Numerous niches for hematopoietic stem cells remain empty during homeostasis

Stem cell homeostasis by integral feedback through the niche

Hematopoiesis is a paradigm for tissue development and renewal from stem cells. Experiments show that the maintenance of hematopoietic stem cells (HSCs) relies on signals from niche cells. However, it is not known how the size of the HSC compartment is set. Competition by HSCs for niche access has been suggested, yet niche cells in the bone marrow outnumber HSCs. Here we propose a cooperative model of HSC homeostasis in which stem and niche cells mutually interact such that niche cells function as negative feedback regulators of HSC proliferation. This model explains puzzling experimental findings, including homeostatic recovery of the HSC compartment after irradiation versus apparent lack of recovery after HSC ablation. We show that bidirectional niche-stem cell regulation has properties of a proportional-integral feedback controller. Moreover, we predict that the outflux of differentiated cells from HSCs can be regulated by the affinity of HSCs for niche cells. Much effort has been devoted to elucidating niche cell signaling to stem cells; our theoretical insights indicate that studying the effect of stem cells on the niche may be equally important for understanding stem cell homeostasis.

Where Hematopoietic Stem Cells Live: The Bone Marrow Niche

Hematopoietic stem cells (HSCs) can sustain the production of blood throughout one's lifetime. However, for proper self-renewal of its own population and differentiation to blood, the HSC requires a specialized microenvironment called the "niche." Recent Advances: Recent studies using novel mouse models have shed new light on the cellular architecture and function of the HSC niche. Here, we review the different cells that constitute the HSC niche and the molecular mechanisms that underlie HSC and niche interaction. We discuss the evidence and potential features that distinguish the HSC niche from other microenvironments in the bone marrow. The relevance of the niche in malignant transformation of the HSCs and harboring cancer metastasis to the bone is also outlined. In addition, we address how the niche may regulate reactive oxygen species levels surrounding the HSCs. Critical Issues and Future Directions: We propose future directions and remaining challenges in investigating the niche of HSCs. We discuss how a better understanding of the HSC niche may help in restoring an aged hematopoietic system, fighting against malignancies, and transplanting purified HSCs safely and effectively into patients. Antioxid. Redox Signal. 00, 000-000.

Quantitative spatial analysis of haematopoiesis-regulating stromal cells in the bone marrow microenvironment by 3D microscopy

Sinusoidal endothelial cells and mesenchymal CXCL12-abundant reticular cells are principal bone marrow stromal components, which critically modulate haematopoiesis at various levels, including haematopoietic stem cell maintenance. These stromal subsets are thought to be scarce and function via highly specific interactions in anatomically confined niches. Yet, knowledge on their abundance, global distribution and spatial associations remains limited. Using three-dimensional quantitative microscopy we show that sinusoidal endothelial and mesenchymal reticular subsets are remarkably more abundant than estimated by conventional flow cytometry. Moreover, both cell types assemble in topologically complex networks, associate to extracellular matrix and pervade marrow tissues. Through spatial statistical methods we challenge previous models and demonstrate that even in the absence of major specific interaction forces, virtually all tissue-resident cells are invariably in physical contact with, or close proximity to, mesenchymal reticular and sinusoidal endothelial cells. We further show that basic structural features of these stromal components are preserved during ageing.

The bone marrow microenvironment modulates haematopoiesis, stem cell maintenance and differentiation. Here, the authors use 3D microscopy to map the topography of haematopoietic stem cell niche stromal components.

Hematopoietic stem cells can differentiate into restricted myeloid progenitors before cell division in mice

Hematopoietic stem cells (HSCs) continuously replenish all blood cell types through a series of differentiation steps and repeated cell divisions that involve the generation of lineage-committed progenitors. However, whether cell division in HSCs precedes differentiation is unclear. To this end, we used an HSC cell-tracing approach and Ki67^RFP knock-in mice, in a non-conditioned transplantation model, to assess divisional history, cell cycle progression, and differentiation of adult HSCs. Our results reveal that HSCs are able to differentiate into restricted progenitors, especially common myeloid, megakaryocyte-erythroid and pre-megakaryocyte progenitors, without undergoing cell division and even before entering the S phase of the cell cycle. Additionally, the phenotype of the undivided but differentiated progenitors correlated with the expression of lineage-specific genes and loss of multipotency. Thus HSC fate decisions can be uncoupled from physical cell division. These results facilitate a better understanding of the mechanisms that control fate decisions in hematopoietic cells.

Dependence of hematopoietic stem cell (HSC) fate on the phase of the cell cycle has not been demonstrated in vivo. Here, the authors find that HSCs can differentiate into a downstream progenitor without physical division, even before progressing into the S phase of the cell cycle.

Niches for Hematopoietic Stem Cells and Their Progeny

Steady-state hematopoietic stem cells' (HSCs) self-renewal and differentiation toward their mature progeny in the adult bone marrow is tightly regulated by cues from the microenvironment. Recent insights into the cellular and molecular constituents have uncovered a high level of complexity. Here, we review emerging evidence showing how HSCs and their progeny are regulated by an interdependent network of mesenchymal stromal cells, nerve fibers, the vasculature, and also other hematopoietic cells. Understanding the interaction mechanisms in these intricate niches will provide great opportunities for HSC-related therapies and immune modulation.

Stem cell niche-specific Ebf3 maintains the bone marrow cavity

Bone marrow is the tissue filling the space between bone surfaces. Hematopoietic stem cells (HSCs) are maintained by special microenvironments known as niches within bone marrow cavities. Mesenchymal cells, termed CXC chemokine ligand 12 (CXCL12)-abundant reticular (CAR) cells or leptin receptor-positive (LepR⁺) cells, are a major cellular component of HSC niches that gives rise to osteoblasts in bone marrow. However, it remains unclear how osteogenesis is prevented in most CAR/LepR⁺ cells to maintain HSC niches and marrow cavities. Here, using lineage tracing, we found that the transcription factor early B-cell factor 3 (Ebf3) is preferentially expressed in CAR/LepR⁺ cells and that Ebf3-expressing cells are self-renewing mesenchymal stem cells in adult marrow. When Ebf3 is deleted in CAR/LepR⁺ cells, HSC niche function is severely impaired, and bone marrow is osteosclerotic with increased bone in aged mice. In mice lacking Ebf1 and Ebf3, CAR/LepR⁺ cells exhibiting a normal morphology are abundantly present, but their niche function is markedly impaired with depleted HSCs in infant marrow. Subsequently, the mutants become progressively more osteosclerotic, leading to the complete occlusion of marrow cavities in early adulthood. CAR/LepR⁺ cells differentiate into bone-producing cells with reduced HSC niche factor expression in the absence of Ebf1/Ebf3 Thus, HSC cellular niches express Ebf3 that is required to create HSC niches, to inhibit their osteoblast differentiation, and to maintain spaces for HSCs.

Extrinsic regulation of hematopoietic stem cells in development, homeostasis and diseases

Lifelong generation of blood and immune cells depends on hematopoietic stem cells (HSCs). Their function is precisely regulated by complex molecular networks that integrate and respond to ever changing physiological demands of the body. Over the past several years, significant advances have been made in understanding the extrinsic regulation of HSCs during development and in homeostasis. Propelled by technical advances in the field, the cellular and molecular components of the microenvironment that support HSCs in vivo are emerging. In addition, the interaction of HSCs with their niches is appreciated as a critical contributor to the pathogenesis of a number of hematologic disorders. Here, we review these advances in detail and highlight the extrinsic regulation of HSCs in the context of development, homeostasis, and diseases. WIREs Dev Biol 2017, 6:e279. doi: 10.1002/wdev.279 For further resources related to this article, please visit the WIREs website.