The study investigates the distinct niches for hematopoietic stem cells (HSCs) and early lymphoid progenitors in bone marrow. Using CXCL12 reporter mice, the authors found that CXCL12 is primarily expressed by perivascular stromal cells and endothelial cells, with lower levels in osteoblasts and some hematopoietic cells. Conditional deletion of CXCL12 from different cell types revealed that endothelial cells are essential for HSC maintenance, while perivascular stromal cells are crucial for maintaining both HSCs and certain early lymphoid progenitors. Deletion of CXCL12 from osteoblasts depleted early lymphoid progenitors but not HSCs or myeloerythroid progenitors. These findings suggest that HSCs reside in a perivascular niche, while early lymphoid progenitors occupy an endosteal niche. The study also confirms the role of SCF in HSC maintenance and highlights the importance of CXCL12 in maintaining the quiescent state of HSCs and lymphoid progenitors.The study investigates the distinct niches for hematopoietic stem cells (HSCs) and early lymphoid progenitors in bone marrow. Using CXCL12 reporter mice, the authors found that CXCL12 is primarily expressed by perivascular stromal cells and endothelial cells, with lower levels in osteoblasts and some hematopoietic cells. Conditional deletion of CXCL12 from different cell types revealed that endothelial cells are essential for HSC maintenance, while perivascular stromal cells are crucial for maintaining both HSCs and certain early lymphoid progenitors. Deletion of CXCL12 from osteoblasts depleted early lymphoid progenitors but not HSCs or myeloerythroid progenitors. These findings suggest that HSCs reside in a perivascular niche, while early lymphoid progenitors occupy an endosteal niche. The study also confirms the role of SCF in HSC maintenance and highlights the importance of CXCL12 in maintaining the quiescent state of HSCs and lymphoid progenitors.