Murine hematopoietic bone marrow is heterogenous in respect to bone-lining cells, hematopoiesis, and release of blood cells. In diaphyseal femoral marrow, bone-lining cells are largely osteoblasts, indifferent endosteum, blood cells, and reticular cells. Hematopoiesis is sustained by rather differentiated progenitors, as myelocytes and polychromatophilic erythroblasts. But in sharply restricted loci within trabeculated bone in the distal medial femoral metaphysis, bone-lining cells are dominated by newly discovered fibroblastic, contractile, stromal barrier cells; activated, multilaminar and branched, enveloping putative stem cells; and extending into the marrow, where they enclose hematopoietic clusters of rather early progenitors, as promyelocytes; and basophilic erythroblasts. Barrier cells may endocytize granules released individually by vicinal differentiating granulocytes, preventing tissue damage. Barrier cells envelope blood vessels and insinuate processes into their wall, augmenting blood-marrow barriers, preventing release of immature cells. Further, extensions of venous sinuses made of extraordinarily thin barrier cell processes receive released blood cells. With heightened hematopoiesis due to mutant hemolytic anemias or after administration of interleukin-1, barrier cells and their associated structures are greatly increased, spreading beyond normally restricted loci. But in microenvironment-deficient mutants, barrier cells are fewer, less activated, and less granulated. Barrier cells thus appear important in hematopoiesis and in the release of blood cells from bone marrow.