The relationship between bone, hemopoietic stem cells, and vasculature

Abstract

A large body of evidence suggests hemopoietic stem cells (HSCs) exist in an endosteal niche close to bone, whereas others suggest that the HSC niche is intimately associated with vasculature. In this study, we show that transplanted hemopoietic stem and progenitor cells (HSPCs) home preferentially to the trabecular-rich metaphysis of the femurs in nonablated mice at all time points from 15 minutes to 15 hours after transplantation. Within this region, they exist in an endosteal niche in close association with blood vessels. The preferential homing of HSPCs to the metaphysis occurs rapidly after transplantation, suggesting that blood vessels within this region may express a unique repertoire of endothelial adhesive molecules. One candidate is hyaluronan (HA), which is highly ex- IntroductionHemopoietic stem cells (HSCs) exist in a specialized microanatomic space within the bone marrow (BM) termed the niche. 1 Within the niche, cues from the surrounding microenvironment maintain the balance between maintenance of the stem cell pool and proliferation. Despite 30 years of research, the precise location of the HSC niche is unclear. Using different phenotypic markers for hemopoietic stem and progenitor cells (HSPCs), many groups have attempted to define the location of the HSC niche based on its cellular constituents. As examples, the endosteal or osteoblastic niche suggests that HSCs are close to or touching osteoblasts, [2][3][4][5] and the vascular niche suggests that HSCs are close to or in contact with the sinusoidal endothelium. 6,7 Considerable debate has ensued regarding the prominence of each of these niches in HSC regulation. However, several critical questions remain unanswered, including the following: Are these separate or overlapping niches? Do HSCs actually have to be in contact with particular cell types for HSC regulation? Is one cell type more important than others in regulating HSC? Of paramount importance to addressing these questions is an understanding of the three-dimensional relationship between HSCs, blood vessel (BVs), and bone.Most recently, intravital 2-photon or confocal microscopy was used to define the HSC niche by investigating the spatial distribution and lodgement of transplanted fluorescently labeled HSPCs. [7][8][9][10] However, the thick cortical bone of long bones presents a challenge for current high resolution imaging modalities as it is impervious to light. A number of innovative methodologies have recently been developed by researchers to permit imaging of HSPC homing to their niche in long bones either in vivo 8,11 or ex vivo. 10,12 These consist of cutting or splitting the bone, 10,12 grinding the cortical bone until semitransparent, 8 or inserting an imaging probe into the BM. 11 As the vasculature of the bone is continuous with the BM, 13 damaging bone disrupts BM vasculature, inducing a stress response. Stress responses include the up-regulation of the chemokine, stromal derived factor 1 (SDF-1), 14,15 and vascular cell adhesion molecule 1 (VCAM-1), 11,16 both of which a...

The role of CD44 in fetal and adult hematopoietic stem cell regulation

“…Previously, hyaluronan had been shown to be highly haematologica | 2016; 101(1) expressed at the endosteal BM surface as well as acting as a negative regulator of HSC proliferation. 18,48 This could also explain why endosteal HSC have significantly higher CD44 expression compared to centrally isolated HSC.…”

“…20 We have previously demonstrated that HSC isolated from the endosteal region home to BM better than their central counterparts as well as preferentially home back to the endosteal region, 38 and that hyaluronan is highly expressed in the endosteal BM region compared to central BM. 18,48 Therefore, without CD44, HSC and progenitors may have impaired interactions with their microenvironment during the homing process. Furthermore, hyaluronan has been shown to act as a chemoattractant via CD44 63 and we did not find any differences in the expression of HAS3, critical in the generation of hyaluronan, in CD44 -/-mouse BM (data not shown).…”

“…40 unlabeled carrier cells were then transplanted into non-ablated pre-reconstituted mice to assess their lodgment 15 h post-transplant both via flow cytometry as well as histologically in paraffin embedded femur sections. 48 The number of cells located in the endosteal (within 12-cell diameters from the bone/BM interface in the diaphysis or in the metaphysis) and central BM regions were counted manually.…”

<em>In Vivo</em> 4-Dimensional Tracking of Hematopoietic Stem and Progenitor Cells in Adult Mouse Calvarial Bone Marrow

“…HSPCs have been imaged for short periods of time through coverslips placed over excised, fractured femurs [11] , however the impact of this harsh preparation of the tissue on HSPCs is not clear and the technique was used uniquely to obtain single time point observations. Fixed bones have been sectioned into serial sections, stained, and the obtained images reconstructed into a 3D model, providing excellent 3D resolution, especially when vascular casts are used [17] , and recently Laser Scanning Cytometry (LaSC) has been used to obtain quantitative measures about HSPCs in situ, highlighted by immunostaining [18] but are unable to provide any temporal information about the cells behavior. The techniques described in this new technique provide a combination of good resolution in 3D, sufficient temporal sampling for monitoring cells in 4D and they are relatively non-invasive, therefore offering an ideal approach to achieve long-term monitoring of HSPCs interacting with the bone marrow microenvironment.…”

Splenic pooling and loss of VCAM-1 causes an engraftment defect in patients with myelofibrosis after allogeneic hematopoietic stem cell transplantation

“…This process is highly regulated by a dynamic interaction of chemokines and adhesion molecules to ensure a purposive homing and engraftment. 22 We recently demonstrated that more than 80% of transplanted murine HSC home to the BM within 5 h. 23 As our clinical data indicate that early engraftment of neutrophils is significantly delayed in MF patients, we used the clearance of the HSC from the PB as a surrogate marker for the homing of HSC. We show that more than 75% of the transplanted CD34 + cells exited the PB within 40 min and the number of circulating CD34 + cells after 22 h was the same as that prior to transplantation.…”