Stage-specific niches of early B-cell development. HSCs move from an endosteal CXCL12⁺ cellular niche toward a vascular CXCL12⁺ niche while differentiating into pre-pro-B cells. At the next step, cells reach the proximal IL-7⁺ niche in the perivascular region, entering the pro-B/pre-BI stage. Finally, at the pre-BCR⁺ pre-BII stage, cells leave the vasculature to enter deeper into the BM, close to GAL1⁺ stromal cells.