Introduction
The skeleton is maintained by diverse cell types of the mesenchymal cell lineage, including bone-forming osteoblasts, fat-forming adipocytes, cartilage-forming chondrocytes and their precursors of stromal cell populations, while housing cells of the haematopoietic lineage within its marrow cavity [ 1 , 2 ]. Beyond its central role in locomotion, the bone has also been recognized for its extensive functions, such as the site of haematopoiesis and the target of mineral homeostasis through endocrine regulations [ 1 , 3 ]. Osteoblasts, the only cells capable of depositing mineralized matrix, play a central role in supporting the skeleton. Osteoblasts need to be replenished by skeletal stem cells (SSCs) to keep building bones, as osteoblasts have only a limited lifespan owing to their specialized function in matrix production and mineralization and their eventual attrition from apoptosis or embedment as osteocytes [ 4 ].
SSCs were first discovered in the bone marrow stroma [ 5 , 6 ]. When isolated ex vivo and grown under cultured conditions, these cells can self-renew and differentiate into osteoblasts, chondrocytes and adipocytes. Fluorescence-activated cell sorting (FACS)-based isolation techniques using appropriate combinations of cell surface markers facilitated the characterization of SSCs [ 7 ]. In addition, in vivo fate-mapping approaches using cell type-specific constitutively active cre drivers, as well as in vivo lineage-tracing approaches using cell type-specific tamoxifen-inducible creER drivers in mice, have also been instrumental for elucidating in vivo functions of SSCs [ 7 – 12 ]. Accumulating evidence indicates that skeletal development, maintenance and regeneration are coordinated by diverse classes of SSCs localized in each compartment of the bone [ 8 , 9 , 13 ].
Long bones are shaped through two different mechanisms of endochondral and intramembranous bone formation. The growth plate is a unique cartilaginous structure located at the end of the endochondral bone. It is an essential component of endochondral bones, composed of three main layers: the resting, proliferating and hypertrophic zones [ 14 ]. In endochondral bone formation, longitudinal axial growth of long bones is driven at least in part by the proliferation and differentiation of growth plate stem cells (GPSCs) residing in the resting zone. Our group previously reported that parathyroid hormone-related protein (PTHrP)-expressing GPSCs are the source of growth plate chondrocytes, osteoblasts and bone marrow stromal cells [ 8 ]. PTHrP + GPSCs first differentiate into proliferating and hypertrophic chondrocytes and, subsequently, into osteoblasts as well as marrow stromal cells beneath the growth plate, including C-X-C chemokine ligand (CXCL)12-abundant reticular (CAR) cells [ 8 ]. The current concept is that SSCs in the resting zone of the growth plate are pivotal to endochondral bone formation.
In intramembrane bone formation, SSCs within the periosteum, termed periosteal stem cells (PSCs), coordinate the growth of long-tubular bone in the transverse axial direction [ 15 , 16 ]. Histologically, the periosteum is the site, where the skeletal muscle is attached, composed of two fibrous and cambium (cellular) layers [ 17 , 18 ] ( figure 1 ). The outer fibrous layer is rich in collagen and elastin and includes linear neuron fibres. In contrast, the inner cambium layer comprises several types of cells, including osteoblast precursor cells and fibroblasts. The cambium layer is highly vascularized with a high density of microvessels [ 18 , 19 ]. Cathepsin K + and Mx1 + aSMA + PSCs within the periosteal inner layer can self-renew and differentiate into tri-lineage cells in transplantation and promote osteogenesis [ 16 , 20 ]. In this process, PSCs directly supply osteoblasts without chondrogenic differentiation.
Relatively less defined are SSCs residing in the endosteum, the counterpart of the periosteum inside of the cortical bone ( figure 1 ). The endosteum includes diverse cell types, including osteoblasts and their precursor cells and endothelial cells [ 7 , 9 ]. The endosteum is a thin layer lining the inner surface of the bone adjoining the bone marrow cavity. Historically, the endosteum has been primarily studied as a haematopoietic stem cell (HSC) niche component [ 3 , 21 ]. However, recent studies show that skeletal stem cells in the endosteum, termed endosteal stem cells (ESCs), coordinate active bone formation, particularly in young bones, while also being involved in initiating and progressing osteosarcoma-like lesions [ 9 ]. Therefore, understanding the characteristics of the endosteum is essential not only for understanding the fundamental mechanism of bone growth and development but also for exploiting a potential therapeutic target of pathological conditions in bone and blood, such as osteosarcoma, osteoporosis and multiple myeloma. This review summarizes the current understanding of the endosteum with an emphasis on the functional roles of endosteal cells in primary and metastatic tumours.
Bone marrow endosteum as a haematopoietic stem and progenitor cell niche
The endosteal and perisinusoidal niches are the two main HSC-supporting niches in the bone marrow microenvironment. The perisinusoidal niche, also called the central or perivascular niche, houses approximately 85% of the HSCs. In comparison, the endosteal niche at the interface between the bone and bone marrow houses the remaining 15% of HSCs [ 3 , 22 ]. The endosteal niche is also enriched with mesenchymal lineage cells, such as osteoblasts and osteoblast progenitor cells. Osteoblasts on the endosteal surface are integral to supporting haematopoietic progenitor cells, which are thought to play multifaceted roles in creating a haematopoiesis-supporting microenvironment, or niche, ensuring the proper regulation of HSC quiescence, self-renewal and differentiation [ 3 , 23 ]. The dynamic interplay between osteoblasts and haematopoietic progenitor cells is essential for sustaining a balanced production of blood cells throughout life [ 3 , 21 , 23 – 25 ]. Endosteal osteoblasts express the receptor for parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP) (PTH1R). Mice expressing a constitutively active PTH1R under the control of the α1(I) collagen promoter active in osteoblastic cells (Col1a1-caPTH1R) show a dramatic increase in osteoblastic cells with an upregulation of a Notch ligand Jagged 1, which supports an increase of HSCs through Notch signalling activation [ 23 ]. Also, conditional deletion of bone morphogenic protein receptor type IA (BMPRIA) by Mx1-cre induces a drastic increase in osteoblasts and HSCs, supporting the notion that osteoblasts are a component of the HSC niche [ 24 ]. The converse experiment of ablating osteoblasts using Col2.3Δtk transgenic mice reduces the HSC number [ 26 ]. Other studies show that HSCs prefer to engraft at the endosteal region after transplantation [ 27 , 28 ]. On the other hand, several studies have shown that the osteoblast number is not correlated with the HSC number. Moreover, CXCL12 and stem cell factor (SCF), two cytokines critical for HSC maintenance, from the endosteum are dispensable for maintaining HSCs. Conditional deletion of CXCL12 in osteoblasts selectively impairs B cell development without affecting the HSC number [ 29 ]. Likewise, conditional deletion of SCF in osteoblasts does affect HSC maintenance [ 30 ]. Cecal ligation and puncture is a widely used model of polymicrobial sepsis. This triggers a systemic inflammatory response, leading to a cytokine storm. In addition to affecting immune cells, cecal ligation and puncture also impact bone marrow mesenchymal cells, leading to the loss of osteoblasts. Osteoblastic ablation induced by cecal ligation reduced the number of T and B lymphocytes and common lymphoid progenitor cells (CLPs) without affecting the HSC number [ 25 ]. This immunodeficient phenotype is rescued by intermittent PTH administration [ 31 ].
Therefore, the precise role that endosteal osteoblasts play in HSC maintenance remains largely unclear. However, multiple lines of evidence support that the endosteal niche is essential for maintaining their downstream haematopoietic progenitor cell populations, including lymphoid progenitor cells, supporting the concept that the endosteum is an essential haematopoiesis-supporting structure.
Bone marrow endosteum in haematological tumour progression
The emerging concept is that the endosteum can also serve as a niche for haematological tumours, including multiple myeloma (MM) and acute myeloid leukaemia (AML) ( figure 2 ). MM is a malignant neoplasm characteriszd by the clonal proliferation of plasma cells within the bone marrow, producing abnormal monoclonal immunoglobulins or light chains [ 32 ]. This accumulation of malignant plasma cells disrupts normal bone marrow function, causing anaemia, immunodeficiency and renal dysfunction. One of the hallmark features of MM is the presence of osteolytic lesions, which result in significant morbidity for patients, including recurrent fractures [ 33 ]. Osteolytic lesions of MM show unique spot-like bone erosion in the cortical bone, called punched-out lesions [ 34 ], distinct from those developing from bone metastases of epithelial cell-derived tumours, such as breast cancer and lung cancer [ 33 , 35 ]. One reason for this unique osteolysis is that MM cells colonize the endosteum. Lawson et al. show that 5TGM1-GFP mouse-derived myeloma cells preferentially colonized endosteal bone surfaces by two-photon microscopy using 1,1'-dioctadecyl−3,3,3',3'-tetramethylindodicarbocyanine (DiD) labelling [ 36 ]. As 5TGM1-GFP DiD hi cells divide, DiD becomes progressively diluted among their descendants. Dormant 5TGM1-GFP DiD hi cells are preferentially localized to the endosteal surface, stay dormant over time and are resistant to chemotherapy. Interestingly, increased osteoclast activity can disrupt the endosteal niche, potentially leading to the reactivation of dormant MM cells and subsequent cortical bone erosion. Conversely, inhibiting osteoclast activity helps maintain the dormancy of MM cells [ 36 ].
AML is a heterogeneous and aggressive haematologic malignancy characterized by the rapid proliferation of abnormal myeloid precursors (blasts) in the bone marrow, blood and other tissues. The pathogenesis of AML involves a complex interplay of genetic, epigenetic and environmental factors that lead to the disruption of normal haematopoiesis and uncontrolled growth of leukaemic cells [ 37 , 38 ]. AML, like MM, is also known to grow and proliferate in the endosteum. AML cells preferentially colonize the osteoblast-rich areas, such as the trabecular bone and endosteum, altering the endosteal niche [ 39 ]. These cells compete with HSCs and occupy the endosteal niche, impairing normal haematopoiesis [ 40 ]. The in vivo AML mouse model shows that vasculature loss is observed in the endosteum but not in the central bone marrow [ 40 ]. This phenotype is also seen in over 80% of BM trephine biopsies from patients with AML [ 40 ]. AML cells inside the endosteum have a higher expression of genes related to inflammatory responses and anti-angiogenic cytokine Cxcl2, downstream of tumour necrosis factor (TNF) signalling pathways [ 40 ]. These cytokines are postulated to disturb the endosteal vasculature. The first-line treatment for AML is chemotherapy, such as cytarabine or anthracyclines. Chemotherapy reaches bone marrow through blood vessels but is ineffective in annihilating endosteum-residing AML cells due to disrupted endosteal vasculature [ 39 , 40 ]. In other words, AML cells may protect themselves from chemotherapy by destroying blood vessels within their own niche in the endosteum.
Clinically, AML is characterized by impaired osteogenesis and decreased osteocalcin levels in the peripheral blood [ 41 ]. In recent years, the critical roles that blood vessels play in bone biology have been unveiled [ 42 ]. Two distinct types of blood vessels, known as type H and type L vessels, have been identified as crucial players in bone homeostasis. Type H vessels are predominantly located in the metaphyseal regions of long bones and near the endosteal surface. In contrast, type L vessels are primarily found in the diaphyseal region of bones. Type H vessels are characterized by high expression of CD31 (also known as PECAM-1) and endomucin, which are markers of endothelial cells and are associated with areas of active bone formation by providing a niche that supports the proliferation and differentiation of osteoprogenitor cells [ 43 ]. During AML pathogenesis, several blood vessels, including CD31 hi Endomucin hi type H vessels, are impaired, associated with osteoblast dysfunction and the reduced number of HSCs [ 40 , 44 , 45 ]. The collapse of both the endosteal and perisinusoidal niches may cause decreased HSCs in AML. Overall, the endosteum provides a crucial niche for haematological tumour cells, where tumour cells preferentially remodel the microenvironment for their survival and progression ( figure 2 ).
Bone marrow endosteum in bone metastasis
Bone is one of the most preferential metastatic sites of malignant tumours such as breast cancer, lung cancer and malignant melanoma [ 35 ]. When cancer cells migrate to the bone marrow, they establish a metastatic site within the bone microenvironment, which will eventually provide a seedling bed for cancer cells to grow and destroy bones, leading to significant morbidity associated with pain, fractures and hypercalcemia. Recent advances in cancer therapies have extended patients' longevity and, conversely, increased the risk of bone metastasis. Cancer cells originating in the primary site often metastasize to the bone through the bloodstream by chemotactic factors such as CXCL12 and receptor activator of nuclear factor-kB ligand (RANKL) [ 46 , 47 ]. The endosteum is rich in osteoblasts, often attracting tumour cells to the endosteum through RANKL [ 48 ]. Cancer cells metastasizing to bone compete with HSCs for niche occupancy and then acquire dormancy [ 49 ]. Wang et al. show that systemically inoculated tumour cells colonize near type I collagen (COL-I) + or alkaline phosphatase (ALP) + cells [ 50 ]. in vitro experiments further reveal that the cell–cell contact between tumour cells and osteoblasts accelerates tumour progression [ 50 ]. E-cadherin is expressed in several types of breast cancer cells and is negatively associated with overall survival and distant metastasis-free survival [ 51 ]; the interaction between E-cadherin on tumour cells and N-cadherin on osteoblasts accelerates tumour proliferation [ 50 ]. After tumour cells migrate to the endosteum, tumour cells stimulate the RANKL expression in osteoblasts through the production of PTHrP, prostaglandin E2, interleukin (IL)-6, IL-1β, enhancing osteoclastic bone resorption [ 48 ]. Subsequently, osteoclast-induced bone resorption releases calcium. Thus, metastatic tumour cells in the endosteal niche are exposed to high calcium levels. Tumour cells express calcium-sensing receptors (CaSRs) that enable them to detect and respond to extracellular calcium levels, acquiring the ability to migrate and proliferate [ 52 ]. Calcium can activate several oncogenic pathways that drive tumour progression. For instance, calcium can influence the expression of genes related to cell cycle regulation, apoptosis and metabolism, further supporting cancer cell growth in bone [ 53 , 54 ]. Besides, the degraded bone matrix induced by osteoclasts also releases TGF-β and IGF, promoting tumour cell proliferation. This interaction among the tumour cells, osteoblasts and osteoclasts develops tumour progression in bone in a process called the ‘vicious cycle.’ In conclusion, the endosteum provides an essential scaffold for bone metastatic tumour cells to grow exponentially and progress into bone-destructing tumours.
Bone marrow endosteal stem cells in bone tumour initiation
SSCs providing a source of osteoblasts in the endosteum have been poorly characterised [ 55 ], with no specific subset of endosteal stem cells (ESCs) identified. Our group has identified that ESCs are characterized by fibroblast growth factor receptor 3 ( Fgfr3 ) expression and osteoblast-chondrocyte transitional (OCT) identities [ 9 ] ( figure 3 ). At a young age, these cells have some hallmarks of osteoblasts and chondrocytes, identified by single-cell RNA-seq analysis of bone marrow stromal cells [ 56 ]. More specifically, single-cell RNA-seq analyses of Prrx1-cre- labelled stromal cells at three weeks (Young) and 18 months (Old) mice (integrated by LIGER [ 57 , 58 ]) reveal three major groups of osteoblasts, chondrocytes and reticular cells. In addition, a small cluster of cells with OCT identities is identified, which express molecular markers of chondrocytes ( Acan ), osteoblasts ( Col1a1 ) and reticular cells ( Cxcl12 ). These OCT cells are abundant at a young age (P21) but depleted at an old age (18 months), suggesting that OCT cells robustly contribute to active osteogenesis during a young age. Single-nucleus ATAC-seq analyses further confirm that OCT cells maintain multipotent states, indicative of their stem cell potential. Fgfr3 is a representative marker of these OCT cells, as it is highly expressed in the OCT cluster. Fgfr3 + cells in the endosteum have strong colony-forming capabilities in vitro , which are often used as proof of stem cell potential.
Osteosarcoma is the most common primary bone tumour, predominantly affecting adolescents and young adults. Osteosarcoma arises from osteoblastic cells that form immature bone or osteoid tissue and primarily occurs in the metaphyseal regions of long bones and the endosteum [ 59 ]. The incidence of osteosarcoma is often linked to mutations in tumour suppressor genes and oncogenes [ 60 ]. Among these, mutations in the TRP53 gene (encoding p53) are the most common, occurring in over 50% of cases. The hypothesis is that Fgfr3 + ESCs initiate osteosarcoma. We found that the deletion of p53, specifically in Fgfr3 + cells with Trp53 -floxed mice, drives Fgfr3 + ESCs to develop into osteosarcoma-like bone tumours, supporting that Fgfr3 + endosteal cells can be a cellular source of osteosarcoma ( figure 3 ). Although conventional osteosarcomas most frequently arise in the metaphyseal regions of long bones, several case reports have documented osteosarcomas originating from the cortical bone in paediatric patients [ 61 , 62 ]. Our findings that Fgfr3 + ESCs can give rise to osteosarcoma-like tumours during skeletal growth suggest that Fgfr3 + ESCs may represent the cellular origin of cortical osteosarcomas in juvenile patients. This insight highlights the potential clinical significance of targeting Fgfr3 + ESCs in the context of paediatric osteosarcoma. In conclusion, Fgfr3 + cells at the endosteum with SSC-like properties can provide a robust source of osteosarcoma-forming cells.
Prospects
The bone marrow endosteum is a critical site for haematopoiesis and osteogenesis, hosting a subgroup of hematopoietic progenitor cells and SSCs. Among these, Fgfr3 + SSC (‘ESCs’) robustly contribute to the generation of bone-forming osteoblasts, as well as to CXCL12-abundant reticular (CAR) cells and osteocytes within the cortical bone. Fgfr3 + cells in the endosteum are heterogeneous, presumably including osteoblast progenitor and precursor cells with varied potential. Moreover, Fgfr3 is expressed not only in the endosteum but also in the periosteum and growth plate cartilage. As a future endeavour, it will be necessary to identify endosteum-specific SSC markers by combining multiple cell surface markers and isolating endosteal stem cells with high purity for further functional evaluation. This endeavour will be necessary to exploit these cells as potential targets for future innovative therapies.
The bone marrow endosteum provides a supportive microenvironment for the metastatic cancer cells and the progression of haematological malignancies. Most studies have focused on the relationship between tumour cells and drug efficacy. In contrast, relatively few have investigated the interactions between tumour cells and endosteal cells, such as osteoblasts and osteoclasts. As a result, the mechanisms behind bone destruction have not been fully elucidated. This knowledge gap primarily stems from the frequent use of human tumour cells in research, necessitating transplantation into immunodeficient hosts such as nude or SCID mice, limiting the ability to obtain mechanistic insights through genetically modified mouse models.
Recent research has revealed that the periosteum acts as a defensive barrier against cancer invasion into bone [ 63 ]. The authors used C57BL/6J-derived head and neck squamous cell carcinoma (HNSCC) cell lines in tumour injection mouse models. They observed that the periosteum highly expresses tissue inhibitor of metalloproteinases 1 (TIMP1). To investigate the role of TIMP1 in the periosteal defence, the researchers used TIMP1 knockout (KO) mice. TIMP1 KO mice exhibited a significant loss of the periosteal defence, allowing cancer cells to infiltrate the bone and proliferate extensively. These findings demonstrate that the periosteum functions as a stromal defence mechanism against cancer invasion. It remains to be determined whether the endosteum functions similarly.
Recent advances in cancer therapies have extended the survival of patients with metastatic bone tumours, MM and AML. However, this has also brought increased attention to the complications of tumor-induced bone destruction, such as bone pain, hypercalcemia, fractures and spinal cord compression, all of which significantly impair patients' quality of life [ 64 , 65 ]. Notably, MM often metastasizes to the calvaria more readily than to long bones such as the femur [ 66 , 67 ]. In some cases, bone destruction facilitates tumour spread into the cranial cavity, leading to extramedullary intracranial growth, which can, in severe cases, result in life-threatening conditions [ 68 ]. Therefore, elucidating the crosstalk between tumour cells and the endosteal microenvironment is crucial not only for deepening our understanding of the mechanisms behind cancer-induced bone pathology, but also for developing novel therapeutic strategies to preserve bone integrity and prevent such devastating disease progression.
Ethics
This work did not require ethical approval from a human subject or animal welfare committee.
Data accessibility
This article has no additional data.
Declaration of AI use
We have not used AI-assisted technologies in creating this article.
Authors’ contributions
Y.N.: conceptualization, investigation, writing—original draft, writing—review and editing; W.O.: conceptualization, writing—review and editing; N.O.: conceptualization, investigation, project administration, resources, supervision, validation, writing—original draft, writing—review and editing.
All authors gave final approval for publication and agreed to be held accountable for the work performed therein.
Conflict of interest declaration
We declare we have no competing interests.
Funding
This work was supported by the National Institutes of Health grants R35DE034348 to N.O., R01DE030630 to N.O. and W.O. and R01DE030416 to W.O.
References
1 . Okamoto K , Nakashima T , Shinohara M , Negishi-Koga T , Komatsu N , Terashima A , Sawa S , Nitta T , Takayanagi H . 2017 Osteoimmunology: the conceptual framework unifying the immune and skeletal systems . Physiol. Rev. 97 , 1295 – 1349 . ( 10.1152/physrev.00036.2016 ) 28814613
2 . Matsushita Y , Ono W , Ono N . 2020 Skeletal stem cells for bone development and repair: diversity matters . Curr. Osteoporos. Rep. 18 , 189 – 198 . ( 10.1007/s11914-020-00572-9 ) 32172443 PMC7255932
3 . Morrison SJ , Scadden DT . 2014 The bone marrow niche for haematopoietic stem cells . Nature 505 , 327 – 334 . ( 10.1038/nature12984 ) 24429631 PMC4514480
4 . Mizoguchi T , Ono N . 2021 The diverse origin of bone-forming osteoblasts . J. Bone Miner. Res. 36 , 1432 – 1447 . ( 10.1002/jbmr.4410 ) 34213032 PMC8338797
5 . Friedenstein AJ , Piatetzky-Shapiro II , Petrakova KV . 1966 Osteogenesis in transplants of bone marrow cells . J. Embryol. Exp. Morphol. 16 , 381 – 390 . 5336210
6 . Friedenstein AJ , Chailakhjan RK , Lalykina KS . 1970 The development of fibroblast colonies in monolayer cultures of guinea-pig bone marrow and spleen cells . Cell Tissue Kinet. 3 , 393 – 403 . ( 10.1111/j.1365-2184.1970.tb00347.x ) 5523063
7 . Ambrosi TH , Longaker MT , Chan CKF . 2019 A revised perspective of skeletal stem cell biology . Front. Cell Dev. Biol. 7 , 189 . ( 10.3389/fcell.2019.00189 ) 31572721 PMC6753172
8 . Mizuhashi K , Ono W , Matsushita Y , Sakagami N , Takahashi A , Saunders TL , Nagasawa T , Kronenberg HM , Ono N . 2018 Resting zone of the growth plate houses a unique class of skeletal stem cells . Nature 563 , 254 – 258 . ( 10.1038/s41586-018-0662-5 ) 30401834 PMC6251707
9 . Matsushita Y et al . 2023 Bone marrow endosteal stem cells dictate active osteogenesis and aggressive tumorigenesis . Nat. Commun. 14 , 2383 . ( 10.1038/s41467-023-38034-2 ) 37185464 PMC10130060
10 . Matsushita Y , Chu AKY , Tsutsumi-Arai C , Orikasa S , Nagata M , Wong SY , Welch JD , Ono W , Ono N . 2022 The fate of early perichondrial cells in developing bones . Nat. Commun. 13 . ( 10.1038/s41467-022-34804-6 ) PMC9705540 36443296
11 . Matsushita Y et al . 2020 A Wnt-mediated transformation of the bone marrow stromal cell identity orchestrates skeletal regeneration . Nat. Commun. 11 , 332 . ( 10.1038/s41467-019-14029-w ) 31949165 PMC6965122
12 . Matsushita Y , Chu AKY , Ono W , Welch JD , Ono N . 2020 Intercellular interactions of an adipogenic CXCL12-expressing stromal cell subset in murine bone marrow . J. Bone Miner. Res. 36 , 1145 – 1158 . ( 10.1002/jbmr.4282 ) PMC8605623 33651379
13 . Bianco P , Cao X , Frenette PS , Mao JJ , Robey PG , Simmons PJ , Wang CY . 2013 The meaning, the sense and the significance: translating the science of mesenchymal stem cells into medicine . Nat. Med. 19 , 35 – 42 . ( 10.1038/nm.3028 ) 23296015 PMC3998103
14 . Kronenberg HM . 2003 Developmental regulation of the growth plate . Nature 423 , 332 – 336 . ( 10.1038/nature01657 ) 12748651
15 . Tsukasaki M et al . 2022 Periosteal stem cells control growth plate stem cells during postnatal skeletal growth . Nat. Commun. 13 , 4166 . ( 10.1038/s41467-022-31592-x ) 35851381 PMC9293991
16 . Debnath S et al . 2018 Discovery of a periosteal stem cell mediating intramembranous bone formation . Nature 562 , 133 – 139 . ( 10.1038/s41586-018-0554-8 ) 30250253 PMC6193396
17 . Bianco P , Robey PG . 2015 Skeletal stem cells . Development 142 , 1023 – 1027 . ( 10.1242/dev.102210 ) 25758217 PMC4360182
18 . Allen MR , Hock JM , Burr DB . 2004 Periosteum: biology, regulation, and response to osteoporosis therapies 35 , 1003 – 1012 . ( 10.1016/j.bone.2004.07.014 ) 15542024
19 . Li C , Fennessy P . 2021 The periosteum: a simple tissue with many faces, with special reference to the antler-lineage periostea . Biol. Direct 16 , 17 . ( 10.1186/s13062-021-00310-w ) 34663443 PMC8522104
20 . Ortinau LC et al . 2019 Identification of functionally distinct Mx1+αSMA+ periosteal skeletal stem cells . Cell Stem Cell 25 , 784 – 796 .( 10.1016/j.stem.2019.11.003 ) 31809737 PMC7055207
21 . Kiel MJ , Morrison SJ . 2008 Uncertainty in the niches that maintain haematopoietic stem cells . Nat. Rev. Immunol. 8 , 290 – 301 . ( 10.1038/nri2279 ) 18323850
22 . Acar M et al . 2015 Deep imaging of bone marrow shows non-dividing stem cells are mainly perisinusoidal . Nature 526 , 126 – 130 . ( 10.1038/nature15250 ) 26416744 PMC4850557
23 . Calvi LM et al . 2003 Osteoblastic cells regulate the haematopoietic stem cell niche . Nature 425 , 841 – 846 . ( 10.1038/nature02040 ) 14574413
24 . Zhang J et al . 2003 Identification of the haematopoietic stem cell niche and control of the niche size . Nature 425 , 836 – 841 . ( 10.1038/nature02041 ) 14574412
25 . Terashima A , Okamoto K , Nakashima T , Akira S , Ikuta K , Takayanagi H . 2016 Sepsis-induced osteoblast ablation causes immunodeficiency . Immunity 44 , 1434 – 1443 . ( 10.1016/j.immuni.2016.05.012 ) 27317262
26 . Visnjic D . 2001 Conditional ablation of the osteoblast lineage in Col2.3deltatk transgenic mice . J. Bone Miner. Res. 16 , 2222 – 2231 . 11760835 10.1359/jbmr.2001.16.12.2222
27 . Lam BS , Cunningham C , Adams GB . 2011 Pharmacologic modulation of the calcium-sensing receptor enhances hematopoietic stem cell lodgment in the adult bone marrow . Blood 117 , 1167 – 1175 . ( 10.1182/blood-2010-05-286294 ) 21076044 PMC3056470
28 . Adams GB et al . 2006 Stem cell engraftment at the endosteal niche is specified by the calcium-sensing receptor . Nature 439 , 599 – 603 . ( 10.1038/nature04247 ) 16382241
29 . Ding L , Morrison SJ . 2013 Haematopoietic stem cells and early lymphoid progenitors occupy distinct bone marrow niches . Nature 495 , 231 – 235 . ( 10.1038/nature11885 ) 23434755 PMC3600153
30 . Ding L , Saunders TL , Enikolopov G , Morrison SJ . 2012 Endothelial and perivascular cells maintain haematopoietic stem cells . Nature 481 , 457 – 462 . ( 10.1038/nature10783 ) 22281595 PMC3270376
31 . Martin TJ . 2014 Bone biology and anabolic therapies for bone: current status and future prospects . J. Bone Metab. 21 , 8 – 20 . ( 10.11005/jbm.2014.21.1.8 ) 24707463 PMC3970295
32 . Kumar SK , Rajkumar V , Kyle RA , van Duin M , Sonneveld P , Mateos MV , Gay F , Anderson KC . 2017 Multiple myeloma . Nat. Rev. Dis. Primers 3 , 17046 . ( 10.1038/nrdp.2017.46 ) 28726797
33 . Roodman GD . 2009 Pathogenesis of myeloma bone disease . Leukemia 23 , 435 – 441 . ( 10.1038/leu.2008.336 ) 19039321
34 . Roodman GD . 1997 Mechanisms of bone lesions in multiple myeloma and lymphoma . Cancer 80 , 1557 – 1563 . ( 10.1002/(sici)1097-0142(19971015)80:8+3.3.co;2-k ) 9362422
35 . Mundy GR . 2002 Metastasis to bone: causes, consequences and therapeutic opportunities . Nat. Rev. Cancer 2 , 584 – 593 . ( 10.1038/nrc867 ) 12154351
36 . Lawson MA et al . 2015 Osteoclasts control reactivation of dormant myeloma cells by remodelling the endosteal niche . Nat. Commun. 6 , 8983 . ( 10.1038/ncomms9983 ) 26632274 PMC4686867
37 . Passegué E , Jamieson CHM , Ailles LE , Weissman IL . 2003 Normal and leukemic hematopoiesis: are leukemias a stem cell disorder or a reacquisition of stem cell characteristics? Proc. Natl Acad. Sci. USA 100 , 11842 – 11849 . ( 10.1073/pnas.2034201100 ) 14504387 PMC304096
38 . Döhner H , Weisdorf DJ , Bloomfield CD . 2015 Acute myeloid leukemia . N. Engl. J. Med. 373 , 1136 – 1152 . ( 10.1056/NEJMra1406184 ) 26376137
39 . Ishikawa F et al . 2007 Chemotherapy-resistant human AML stem cells home to and engraft within the bone-marrow endosteal region . Nat. Biotechnol. 25 , 1315 – 1321 . ( 10.1038/nbt1350 ) 17952057
40 . Duarte D et al . 2018 Inhibition of endosteal vascular niche remodeling rescues hematopoietic stem cell loss in AML . Cell Stem Cell 22 , 64 – 77 .( 10.1016/j.stem.2017.11.006 ) 29276143 PMC5766835
41 . Chen Y et al . 2020 Acute myeloid leukemia induced remodelling of the human bone marrow niche predicts clinical outcome . Blood 4 , 5257 – 5268 . 10.1182/bloodadvances.2020001808 PMC7594397 33108453
42 . Chen M , Li Y , Huang X , Gu Y , Li S , Yin P , Zhang L , Tang P . 2021 Skeleton-vasculature chain reaction: a novel insight into the mystery of homeostasis . Bone Res. 9 , 21 . ( 10.1038/s41413-021-00138-0 ) 33753717 PMC7985324
43 . Kusumbe AP , Ramasamy SK , Adams RH . 2014 Coupling of angiogenesis and osteogenesis by a specific vessel subtype in bone . Nature 507 , 323 – 328 . ( 10.1038/nature13145 ) 24646994 PMC4943525
44 . Passaro D et al . 2017 Increased vascular permeability in the bone marrow microenvironment contributes to disease progression and drug response in acute myeloid leukemia . Cancer Cell 32 , 324 – 341 .( 10.1016/j.ccell.2017.08.001 ) 28870739 PMC5598545
45 . Kumar B , Chen CC . 2018 Acute myeloid leukemia remodels endosteal vascular niche into a leukemic niche . Stem Cell Investig. 5 , 34 . ( 10.21037/sci.2018.09.05 ) PMC6232068 30498745
46 . Asano T et al . 2019 Soluble RANKL is physiologically dispensable but accelerates tumour metastasis to bone . Nat. Metab. 1 , 868 – 875 . ( 10.1038/s42255-019-0104-1 ) 32694743
47 . Nakai Y , Okamoto K , Terashima A , Ehata S , Nishida J , Imamura T , Ono T , Takayanagi H . 2019 Efficacy of an orally active small-molecule inhibitor of RANKL in bone metastasis . Bone Res. 7 , 1 . ( 10.1038/s41413-018-0036-5 ) 30622830 PMC6315020
48 . Weilbaecher KN , Guise TA , McCauley LK . 2011 Cancer to bone: a fatal attraction . Nat. Rev. Cancer 11 , 411 – 425 . ( 10.1038/nrc3055 ) 21593787 PMC3666847
49 . Shiozawa Y . 2011 Human prostate cancer metastases target the hematopoietic stem cell niche to establish footholds in mouse bone marrow . J. Clin. Investig 121 , 1298 – 1312 . ( 10.1172/JCI43414 ) 21436587 PMC3069764
50 . Wang H et al . 2015 The osteogenic niche promotes early-stage bone colonization of disseminated breast cancer cells . Cancer Cell 27 , 193 – 210 . ( 10.1016/j.ccell.2014.11.017 ) 25600338 PMC4326554
51 . Russo GC et al . 2024 E-cadherin interacts with EGFR resulting in hyper-activation of ERK in multiple models of breast cancer . Oncogene 43 , 1445 – 1462 . ( 10.1038/s41388-024-03007-2 ) 38509231
52 . Wang H et al . 2018 The osteogenic niche is a calcium reservoir of bone micrometastases and confers unexpected therapeutic vulnerability . Cancer Cell 34 , 823 – 839 .( 10.1016/j.ccell.2018.10.002 ) 30423299 PMC6239211
53 . Moon DO . 2023 Calcium’s role in orchestrating cancer apoptosis: mitochondrial-centric perspective . Int. J. Mol. Sci. 24 , 8982 . ( 10.3390/ijms24108982 ) 37240331 PMC10218825
54 . Monteith GR , Prevarskaya N , Roberts-Thomson SJ . 2017 The calcium–cancer signalling nexus . Nat. Rev. Cancer 17 , 367 – 380 . ( 10.1038/nrc.2017.18 ) 28386091
55 . Siclari VA , Zhu J , Akiyama K , Liu F , Zhang X , Chandra A , Nah HD , Shi S , Qin L . 2013 Mesenchymal progenitors residing close to the bone surface are functionally distinct from those in the central bone marrow . Bone 53 , 575 – 586 . ( 10.1016/j.bone.2012.12.013 ) 23274348 PMC3674849
56 . Logan M , Martin JF , Nagy A , Lobe C , Olson EN , Tabin CJ . 2002 Expression of Cre recombinase in the developing mouse limb bud driven by a Prxl enhancer . Genesis 33 , 77 – 80 . ( 10.1002/gene.10092 ) 12112875
57 . Liu J , Gao C , Sodicoff J , Kozareva V , Macosko EZ , Welch JD . 2020 Jointly defining cell types from multiple single-cell datasets using LIGER . Nat. Protoc. 15 , 3632 – 3662 . ( 10.1038/s41596-020-0391-8 ) 33046898 PMC8132955
58 . Welch JD , Kozareva V , Ferreira A , Vanderburg C , Martin C , Macosko EZ . 2019 Single-cell multi-omic integration compares and contrasts features of brain cell identity . Cell 177 , 1873 – 1887 .( 10.1016/j.cell.2019.05.006 ) 31178122 PMC6716797
59 . Yen CH , Chang CY , Teng MMH , Wu HTH , Chen PCH , Chiou HJ , Chiu NC . 2009 Different and identical features of chondroblastic osteosarcoma and chondrosarcoma: highlights on radiography and magnetic resonance imaging . J. Chin. Med. Assoc. 72 , 76 – 82 . ( 10.1016/s1726-4901(09)70027-9 ) 19251535
60 . Ritter J , Bielack SS . 2010 Osteosarcoma . Ann. Oncol. 21 , vii320 – vii325 . ( 10.1093/annonc/mdq276 ) 20943636
61 . Hasegawa T . 1999 Intracortical osteoblastic osteosarcoma with oncogenic rickets . Skelet. Radiol. 28 , 41 – 45 . 10.1007/s002560050470 10068074
62 . Griffith JF . 1998 Intracortical osteosarcoma . Skelet. Radiol. 27 , 228 – 232 . 10.1007/s002560050372 9592909
63 . Nakamura K et al . 2024 The periosteum provides a stromal defence against cancer invasion into the bone . Nature 634 , 474 – 481 . ( 10.1038/s41586-024-07822-1 ) 39169177
64 . Roodman GD . 2004 Mechanisms of bone metastasis . Discov. Med 4 , 144 – 148 . ( 10.1056/NEJMra030831 ) 20704976
65 . Stewart AF . 2005 Clinical practice. Hypercalcemia associated with cancer . N. Engl. J. Med. 352 , 373 – 379 . ( 10.1056/NEJMcp042806 ) 15673803
66 . Gomez CK , Schiffman SR , Bhatt AA . 2018 Radiological review of skull lesions . Insights Imaging 9 , 857 – 882 . ( 10.1007/s13244-018-0643-0 ) 30232767 PMC6206383
67 . Hershkovich O , Sakhnini M , Barkay G , Liberman B , Friedlander A , Lotan R . 2024 Proximal femoral multiple myeloma pathological fractures, impending and actual fractures - a patient survival study . BMC Cancer 24 , 917 . ( 10.1186/s12885-024-12681-0 ) 39080587 PMC11290202
68 . Hsu WL , Guo JH , Hung YY , Cho DY , Chen DC . 2020 Multiple myeloma with intracranial and spinal intradural metastasis: a case report . Biomedicine 10 , 45 – 49 . ( 10.37796/2211-8039.1079 ) PMC7721472 33854927