ANGPTL2-producing endothelium supports the localization of HSCs in BM niches. (A) Representative images of the localization of LT-HSCs in the BM of Angptl2^fl/fl or Cdh5-Cre; Angptl2^fl/fl mice. BM sections were stained to reveal Lin⁻CD150⁺CD48⁻CD41⁻ LT-HSCs (red, with arrowhead), laminin-positive vessels (green), and Lin⁺CD41⁺CD48⁺ differentiated hematopoietic cells (gray). Scale bar, 10 μm. (B) Quantification of the frequencies of LT-HSCs localized at the indicated distance to vessels. n = 225-256 cells per group. (C) The percentages of LT-HSCs in the BM of Angptl2^fl/fl and Cdh5-Cre; Angptl2^fl/fl mice adjacent to laminin-positive endothelial cells (within 10 μm) are shown. A total of 225 to 256 LT-HSCs were counted (n = 4). (D) The cell cycle status of LT-HSCs from Angptl2^fl/fl and Cdh5-Cre; Angptl2^fl/fl mice was determined by staining with Hoechst 33342 and pyronin Y. (E) The percentages of the G₀ fraction in panel D are shown (n = 4). (F) The apoptotic status of LT-HSCs from Angptl2^fl/fl or Cdh5-Cre; Angptl2^fl/fl mice was measured by using Annexin V and 7-AAD staining (n = 4). (G) BM cells from Angptl2^fl/fl and Cdh5-Cre; Angptl2^fl/fl mice were labeled with CFSE and transplanted into lethally irradiated recipient mice. The percentage of CFSE⁺ cells in the BM, spleen and liver or CFSE⁺ LSK cells in the BM was determined by flow cytometric analysis 16 hours later (n = 5. *P < .05; ***P < .001.