This study investigates the relationship between hematopoietic stem and progenitor cells (HSPCs) and their microenvironment in the bone marrow niche using high-resolution confocal and two-photon video imaging in living mice. The researchers found that HSPCs localize to subdomains of bone-marrow microvessels where the chemokine CXCL12 is abundant. They observed that HSPCs are nonrandomly and dynamically positioned within the complex tissue, influenced by both cell autonomous and non-autonomous factors. Different HSPC subsets localized to distinct locations based on their differentiation stage. When physiological challenges drove engraftment or expansion, HSPCs assumed positions close to bone and osteoblasts. The study also demonstrated that the previously proposed dichotomy between osteoblast and perivascular niches is not anatomically feasible in the calvarium, as osteoblasts are perivascular. The findings highlight the dynamic nature of the bone marrow microenvironment and the influence of both cell intrinsic and niche intrinsic variables on HSPC localization.This study investigates the relationship between hematopoietic stem and progenitor cells (HSPCs) and their microenvironment in the bone marrow niche using high-resolution confocal and two-photon video imaging in living mice. The researchers found that HSPCs localize to subdomains of bone-marrow microvessels where the chemokine CXCL12 is abundant. They observed that HSPCs are nonrandomly and dynamically positioned within the complex tissue, influenced by both cell autonomous and non-autonomous factors. Different HSPC subsets localized to distinct locations based on their differentiation stage. When physiological challenges drove engraftment or expansion, HSPCs assumed positions close to bone and osteoblasts. The study also demonstrated that the previously proposed dichotomy between osteoblast and perivascular niches is not anatomically feasible in the calvarium, as osteoblasts are perivascular. The findings highlight the dynamic nature of the bone marrow microenvironment and the influence of both cell intrinsic and niche intrinsic variables on HSPC localization.