Specification and emergence of HSCs via EHT in the human AGM region. Schematic representation of the establishment of HSC-forming HE in the human AGM region. At CS14/15, an arterially specified HOXA-patterned EC gives rise to an arterial subset termed pre-HE, characterized by ALDH1A1 (RA-signaling) and IL-33 expression. HE is specified from pre-HE by the hematopoietic transcription factor RUNX1 in cells that upregulate KCNK17, CD44, and WNT inhibitors (DKK1), and gives rise to SPINK2⁺CD45⁺ nascent HSCs. In the earlier embryo (CS10-11), HE that generates HSC-independent progenitors is specified from immature endothelial precursors that express embryonic genes (LIN28A) and lack HOXA patterning and arterial identity. (Right) Immunofluorescence validation of stage-specific markers indicate the emergence of human HSCs from IAHC in sections of CS15 human AGM. Modified from Calvanese et al.¹⁵