(s) (eg, some mobilization factors) indicated with a question mark (?) to modulate HSC maintenance and/or differentiation.