Requirements for α4 integrins throughout embryonic and adult hematopoiesis. A collective summary of works illustrates the requirements for α4 integrin, depending on developmental stage and context. When HSCs develop through the fetal stage into adulthood in the absence of α4 integrin, their differentiation program becomes progressively corrupt (pathway “X”).^34,35  However, if the fetal stage is bypassed by transplanting early embryonic α4 integrin^–/– HSCs directly into the adult, the hematopoietic deficiency is avoided (pathway “✓”). If normal hematopoiesis is established in the BM, α4 integrin ablation results in only minor defects (not shown).³⁷  Cumulatively, these data suggest that during the fetal/neonatal stage(s) HSC development is strongly α4 integrin dependent.