Hematopoietic stem cells (HSCs) can differentiate into several types of hematopoietic cells, such as erythrocytes, megakaryocytes, lymphocytes, neutrophils, or macrophages, and also undergo self-renewal to sustain hematopoiesis throughout an organism's lifetime. HSCs emerge and expand during mouse embryogenesis. HSC regulation is governed by two types of activity: intrinsic activity programmed primarily by cell autonomous gene expression, and extrinsic factors, which originate from the so-called niche cells surrounding HSCs. Previously, we reported that endothelial niche cells regulate HSC generationat aorta-gonad- mesonephros region and placenta, and that hepatoblastic niche cells regulate HSC differentiation in mouse embryonic liver. In the course of those studies, we employed immunohistochemistry,flow cytometry, and the laser capture microdissection system to assess embryonic regulation of the mouse hematopoietic niche.