Arterioles and sinusoids of the bone marrow (BM) are accompanied by stromal cells that express nerve/glial antigen 2 (NG2) and leptin receptor (LepR), and constitute specialised niches that regulate quiescence and proliferation of haematopoietic stem cells (HSCs). However, how niche cells differentially regulate HSC functions remains unknown. Here, we show that the effects of cytokines regulating HSC functions are dependent on the producing cell sources. Deletion of chemokine C-X-C motif ligand 12 (Cxcl12) or stem cell factor (Scf) from all perivascular cells marked by Nestin-GFP dramatically depleted BM HSCs. Selective Cxcl12 deletion from arteriolar NG2+ cells, but not from sinusoidal LepR+ cells, caused HSC reductions and altered HSC localisation in BM. By contrast, deletion of Scf in LepR+ cells, but not NG2+ cells, led to reductions in BM HSC numbers. These results uncover distinct contributions of cytokines derived from perivascular cells in separate vascular niches to HSC maintenance.

Aging of hematopoietic stem cells (HSCs) is associated with a decline in their regenerative capacity and multi-lineage differentiation potential, contributing to the development of blood disorders. The bone marrow microenvironment has recently been suggested to influence HSC aging, but the underlying mechanisms remain largely unknown. Here, we show that HSC aging critically depends on bone marrow innervation by the sympathetic nervous system (SNS), as loss of SNS nerves or adrenoreceptor β3 (ADRβ3) signaling in the bone marrow microenvironment of young mice led to premature HSC aging, as evidenced by appearance of HSC phenotypes reminiscent of physiological aging. Strikingly, supplementation of a sympathomimetic acting selectively on ADRβ3 to old mice significantly rejuvenated the in vivo function of aged HSCs, suggesting that the preservation or restitution of bone marrow SNS innervation during aging may hold the potential for new HSC rejuvenation strategies.

Hematopoietic stem cells (HSCs) develop in discrete anatomical niches, migrating during embryogenesis from the aorta-gonad-mesonephros (AGM) region to the fetal liver, and finally to the bone marrow, where most HSCs reside throughout adult life. These niches provide supportive microenvironments that specify, expand and maintain HSCs. Understanding the constituents and molecular regulation of HSC niches is of considerable importance as it could shed new light on the mechanistic principles of HSC emergence and maintenance, and provide novel strategies for regenerative medicine. However, controversy exists concerning the cellular complexity of the bone marrow niche, and our understanding of the different HSC niches during development remains limited. In this Review, we summarize and discuss what is known about the heterogeneity of the HSC niches at distinct stages of their ontogeny, from the embryo to the adult bone marrow, drawing predominantly on data from mouse studies.

The bone marrow (BM) niche comprises multiple cell types that regulate hematopoietic stem/progenitor cell (HSPC) migration out of the niche and into the circulation. Here, we demonstrate that osteocytes, the major cellular component of mature bone, are regulators of HSPC egress. Granulocyte colony-stimulating factor (G-CSF), used clinically to mobilize HSPCs, induces changes in the morphology and gene expression of the osteocytic network that precedes changes in osteoblasts. This rapid response is likely under control of the sympathetic nervous system, since osteocytes express the β2-adrenergic receptor and surgical sympathectomy prevents it. Mice with targeted ablation of osteocytes or a disrupted osteocyte network have comparable numbers of HSPCs in the BM but fail to mobilize HSPCs in response to G-CSF. Taken together, these results indicate that the BM/bone niche interface is critically controlled from inside of the bone matrix and establish an important physiological role for skeletal tissues in hematopoietic function.

Intravascular lymphoma (IVL) is a rare type of extranodal lymphoma in which the lymphoma cells proliferate exclusively in the lumina of small vessels. The diagnosis of IVL requires histological confirmation. Although random skin biopsy from healthy-appearing skin in patients with suspected IVL appeared to be useful, the sensitivity of this method for the diagnosis of IVL remains unknown. We performed a random skin biopsy from 12 consecutive cases of IVL diagnosed at our institution over the past 4 years and evaluate its relevance of clinical and laboratory characteristics, presence or absence of skin lesions, and bone marrow involvement. All 12 patients were diagnosed antemortem by either random skin biopsy or bone marrow biopsy and treated with rituximab-containing chemotherapy. Random skin biopsy was performed in all 12 patients, and the results were positive in ten patients (83.3%). Erythematous skin lesions were seen in 3 of 12 patients, but biopsy was positive for lymphoma lesion in two patients. Bone marrow invasion was seen in 11 of the 12 patients (91.6%) by bone marrow smear and/or flow cytometric analysis, but was detected in only half of the patients by trephine biopsy. We concluded that random skin biopsy from normal-appearing skin is highly sensitive in the diagnosis of IVL comparable to bone marrow trephine biopsy. It should be performed irrespective of the presence or absence of skin lesions in patients who were suspicious of IVL.

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Osteocytes act as mechanosensors to control local bone volume. However, their roles in the homeostasis of remote organs are largely unknown. We show that ablation of osteocytes in mice (osteocyte-less [OL] mice) leads to severe lymphopenia, due to lack of lymphoid-supporting stroma in both the bone marrow and thymus, and complete loss of white adipose tissues. These effects were reversed when osteocytes were replenished within the bone. In contrast, neither in vivo supply of T cell progenitors and humoral factors via shared circulation with a normal parabiotic partner nor ablation of specific hypothalamic nuclei rescued thymic atrophy and fat loss in OL mice. Furthermore, ablation of the hypothalamus in OL mice led to hepatic steatosis, which was rescued by parabiosis with normal mice. Our results define a role for osteocytes as critical regulators of lymphopoiesis and fat metabolism and suggest that bone acts as a central regulator of multiple organs.

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