Endogenous bone marrow MSCs are dynamic, fate-restricted participants in bone maintenance and regeneration

Bmi1 Suppresses Adipogenesis in the Hematopoietic Stem Cell Niche

Bone marrow stromal cells (BMSCs) that express high levels of stem cell factor (SCF) and CXC chemokine ligand 12 (CXCL12) are one crucial component of the hematopoietic stem cell (HSC) niche. While the secreted factors produced by BMSCs to support HSCs have been well described, little is known regarding the transcriptional regulators controlling the cell fate of BMSCs and thus indirectly maintaining HSCs. BMI1 is a polycomb group protein that regulates HSCs both cell intrinsically and extrinsically, but it is unknown in which cell type and how BMI1 functions to maintain HSCs extrinsically. Here we show that Bmi1 maintains HSCs by preventing adipogenic differentiation of BMSCs. Bmi1 is highly expressed in BMSCs but becomes downregulated upon adipogenic differentiation and during aging. Deleting Bmi1 from BMSCs increased marrow adipocytes, induced HSC quiescence and depletion, and impaired hematopoiesis. We found that BMI1 repressed multiple developmental programs in BMSCs by safeguarding the repressive epigenetic marks histone H2A ubiquitylation and H3 lysine 27 trimethylation. We identified a novel adipogenic program governed by Pax3, which BMI1 repressed in BMSCs. Our results establish Bmi1 as a critical regulator of BMSC cell fate that suppresses marrow adipogenesis to create a supportive niche for HSCs.

Subchondral bone remodelling in osteoarthritis

Subchondral bone remodelling is an integral part of osteoarthritis and involves the development of subchondral sclerosis seen on plain imaging, along with osteophyte formation.

The development of these changes is due to persistent abnormal mechanical stresses which create a cellular and biomolecular response to microfractures in the subchondral bone and osteochondral junction.

An early sign is bone marrow lesions seen on MRI scanning. Healing can occur at this stage by correcting the abnormal loads. Persistence leads to what is thought to be a delayed union or nonunion response by the bone.

Microfractures of the osteochondral junction, coupled with articular cartilage fissuring and loss, allows synovial fluid to penetrate the subchondral bone along with cytokines and other molecules reacting with the bone cells to increase the pathological effects.

This review gives an overview of the current thoughts on subchondral bone remodelling in osteoarthritis that is aimed at orthopaedic surgeons to help in the understanding of the pathogenesis of osteoarthritis and the role of surgical management.

Cite this article: EFORT Open Rev 2019;4 DOI: 10.1302/2058-5241.4.180102

New Insights on Properties and Spatial Distributions of Skeletal Stem Cells

Skeletal stem cells (SSCs) are postnatal self-renewing, multipotent, and skeletal lineage-committed progenitors that are capable of giving rise to cartilage, bone, and bone marrow stroma including marrow adipocytes and stromal cells in vitro and in an exogenous environment after transplantation in vivo. Identifying and isolating defined SSCs as well as illuminating their spatiotemporal properties contribute to our understating of skeletal biology and pathology. In this review, we revisit skeletal stem cells identified most recently and systematically discuss their origin and distributions.

Elimination of senescent osteoclast progenitors has no effect on the age-associated loss of bone mass in mice

Both an increase in osteoclast and a decrease in osteoblast numbers contribute to skeletal aging. Markers of cellular senescence, including expression of the cyclin inhibitor p16, increase with aging in several bone cell populations. The elimination of p16-expressing cells in old mice, using the INK-ATTAC transgene, increases bone mass indicating that senescent cells contribute to skeletal aging. However, the identity of the senescent cells and the extent to which ablation of p16-expressing cells may prevent skeletal aging remain unknown. Using mice expressing the p16-3MR transgene, we examined whether elimination of p16-expressing cells between 12 and 24 months of age could preserve bone mass; and whether elimination of these cells from 20 to 26 months of age could restore bone mass. The activation of the p16-3MR transgene by ganciclovir (GCV) greatly diminished p16 levels in the brain, liver, and osteoclast progenitors from the bone marrow. The age-related increase in osteoclastogenic potential of myeloid cells was also abrogated by GCV. However, GCV did not alter p16 levels in osteocytes-the most abundant cell type in bone-and had no effect on the skeletal aging of p16-3MR mice. These findings indicate that the p16-3MR transgene does not eliminate senescent osteocytes but it does eliminate senescent osteoclast progenitors and senescent cells in other tissues, as described previously. Elimination of senescent osteoclast progenitors, in and of itself, has no effect on the age-related loss of bone mass. Hence, other senescent cell types, such as osteocytes, must be the seminal culprits.

A Cellular Taxonomy of the Bone Marrow Stroma in Homeostasis and Leukemia

Stroma is a poorly defined non-parenchymal component of virtually every organ with key roles in organ development, homeostasis, and repair. Studies of the bone marrow stroma have defined individual populations in the stem cell niche regulating hematopoietic regeneration and capable of initiating leukemia. Here, we use single-cell RNA sequencing (scRNA-seq) to define a cellular taxonomy of the mouse bone marrow stroma and its perturbation by malignancy. We identified seventeen stromal subsets expressing distinct hematopoietic regulatory genes spanning new fibroblastic and osteoblastic subpopulations including distinct osteoblast differentiation trajectories. Emerging acute myeloid leukemia impaired mesenchymal osteogenic differentiation and reduced regulatory molecules necessary for normal hematopoiesis. These data suggest that tissue stroma responds to malignant cells by disadvantaging normal parenchymal cells. Our taxonomy of the stromal compartment provides a comprehensive bone marrow cell census and experimental support for cancer cell crosstalk with specific stromal elements to impair normal tissue function and thereby enable emergent cancer.

Mitochondrial Regulation of Stem Cells in Bone Homeostasis

Mitochondria have emerged as key contributors to the organismal homeostasis, in which mitochondrial regulation of stem cells is becoming increasingly important. Originated from mesenchymal stem cell (MSC) and hematopoietic stem cell (HSC) lineage commitments and interactions, bone is a representative organ where the mitochondrial essentiality to stem cell function has most recently been discovered, underlying skeletal health, aging, and diseases. Furthermore, mitochondrial medications based on modulating stem cell specification are emerging to provide promising therapies to counteract bone aging and pathologies. Here we review the cutting-edge knowledge regarding mitochondrial regulation of stem cells in bone homeostasis, highlighting mechanistic insights as well as mitochondrial strategies for augmented bone healing and tissue regeneration.

Mesenchymal stem cells targeting PI3K/AKT pathway in leukemic model

Mesenchymal stem cells have therapeutic properties that are related to their potentials for trans-differentiation, immunomodulation, anti-inflammatory, inhibitory effect on tumor proliferation, and induction of apoptosis. This study was performed to analyze the role of mesenchymal stem cells as an alternative for cellular signaling growth factors involved in the pathogenesis of leukemogenesis in rats. Treatment of rats with 7,12-dimethyl benz [a] anthracene induced leukemogenesis appeared as a significant decrease in hematological parameters with concomitant significant increase in bone marrow oxidative and inflammatory indices (transforming growth factor beta and interleukin-6) in comparison with normal groups. On the contrary, Western immunoblotting showed a significant increase in the signaling growth factors: PI3K, AKT, mTOR proteins and a significant decrease in PTEN in 7,12-dimethyl benz [a] anthracene-treated group. In addition, a significant increase in the transcript levels of B cell lymphoma-2 protein gene in the 7,12-dimethyl benz [a] anthracene group, while that of C-X-C motif chemokine receptor-4 and B cell lymphoma-2 protein associated x-protein were significantly downregulated compared to controls. Meanwhile, therapeutic mesenchymal stem cells treatment predict a significant improvement versus 7,12-dimethyl benz [a] anthracene group through the modulation of growth factors that confront bone marrow dysplasia. In the same direction treatment of 7,12-dimethyl benz [a] anthracene group with mesenchymal stem cells, it induced apoptosis and increased the homing efficacy to bone marrow. In conclusion, mesenchymal stem cells improve hematopoiesis and alleviate inflammation, and modulated PI3K/AKT signaling pathway contributed to experimental leukemogenesis.

Bone marrow mesenchymal stem cells: Aging and tissue engineering applications to enhance bone healing

Bone has well documented natural healing capacity that normally is sufficient to repair fractures and other common injuries. However, the properties of bone change throughout life, and aging is accompanied by increased incidence of bone diseases and compromised fracture healing capacity, which necessitate effective therapies capable of enhancing bone regeneration. The therapeutic potential of adult mesenchymal stem cells (MSCs) for bone repair has been long proposed and examined. Actions of MSCs may include direct differentiation to become bone cells, attraction and recruitment of other cells, or creation of a regenerative environment via production of trophic growth factors. With systemic aging, MSCs also undergo functional decline, which has been well investigated in a number of recent studies. In this review, we first describe the changes in MSCs during aging and discuss how these alterations can affect bone regeneration. We next review current research findings on bone tissue engineering, which is considered a promising and viable therapeutic solution for structural and functional restoration of bone. In particular, the importance of MSCs and bioscaffolds is highlighted. Finally, potential approaches for the prevention of MSC aging and the rejuvenation of aged MSC are discussed.

Lineage Tracing Reveals a Subset of Reserve Muscle Stem Cells Capable of Clonal Expansion under Stress

Stem cell heterogeneity is recognized as functionally relevant for tissue homeostasis and repair. The identity, context dependence, and regulation of skeletal muscle satellite cell (SC) subsets remains poorly understood. We identify a minor subset of Pax7⁺ SCs that is indelibly marked by an inducible Mx1-Cre transgene in vivo, is enriched for Pax3 expression, and has reduced ROS (reactive oxygen species) levels. Mx1⁺ SCs possess potent stem cell activity upon transplantation but minimally contribute to endogenous muscle repair, due to their relative low abundance. In contrast, a dramatic clonal expansion of Mx1⁺ SCs allows extensive contribution to muscle repair and niche repopulation upon selective pressure of radiation stress, consistent with reserve stem cell (RSC) properties. Loss of Pax3 in RSCs increased ROS content and diminished survival and stress tolerance. These observations demonstrate that the Pax7⁺ SC pool contains a discrete population of radiotolerant RSCs that undergo clonal expansion under severe stress.

Mesenchymal stem cells: From regeneration to cancer

Mesenchymal stem cells (MSCs) are multipotent tissue stem cells that differentiate into a number of mesodermal tissue types, including osteoblasts, adipocytes, chondrocytes and myofibroblasts. MSCs were originally identified in the bone marrow (BM) of humans and other mammals, but recent studies have shown that they are multilineage progenitors in various adult organs and tissues. MSCs that localize at perivascular sites function to rapidly respond to external stimuli and coordinate with the vascular and immune systems to accomplish the wound healing process. Cancer, considered as wounds that never heal, is also accompanied by changes in MSCs that parallels the wound healing response. MSCs are now recognized as key players at distinct steps of tumorigenesis. In this review, we provide an overview of the function of MSCs in wound healing and cancer progression with the goal of providing insight into the development of novel MSC-manipulating strategies for clinical cancer treatment.

A tale of the good and bad: Cell senescence in bone homeostasis and disease

Historically, cellular senescence has been viewed as an irreversible cell-cycle arrest process with distinctive phenotypic alterations that were implicated primarily in aging and tumor suppression. Recent discoveries suggest that cellular senescence represents a series of diverse, dynamic, and heterogeneous cellular states with the senescence-associated secretory phenotype (SASP). Although senescent cells typically contribute to aging and age-related diseases, accumulating evidence has shown that they also have important physiological functions during embryonic development, late pubertal bone growth cessation, and adulthood tissue remodeling. Here, we review the recent research on cellular senescence and SASP, highlighting the key pathways that mediate senescence cell-cycle arrest and initiate SASP. We also summarize recent literature on the role of cellular senescence in maintaining bone homeostasis and mediating age-associated osteoporosis, discussing both the beneficial and adverse roles of cellular senescence in bone during different physiological stages, including bone development, childhood bone growth, adulthood bone remodeling, and bone aging.

Sox9+ messenger cells orchestrate large-scale skeletal regeneration in the mammalian rib

Most bones in mammals display a limited capacity for natural large-scale repair. The ribs are a notable exception, yet the source of their remarkable regenerative ability remains unknown. Here, we identify a Sox9-expressing periosteal subpopulation that orchestrates large-scale regeneration of murine rib bones. Deletion of the obligate Hedgehog co-receptor, Smoothened, in Sox9-expressing cells prior to injury results in a near-complete loss of callus formation and rib bone regeneration. In contrast to its role in development, Hedgehog signaling is dispensable for the proliferative expansion of callus cells in response to injury. Instead, Sox9-positive lineage cells require Hh signaling to stimulate neighboring cells to differentiate via an unknown signal into a skeletal cell type with dual chondrocyte/osteoblast properties. This type of callus cell may be critical for bridging large bone injuries. Thus despite contributing to only a subset of callus cells, Sox9-positive progenitors play a major role in orchestrating large-scale bone regeneration.

Editorial note

This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Glutamine Metabolism Regulates Proliferation and Lineage Allocation in Skeletal Stem Cells

Skeletal stem cells (SSCs) are postulated to provide a continuous supply of osteoblasts throughout life. However, under certain conditions, the SSC population can become incorrectly specified or is not maintained, resulting in reduced osteoblast formation, decreased bone mass, and in severe cases, osteoporosis. Glutamine metabolism has emerged as a critical regulator of many cellular processes in diverse pathologies. The enzyme glutaminase (GLS) deaminates glutamine to form glutamate-the rate-limiting first step in glutamine metabolism. Using genetic and metabolic approaches, we demonstrate GLS and glutamine metabolism are required in SSCs to regulate osteoblast and adipocyte specification and bone formation. Mechanistically, transaminase-dependent α-ketoglutarate production is critical for the proliferation, specification, and differentiation of SSCs. Collectively, these data suggest stimulating GLS activity may provide a therapeutic approach to expand SSCs in aged individuals and enhance osteoblast differentiation and activity to increase bone mass.

The mesenchymal niche in MDS

Myelodysplastic syndrome (MDS) is characterized by bone marrow failure and a strong propensity for leukemic evolution. Somatic mutations are critical early drivers of the disorder, but the factors enabling the emergence, selection, and subsequent leukemic evolution of these “leukemia-poised” clones remain incompletely understood. Emerging data point at the mesenchymal niche as a critical contributor to disease initiation and evolution. Disrupted inflammatory signaling from niche cells may facilitate the occurrence of somatic mutations, their selection, and subsequent clonal expansion. This review summarizes the current concepts about “niche-facilitated” bone marrow failure and leukemic evolution, their underlying molecular mechanisms, and clinical implications for future innovative therapeutic targeting of the niche in MDS.

Mesenchymal lineage cells and their importance in B lymphocyte niches

Early B lymphopoiesis occurs in the bone marrow and is reliant on interactions with numerous cell types in the bone marrow microenvironment, particularly those of the mesenchymal lineage. Each cellular niche that supports the distinct stages of B lymphopoiesis is unique. Different cell types and signaling molecules are important for the progressive stages of B lymphocyte differentiation. Cells expressing CXCL12 and IL-7 have long been recognized as having essential roles in facilitating progression through stages of B lymphopoiesis. Recently, a number of other factors that extrinsically mediate B lymphopoiesis (positively or negatively) have been identified. In addition, the use of transgenic mouse models to delete specific genes in mesenchymal lineage cells has further contributed to our understanding of how B lymphopoiesis is regulated in the bone marrow. This review will cover the current understanding of B lymphocyte niches in the bone marrow and key extrinsic molecules and signaling pathways involved in these niches, with a focus on how mesenchymal lineage cells regulate B lymphopoiesis.

Digit Tip Injuries: Current Treatment and Future Regenerative Paradigms

Over the past several decades there has been a profound increase in the understanding of tissue regeneration, driven largely by the observance of the tremendous regenerative capacity in lower order life forms, such as hydra and urodeles. However, it is known that humans and other mammals retain the ability to regenerate the distal phalanges of the digits after amputation. Despite the increased knowledge base on model organisms regarding regenerative paradigms, there is a lack of application of regenerative medicine techniques in clinical practice in regard to digit tip injury. Here, we review the current understanding of digit tip regeneration and discuss gaps that remain in translating regenerative medicine into clinical treatment of digit amputation.

Mesenchymal Stem Cells in Renal Fibrosis: The Flame of Cytotherapy

Renal fibrosis, as the fundamental pathological process of chronic kidney disease (CKD), is a pathologic extension of the normal wound healing process characterized by endothelium injury, myofibroblast activation, macrophage migration, inflammatory signaling stimulation, matrix deposition, and remodelling. Yet, the current method of treating renal fibrosis is fairly limited, including angiotensin-converting enzyme inhibition, angiotensin receptor blockade, optimal blood pressure control, and sodium bicarbonate for metabolic acidosis. MSCs are pluripotent adult stem cells that can differentiate into various types of tissue lineages, such as the cartilage (chondrocytes), bone (osteoblasts), fat (adipocytes), and muscle (myocytes). Because of their many advantages like ubiquitous sources, convenient procurement and collection, low immunogenicity, and low adverse effects, with their special identification markers, mesenchymal stem MSC-based therapy is getting more and more attention. Based on the mechanism of renal fibrosis, MSCs mostly participate throughout the renal fibrotic process. According to the latest and overall literature reviews, we aim to elucidate the antifibrotic mechanisms and effects of diverse sources of MSCs on renal fibrosis, assess their efficacy and safety in preliminarily clinical application, answer the controversial questions, and provide novel ideas into the MSC cellular therapy of renal fibrosis.

Withdrawal of parathyroid hormone after prolonged administration leads to adipogenic differentiation of mesenchymal precursors in vivo

Intermittent PTH-like drugs are the only approved so-called anabolic agent that increases bone mass in both mice and humans. It is well documented that PTH targets mature cells of the osteoblast lineage, with only indirect evidence of its actions on early cells of the osteoblast lineage. Using a triple transgenic mouse model that allowed labeling of very early cells of the osteoblast lineage, we traced the progeny of these into osteoblast lineage in adult mice. These early cells expressed PTH1R and multiplied when PTH (1-34) was administered daily. We also showed that the early mesenchymal cells showed accelerated differentiation into mature osteocalcin-positive osteoblasts and osteocytes. Rather surprisingly, when teriparatide administration was stopped, these early mesenchymal precursors differentiated into adipocytes. We showed that the adipogenic differentiation is accompanied by a decrease in wnt signaling in osteoblast precursors. In this review, we discuss the possible clinical relevance of this finding and the possible molecular mechanisms that contribute to this phenotype in vivo.

Bioluminescence Imaging as a Tool for Poxvirus Biology

Bioluminescence imaging, with luciferase as a reporter-encoding gene, has been successfully and widely used for studies to follow viral infection in an organism and to measure therapeutic efficacy of antiviral agents in small animal models. Bioluminescence is produced by the reaction of a luciferase enzyme stably inserted into the viral genome with a defined substrate systemically delivered into the animal. The light emitted is captured allowing the detection of viral infection sites and the quantification of viral replication in the context of tissues of a living animal. The goal of this chapter is to provide a technical background for the evaluation of poxvirus infection in cells and animals through bioluminescence imaging technology using luciferase-expressing recombinant poxviruses.

Identification of endogenous migratory MSC-like cells and their interaction with the implant materials guiding osteochondral defect repair

Abundant BMSCs and MSC-like cells move up to the defect area and interact with the implant materials, guiding the osteochondral defect repair.

Atrophic nonunion stromal cells form bone and recreate the bone marrow environment in vivo

Introduction:

Nonunion is a challenging condition in orthopaedics as its etiology is not fully understood. Clinical interventions currently aim to stimulate both the biological and mechanical aspects of the bone healing process by using bone autografts and surgical fixation. However, recent observations showed that atrophic nonunion tissues contain putative osteoprogenitors, raising the hypothesis that its reactivation could be explored to achieve bone repair.

Methods:

Here we characterized atrophic nonunion stromal cells (NUSC) in vitro, using bone marrow stromal cells (BMSC) and osteoblasts as controls cells of the osteoblastic lineage, and evaluated its ability to form bone in vivo.

Results:

NUSC had proliferative and senescence rates comparable to BMSC and osteoblasts, and homogeneously expressed the osteolineage markers CD90 and CD73. Regarding CD105 and CD146 expression, NUSC were closely related to osteoblasts, both with an inferior percentage of CD105⁺/CD146⁺ cells as compared to BMSC. Despite this, NUSC differentiated along the osteogenic and adipogenic lineages in vitro; and when transplanted subcutaneously into immunocompromised mice, new bone formation and hematopoietic marrow were established.

Conclusions:

This study demonstrates that NUSC are osteogenically competent, supporting the hypothesis that their endogenous reactivation could be a strategy to stimulate the bone formation while reducing the amount of bone autograft requirements.

Mesenchymal Stromal Cells: Role in the BM Niche and in the Support of Hematopoietic Stem Cell Transplantation

Mesenchymal stromal cells (MSCs) are key elements in the bone marrow (BM) niche where they interact with hematopoietic stem progenitor cells (HSPCs) by offering physical support and secreting soluble factors, which control HSPC maintenance and fate. Although necessary for their maintenance, MSCs are a rare population in the BM, they are plastic adherent and can be ex vivo expanded to reach numbers adequate for clinical use. In light of HSPC supportive properties, MSCs have been employed in phase I/II clinical trials of hematopoietic stem cell transplantation (HSCT) to facilitate engraftment of hematopoietic stem cells (HSCs). Moreover, they have been utilized to expand ex vivo HSCs before clinical use. The available clinical evidence from these trials indicate that MSC administration is safe, as no acute and long-term adverse events have been registered in treated patients, and may be efficacious in promoting hematopoietic engraftment after HSCT. In this review, we critically discuss the role of MSCs as component of the BM niche, as recent advances in defining different mesenchymal populations in the BM have considerably increased our understanding of this complex environment. Moreover, we will revise published literature on the use of MSCs to support HSC engraftment and expansion, as well as consider potential new MSC application in the clinical context of ex vivo gene therapy with autologous HSC.

Attenuation of PKCδ enhances metabolic activity and promotes expansion of blood progenitors

A finely tuned balance of self‐renewal, differentiation, proliferation, and survival governs the pool size and regenerative capacity of blood‐forming hematopoietic stem and progenitor cells (HSPCs). Here, we report that protein kinase C delta (PKCδ) is a critical regulator of adult HSPC number and function that couples the proliferative and metabolic activities of HSPCs. PKCδ‐deficient mice showed a pronounced increase in HSPC numbers, increased competence in reconstituting lethally irradiated recipients, enhanced long‐term competitive advantage in serial transplantation studies, and an augmented HSPC recovery during stress. PKCδ‐deficient HSPCs also showed accelerated proliferation and reduced apoptosis, but did not exhaust in serial transplant assays or induce leukemia. Using inducible knockout and transplantation models, we further found that PKCδ acts in a hematopoietic cell‐intrinsic manner to restrict HSPC number and bone marrow regenerative function. Mechanistically, PKCδ regulates HSPC energy metabolism and coordinately governs multiple regulators within signaling pathways implicated in HSPC homeostasis. Together, these data identify PKCδ as a critical regulator of HSPC signaling and metabolism that acts to limit HSPC expansion in response to physiological and regenerative demands.

Alkaline Phosphatase Controls Lineage Switching of Mesenchymal Stem Cells by Regulating the LRP6/GSK3β Complex in Hypophosphatasia

Lineage differentiation of bone marrow mesenchymal stem cells (BMMSCs) is the key to bone-fat reciprocity in bone marrow. To date, the regulators of BMMSC lineage switching have all been identified to be transcription factors, and researchers have not determined whether other genes control this process. This study aims to reveal a previously unknown role of tissue-nonspecific alkaline phosphatase (TNSALP) in controlling BMMSC lineage selection.

Methods: We compared the characteristics of cultured BMMSCs from patients with hypophosphatasia (HPP), which is caused by mutations in the liver/bone/kidney alkaline phosphatase (ALPL) gene, and an ALPL knockout (ko) mouse model. We performed ALPL downregulation and overexpression experiments to investigate the regulatory role of ALPL in BMMSC lineage switching. Using the PathScan array, coimmunoprecipitation experiments and pathway-guided small molecule treatments, we explored the possible mechanism underlying the regulatory effects of ALPL on cell differentiation and evaluated its therapeutic effect on ALPL ko mice.

Results: BMMSCs from both patients with HPP and ALPL ko mice exhibited defective lineage differentiation, including a decrease in osteogenic differentiation and a parallel increase in adipogenic differentiation. Mechanistically, TNSALP directly interacted with LRP6 and regulated the phosphorylation of GSK3β, subsequently resulting in lineage switching of BMMSCs. Re-phosphorylation of GSK3β induced by LiCl treatment restored differentiation of BMMSCs and attenuated skeletal deformities in Alpl^+/- mice.

Conclusion: Based on our findings, TNSALP acts as a signal regulator to control lineage switching of BMMSCs by regulating the LRP6/GSK3β cascade.

Modeling human RNA spliceosome mutations in the mouse: not all mice were created equal

Myelodysplastic syndromes (MDS) and related myelodysplastic/myeloproliferative neoplasms (MDS/MPNs) are clonal stem cell disorders, primarily affecting patients over 65 years of age. Mapping of the MDS and MDS/MPN genome identified recurrent heterozygous mutations in the RNA splicing machinery, with the SF3B1, SRSF2, and U2AF1 genes being frequently mutated. To better understand how spliceosomal mutations contribute to MDS pathogenesis in vivo, numerous groups have sought to establish conditional murine models of SF3B1, SRSF2, and U2AF1 mutations. The high degree of conservation of hematopoiesis between mice and human and the well-established phenotyping and genetic modification approaches make murine models an effective tool with which to study how a gene mutation contributes to disease pathogenesis. The murine models of spliceosomal mutations described to date recapitulate human MDS or MDS/MPN to varying extents. Reasons for the differences in phenotypes reported between alleles of the same mutation are varied, but the nature of the genetic modification itself and subsequent analysis methods are important to consider. In this review, we summarize recently reported murine models of SF3B1, SRSF2, and U2AF1 mutations, with a particular focus on the genetically engineered modifications underlying the models and the experimental approaches applied.

Elevated extracellular calcium ions promote proliferation and migration of mesenchymal stem cells via increasing osteopontin expression

Supplementation of mesenchymal stem cells (MSCs) at sites of bone resorption is required for bone homeostasis because of the non-proliferation and short lifespan properties of the osteoblasts. Calcium ions (Ca2+) are released from the bone surfaces during osteoclast-mediated bone resorption. However, how elevated extracellular Ca2+ concentrations would alter MSCs behavior in the proximal sites of bone resorption is largely unknown. In this study, we investigated the effect of extracellular Ca2+ on MSCs phenotype depending on Ca2+ concentrations. We found that the elevated extracellular Ca2+ promoted cell proliferation and matrix mineralization of MSCs. In addition, MSCs induced the expression and secretion of osteopontin (OPN), which enhanced MSCs migration under the elevated extracellular Ca2+ conditions. We developed in vitro osteoclast-mediated bone resorption conditions using mouse calvaria bone slices and demonstrated Ca2+ is released from bone resorption surfaces. We also showed that the MSCs phenotype, including cell proliferation and migration, changed when the cells were treated with a bone resorption-conditioned medium. These findings suggest that the dynamic changes in Ca2+ concentrations in the microenvironments of bone remodeling surfaces modulate MSCs phenotype and thereby contribute to bone regeneration.

Establishment of stably expandable induced myogenic stem cells by four transcription factors

Life-long regeneration of healthy muscle by cell transplantation is an ideal therapy for patients with degenerative muscle diseases. Yet, obtaining muscle stem cells from patients is very limited due to their exhaustion in disease condition. Thus, development of a method to obtain healthy myogenic stem cells is required. Here, we showed that the four transcription factors, Six1, Eya1, Esrrb, and Pax3, converts fibroblasts into induced myogenic stem cells (iMSCs). The iMSCs showed effective differentiation into multinucleated myotubes and also higher proliferation capacity than muscle derived stem cells both in vitro and in vivo. The iMSCs do not lose their proliferation capacity though the passaging number is increased. We further isolated CD106-negative and α7-integrin-positive iMSCs (sort-iMSCs) showing higher myogenic differentiation capacity than iMSCs. Moreover, genome-wide transcriptomic analysis of iMSCs and sort-iMSCs, followed by network analysis, revealed the genes and signaling pathways associated with enhanced proliferation and differentiation capacity of iMSCs and sort-iMSCs, respectively. The stably expandable iMSCs provide a new source for drug screening and muscle regenerative therapy for muscle wasting disease.

Blastema formation and periosteal ossification in the regenerating adult mouse digit

While mammals cannot regenerate amputated limbs, mice and humans have regenerative ability restricted to amputations transecting the digit tip, including the terminal phalanx (P3). In mice, the regeneration process is epimorphic and mediated by the formation of a blastema comprised of undifferentiated proliferating cells that differentiate to regenerate the amputated structures. Blastema formation distinguishes the regenerative response from a scar-forming healing response. The mouse digit tip serves as a preclinical model to investigate mammalian blastema formation and endogenous regenerative capabilities. We report that P3 blastema formation initiates prior to epidermal closure and concurrent with the bone histolytic response. In this early healing response, proliferation and cells entering the early stages of osteogenesis are localized to the periosteal and endosteal bone compartments. After the completion of stump bone histolysis, epidermal closure is completed and cells associated with the periosteal and endosteal compartments blend to form the blastema proper. Osteogenesis associated with the periosteum occurs as a polarized progressive wave of new bone formation that extends from the amputated stump and restores skeletal length. Bone patterning is restored along the proximal-distal and medial digit axes, but is imperfect in the dorsal-ventral axis with the regeneration of excessive new bone that accounts for the enhanced regenerated bone volume noted in previous studies. Periosteum depletion studies show that this compartment is required for the regeneration of new bone distal to the original amputation plane. These studies provide evidence that blastema formation initiates early in the healing response and that the periosteum is an essential tissue for successful epimorphic regeneration in mammals.

The Majority of CD45- Ter119- CD31- Bone Marrow Cell Fraction Is of Hematopoietic Origin and Contains Erythroid and Lymphoid Progenitors

The non-hematopoietic cell fraction of the bone marrow (BM) is classically identified as CD45^- Ter119^- CD31^- (herein referred to as triple-negative cells or TNCs). Although TNCs are believed to contain heterogeneous stromal cell populations, they remain poorly defined. Here we showed that the vast majority of TNCs (∼85%) have a hematopoietic rather than mesenchymal origin. Single cell RNA-sequencing revealed erythroid and lymphoid progenitor signatures among CD51^- TNCs. Ly6D⁺ CD44⁺ CD51^- TNCs phenotypically and functionally resembled CD45⁺ pro-B lymphoid cells, whereas Ly6D^- CD44⁺ CD51^- TNCs were enriched in previously unappreciated stromal-dependent erythroid progenitors hierarchically situated between preCFU-E and proerythroblasts. Upon adoptive transfer, CD44⁺ CD51^- TNCs contributed to repopulate the B-lymphoid and erythroid compartments. CD44⁺ CD51^- TNCs also expanded during phenylhydrazine-induced acute hemolysis or in a model of sickle cell anemia. These findings thus uncover physiologically relevant new classes of stromal-associated functional CD45^- hematopoietic progenitors.

Alpl prevents bone ageing sensitivity by specifically regulating senescence and differentiation in mesenchymal stem cells

Mutations in the liver/bone/kidney alkaline phosphatase (Alpl) gene cause hypophosphatasia (HPP) and early-onset bone dysplasia, suggesting that this gene is a key factor in human bone development. However, how and where Alpl acts in bone ageing is largely unknown. Here, we determined that ablation of Alpl induces prototypical premature bone ageing characteristics, including bone mass loss and marrow fat gain coupled with elevated expression of p16INK4A (p16) and p53 due to senescence and impaired differentiation in mesenchymal stem cells (MSCs). Mechanistically, Alpl deficiency in MSCs enhances ATP release and reduces ATP hydrolysis. Then, the excessive extracellular ATP is, in turn, internalized by MSCs and causes an elevation in the intracellular ATP level, which consequently inactivates the AMPKα pathway and contributes to the cell fate switch of MSCs. Reactivating AMPKα by metformin treatment successfully prevents premature bone ageing in Alpl+/- mice by improving the function of endogenous MSCs. These results identify a previously unknown role of Alpl in the regulation of ATP-mediated AMPKα alterations that maintain MSC stemness and prevent bone ageing and show that metformin offers a potential therapeutic option.

T cell immunomodulation by clinically used allogeneic human cancellous bone fragments: a potential novel immunotherapy tool

Multipotential stromal cells (MSCs) demonstrate strong immunomodulation capabilities following culture expansion. We have previously demonstrated that human cancellous bone fragments (CBFs) clinically used as viable allografts for spinal fusion have resident MSCs that exhibit T cell immunomodulation after monolayer expansion. This study investigated the immunomodulatory ability of these CBFs without MSC culture-expansion. CD4 positive T cells were induced to proliferate using CD3/CD28 stimulation and added to CBFs at different ratios of T cells per gram of CBF. A dose-dependent suppressive effect on T cell proliferation was evident and correlated with increased culture supernatant levels of TGF-ß1, but not PGE2. CBF-driven immunosuppression was reduced in co-cultures with TGF-ß neutralising antibodies and was higher in cell contact compared to non-contact cultures. CBF gene expression profile identified vascular cell adhesion molecule-1, bone marrow stromal antigen 2/CD317 and other interferon signalling pathway members as potential immunomodulatory mediators. The CD317 molecule was detected on the surface of CBF-resident cells confirming the gene expression data. Taken together, these data demonstrate that human clinically used CBFs are inherently immunomodulatory and suggest that these viable allografts may be used to deliver therapeutic immunomodulation for immune-related diseases.

The Spectrum of Fundamental Basic Science Discoveries Contributing to Organismal Aging

Aging research has undergone unprecedented advances at an accelerating rate in recent years, leading to excitement in the field as well as opportunities for imagination and innovation. Novel insights indicate that, rather than resulting from a preprogrammed series of events, the aging process is predominantly driven by fundamental non-adaptive mechanisms that are interconnected, linked, and overlap. To varying degrees, these mechanisms also manifest with aging in bone where they cause skeletal fragility. Because these mechanisms of aging can be manipulated, it might be possible to slow, delay, or alleviate multiple age-related diseases and their complications by targeting conserved genetic signaling pathways, controlled functional networks, and basic biochemical processes. Indeed, findings in various mammalian species suggest that targeting fundamental aging mechanisms (eg, via either loss-of-function or gain-of-function mutations or administration of pharmacological therapies) can extend healthspan; ie, the healthy period of life free of chronic diseases. In this review, we summarize the evidence supporting the role of the spectrum of fundamental basic science discoveries contributing to organismal aging, with emphasis on mammalian studies and in particular aging mechanisms in bone that drive skeletal fragility. These mechanisms or aging hallmarks include: genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication. Because these mechanisms are linked, interventions that ameliorate one hallmark can in theory ameliorate others. In the field of bone and mineral research, current challenges include defining the relative contributions of each aging hallmark to the natural skeletal aging process, better understanding the complex interconnections among the hallmarks, and identifying the most effective therapeutic strategies to safely target multiple hallmarks. Based on their interconnections, it may be feasible to simultaneously interfere with several fundamental aging mechanisms to alleviate a wide spectrum of age-related chronic diseases, including osteoporosis. © 2018 American Society for Bone and Mineral Research.

Pituitary Diseases and Bone

Neuroendocrinology of bone is a new area of research based on the evidence that pituitary hormones may directly modulate bone remodeling and metabolism. Skeletal fragility associated with high risk of fractures is a common complication of several pituitary diseases such as hypopituitarism, Cushing disease, acromegaly, and hyperprolactinemia. As in other forms of secondary osteoporosis, pituitary diseases generally affect bone quality more than bone quantity, and fractures may occur even in the presence of normal or low-normal bone mineral density as measured by dual-energy X-ray absorptiometry, making difficult the prediction of fractures in these clinical settings. Treatment of pituitary hormone excess and deficiency generally improves skeletal health, although some patients remain at high risk of fractures, and treatment with bone-active drugs may become mandatory. The aim of this review is to discuss the physiological, pathophysiological, and clinical insights of bone involvement in pituitary diseases.

Origin of Reparative Stem Cells in Fracture Healing

PURPOSE OF REVIEW

The identity and functional roles of stem cell population(s) that contribute to fracture repair remains unclear. This review provides a brief history of mesenchymal stem cell (MSCs) and provides an updated view of the many stem/progenitor cell populations contributing to fracture repair.

RECENT FINDINGS

Functional studies show MSCs are not the multipotential stem cell population that form cartilage and bone during fracture repair. Rather, multiple studies have confirmed the periosteum is the primary source of stem/progenitor cells for fracture repair. Newer work is also identifying other stem/progenitor cells that may also contribute to healing. Although the heterogenous periosteal cells migrate to the fracture site and contribute directly to callus formation, other cell populations are involved. Pericytes and bone marrow stromal cells are now thought of as key secretory centers that mostly coordinate the repair process. Other populations of stem/progenitor cells from the muscle and transdifferentiated chondroctyes may also contribute to repair, and their functional role is an area of active research.

A Muscle Stem Cell Support Group: Coordinated Cellular Responses in Muscle Regeneration

Skeletal muscle has an extraordinary regenerative capacity due to the activity of tissue-specific muscle stem cells. Consequently, these cells have received the most attention in studies investigating the cellular processes of skeletal muscle regeneration. However, efficient capacity to rebuild this tissue also depends on additional cells in the local milieu, as disrupting their normal contributions often leads to incomplete regeneration. Here, we review these additional cells that contribute to the regenerative process. Understanding the complex interactions between and among these cell populations has the potential to lead to therapies that will help promote normal skeletal muscle regeneration under conditions in which this process is suboptimal.

Molecular mechanisms of glucocorticoids on skeleton and bone regeneration after fracture

Glucocorticoid hormones (GCs) have profound effects on bone metabolism. Via their nuclear hormone receptor - the GR - they act locally within bone cells and modulate their proliferation, differentiation, and cell death. Consequently, high glucocorticoid levels - as present during steroid therapy or stress - impair bone growth and integrity, leading to retarded growth and glucocorticoid-induced osteoporosis, respectively. Because of their profound impact on the immune system and bone cell differentiation, GCs also affect bone regeneration and fracture healing. The use of conditional-mutant mouse strains in recent research provided insights into the cell-type-specific actions of the GR. However, despite recent advances in system biology approaches addressing GR genomics in general, little is still known about the molecular mechanisms of GCs and GR in bone cells. Here, we review the most recent findings on the molecular mechanisms of the GR in general and the known cell-type-specific actions of the GR in mesenchymal cells and their derivatives as well as in osteoclasts during bone homeostasis, GC excess, bone regeneration and fracture healing.

Clonal Analysis Delineates Transcriptional Programs of Osteogenic and Adipogenic Lineages of Adult Mouse Skeletal Progenitors

Bone, cartilage, and marrow adipocytes are generated by skeletal progenitors, but the relationships between lineages and mechanisms controlling their differentiation are poorly understood. We established mouse clonal skeletal progenitors with distinct differentiation properties and analyzed their transcriptome. Unipotent osteogenic and adipogenic cells expressed specific transcriptional programs, whereas bipotent clones combined expression of those genes and did not show a unique signature. We tested potential regulators of lineage commitment and found that in the presence of interferon-γ (IFNγ) adipogenic clones can be induced to osteogenesis and that their adipogenic capacity is inhibited. Analysis of IFNγ-regulated genes showed that lineage signatures and fate commitment of skeletal progenitors were controlled by EGR1 and EGR2. Knockdown experiments revealed that EGR1 is a positive regulator of the adipogenic transcriptional program and differentiation capacity, whereas EGR2 inhibits the osteogenic program and potency. Therefore, our work revealed transcriptional signatures of osteogenic and adipogenic lineages and mechanism triggering cell fate.

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Bone marrow osteo- and adipogenic progenitors have specific transcriptional profiles

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Bipotent progenitors combine expression of osteogenic and adipogenic programs

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IFNγ inhibits adipogenesis and induces osteogenesis via downregulation of Egr1/Egr2

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Egr1 maintains adipogenic and Egr2 suppresses osteogenic lineage commitment

Nestin-expressing progenitor cells: function, identity and therapeutic implications

The neuroepithelial stem cell protein, or Nestin, is a cytoskeletal intermediate filament initially characterized in neural stem cells. However, current extensive evidence obtained in in vivo models and humans shows presence of Nestin+ cells with progenitor and/or regulatory functions in a number of additional tissues, remarkably bone marrow. This review presents the current knowledge on the role of Nestin in essential stem cell functions, including self-renewal/proliferation, differentiation and migration, in the context of the cytoskeleton. We further discuss the available in vivo models for the study of Nestin+ cells and their progeny, their function and elusive nature in nervous system and bone marrow, and their potential mechanistic role and promising therapeutic value in preclinical models of disease. Future improved in vivo models and detection methods will allow to determine the true essence of Nestin+ cells and confirm their potential application as therapeutic target in a range of diseases.

Targeting skeletal endothelium to ameliorate bone loss

Recent studies have identified a specialized subset of CD31hiendomucinhi (CD31hiEMCNhi) vascular endothelium that positively regulates bone formation. However, it remains unclear how CD31hiEMCNhi endothelium levels are coupled to anabolic bone formation. Mice with an osteoblast-specific deletion of Shn3, which have markedly elevated bone formation, demonstrated an increase in CD31hiEMCNhi endothelium. Transcriptomic analysis identified SLIT3 as an osteoblast-derived, SHN3-regulated proangiogenic factor. Genetic deletion of Slit3 reduced skeletal CD31hiEMCNhi endothelium, resulted in low bone mass because of impaired bone formation and partially reversed the high bone mass phenotype of Shn3-/- mice. This coupling between osteoblasts and CD31hiEMCNhi endothelium is essential for bone healing, as shown by defective fracture repair in SLIT3-mutant mice and enhanced fracture repair in SHN3-mutant mice. Finally, administration of recombinant SLIT3 both enhanced bone fracture healing and counteracted bone loss in a mouse model of postmenopausal osteoporosis. Thus, drugs that target the SLIT3 pathway may represent a new approach for vascular-targeted osteoanabolic therapy to treat bone loss.

What is a stem cell?

The historical roots of the stem cell concept are traced with respect to its usage in embryology and in hematology. The modern consensus definition of stem cells, comprising both pluripotent stem cells in culture and tissue-specific stem cells in vivo, is explained and explored. Methods for identifying stem cells are discussed with respect to cell surface markers, telomerase, label retention and transplantability, and properties of the stem cell niche are explored. The CreER method for identifying stem cells in vivo is explained, as is evidence in favor of a stochastic rather than an obligate asymmetric form of cell division. In conclusion, it is found that stem cells do not possess any unique and specific molecular markers; and stem cell behavior depends on the environment of the cell as well as the stem cell's intrinsic qualities. Furthermore, the stochastic mode of division implies that stem cell behavior is a property of a cell population not of an individual cell. In this sense, stem cells do not exist in isolation but only as a part of multicellular system. This article is categorized under: Adult Stem Cells, Tissue Renewal, and Regeneration > Tissue Stem Cells and Niches Adult Stem Cells, Tissue Renewal, and Regeneration > Methods and Principles Adult Stem Cells, Tissue Renewal, and Regeneration > Environmental Control of Stem Cells.

Isolation and functional assessment of mouse skeletal stem cell lineage

There are limited methods available to study skeletal stem, progenitor, and progeny cell activity in normal and diseased contexts. Most protocols for skeletal stem cell isolation are based on the extent to which cells adhere to plastic or whether they express a limited repertoire of surface markers. Here, we describe a flow cytometry-based approach that does not require in vitro selection and that uses eight surface markers to distinguish and isolate mouse skeletal stem cells (mSSCs); bone, cartilage, and stromal progenitors (mBCSPs); and five downstream differentiated subtypes, including chondroprogenitors, two types of osteoprogenitors, and two types of hematopoiesis-supportive stroma. We provide instructions for the optimal mechanical and chemical digestion of bone and bone marrow, as well as the subsequent flow-cytometry-activated cell sorting (FACS) gating schemes required to maximally yield viable skeletal-lineage cells. We also describe a methodology for renal subcapsular transplantation and in vitro colony-formation assays on the isolated mSSCs. The isolation of mSSCs can be completed in 9 h, with at least 1 h more required for transplantation. Experience with flow cytometry and mouse surgical procedures is recommended before attempting the protocol. Our system has wide applications and has already been used to study skeletal response to fracture, diabetes, and osteoarthritis, as well as hematopoietic stem cell-niche interactions in the bone marrow.

Simulated microgravity induces a cellular regression of the mature phenotype in human primary osteoblasts

Decreased mechanical loading on bones, such as prolonged bed rest and microgravity during space flights, leads to the development of an osteoporotic-like phenotype. Although osteoblast hypo-functionality is reported to be involved in the progression of bone pathological conditions, the cellular mechanisms of this process remain largely unknown. The combined application of mass spectrometry "-omics" and histochemical and ultrastructural approaches have been employed to investigate the effects of the gravitational unloading on human bone-cell biology. Here we show, ex vivo, that simulated microgravity (Sμg) on human primary osteoblasts (hpOB) induces an alteration of pro-osteogenic determinants (i.e., cell morphology and deposit of hydroxyapatite crystals), accompanied by a downregulation of adhesive proteins and bone differentiation markers (e.g., integrin beta-1, protein folding Crystallin Alpha B (CRYα-B), runt-related transcription factor 2 (RUNX-2), bone morphogenic protein-2 (BMP-2), and receptor activator of nuclear factor kappa-B ligand (RANK-L)), indicating an impairment of osteogenesis. Further, we observed for the first time that Sμg can trigger a transition toward a mesenchymal-like phenotype, in which a mature osteoblast displays an hampered vitamin A metabolism, loses adhesive molecules, gains mesenchymal components (e.g., pre-osteoblast state marker CD44), morphological protrusions (filopodium-like), enhances GTPase activities, which in turn allows it to acquire migrating properties. Although this phenotypic conversion is not complete and can be reversible, Sμg environment proves a plasticity potential hidden on Earth. Overall, our results suggest that Sμg can be a powerful physical cue for triggering ex vivo a dedifferentiation impulse on hpOBs, opening a new scenario of possible innovative therapeutical biomechanical strategies for the treatment of osteo-degenerative diseases.

The periosteum-like effect of fresh human amniotic membrane on bone regeneration in a rabbit critical-sized defect model

OBJECTIVE

The purpose of this study was to investigate the effect of fresh human amniotic membrane (FHAM) as a substitute of periosteum to enhance bone regeneration in critical-sized defects.

METHODS

Tibial diaphyseal bone defects were created in forty New Zealand white rabbits and treated with FHAM or left empty. Treatment groups consisted of: FHAM implanted in the place of removed periosteum (FHAMP group); FHFAM implanted to fill the entire defect (FHAMF group) compared to negative control group; empty defect with removing the periosteum (NC group) and positive control group; and empty defect without removing the periosteum (PC group). Bone regeneration was evaluated by radiographic, micro-computed tomography (μ-CT) and histological analyses at 4 and 8weeks post-surgery.

RESULTS

Radiographic and μ-CT analysis demonstrated clearly enhanced new bone formation in positive control group (PC) and FHAMP group compared to negative control group (NC) and FHAMF group. Histological staining exhibited remaining woven bones and cartilage matrix in the FHAMP group, immature lamellar bone with medellury cavity and marrow bone formation in PC group from 4weeks post-operatively. For FHAMF group, a little new bone formation was detected only from 8weeks post-operatively, and an absence of any sign of healing in NC group at both time points.

CONCLUSION

The results provide that FHAM increases bone regeneration in critical-sized defects when it is implanted in the place of the removed periosteum, but its additive effect does not have the same effect of the natural periosteum.

Stem cell-based therapeutic strategies for cartilage defects and osteoarthritis

The gold standard cell therapy for repair of articular cartilage defects is autologous chondrocyte implantation, with good outcomes long-term. Mesenchymal stromal/stem cells (MSCs) from bone marrow or connective tissues such as fat are being pursued as alternatives for cartilage repair, and are trialled via intra-articular administration in patients with knee osteoarthritis. Early-phase clinical studies concur on safety and provide some promising insight into efficacy, but the mechanism of action remains unclear. Recent studies implicate extracellular vesicles as important mediators of MSC action, offering exciting therapeutic prospects. Our increasing understanding of the mechanisms underlying intrinsic articular cartilage maintenance and repair fosters hope that novel/repurposed therapeutics could elicit repair through activation of endogenous stem/progenitor cells to maintain healthy joints and prevent osteoarthritis.

In Vivo 3D Histomorphometry Quantifies Bone Apposition and Skeletal Progenitor Cell Differentiation

Histomorphometry and Micro-CT are commonly used to assess bone remodeling and bone microarchitecture. These approaches typically require separate cohorts of animals to analyze 3D morphological changes and involve time-consuming immunohistochemistry preparation. Intravital Microscopy (IVM) in combination with mouse genetics may represent an attractive option to obtain bone architectural measurements while performing longitudinal monitoring of dynamic cellular processes in vivo. In this study we utilized two-photon, multicolor fluorescence IVM together with a lineage tracing reporter mouse model to image skeletal stem cells (SSCs) in their calvarial suture niche and analyze their differentiation fate after stimulation with an agonist of the canonical Wnt pathway (recombinant Wnt3a). Our in vivo histomorphometry analyses of bone formation, suture volume, and cellular dynamics showed that recombinant Wnt3a induces new bone formation, differentiation and incorporation of SSCs progeny into newly forming bone. IVM technology can therefore provide additional dynamic 3D information to the traditional static 2D histomorphometry.