Immunofluorescent staining shows that in situ fetal lung EHT is directed toward the interstitium. Twelve μm sections of a fixed frozen postconception week-20 human fetal lung stained with the broad hematopoietic marker CD45, the EHT marker CD41, the endothelial marker VECAD, and the nuclear stain Hoechst. The lumen of a large vessel lined with VECAD⁺ staining is highlighted with a “+,” and the lumen of a VECAD^– developing epithelial space is highlighted with a “∗.” (A-B) Cells coexpressing VECAD, CD41, and CD45 are highlighted with yellow arrows showing developing hematopoietic clusters oriented toward the lung interstitium. Anucleate platelets single-positive for CD41 are highlighted with green arrows. Differentiated hematopoietic cells only positive for CD45 are highlighted with white arrows. A wide-field image of section (A), a section including ECAD staining, and a control stain is included in supplemental Figures 8, 10, and 11.