Research Progress of Paradoxical Bone Formation in Osteoporosis Treatment

YANG Yang¹, GAO Xi²*

¹The First Clinical Medical College, Heilongjiang University of Chinese Medicine, Harbin 150040, China
²The First Affiliated Hospital of Heilongjiang University of Chinese Medicine, Harbin 150040, China

*Corresponding author: GAO Xi, Chief physician/Doctoral supervisor; E-mail: gaoxi2025@163.com

Abstract: Osteoporosis is a common clinical bone disease caused by multiple factors. Its primary mechanism involves altering the body's inflammatory microenvironment, which promotes increased osteoclast generation and leads to enhanced bone resorption and decreased bone mass. Paradoxical bone formation is a process that regulates bone formation and increases bone mass by modulating apoptosis in a subset of osteoblasts, thereby affecting macrophage efferocytosis to promote osteoblast differentiation. Osteoblasts primarily participate in bone formation, while osteoclasts mediate bone resorption, and together they regulate bone homeostasis. Under normal homeostatic conditions, approximately 50% of osteoblasts in remodeling bone undergo apoptosis. When partial osteoblast apoptosis triggers paradoxical bone formation, macrophages are recruited and perform efferocytosis, polarizing into the M2 phenotype. M2 macrophages regulate osteoblast differentiation and inhibit osteoclast generation, suppressing bone resorption while enabling fresh osteoblasts to rapidly occupy the positions of original senescent osteoblasts to continue participating in bone formation. Since fresh osteoblasts produce greater bone formation than senescent ones, bone mass increases significantly compared to pre-apoptosis levels. Promoting partial osteoblast apoptosis in osteoporotic organisms may thus reverse bone loss, making paradoxical bone formation a promising new therapeutic direction for osteoporosis. Therefore, this paper proposes treating osteoporosis through "paradoxical bone formation" and analyzes its underlying mechanisms to provide novel insights for osteoporosis research and treatment.

Keywords: Osteoporosis; Paradoxical bone formation; Osteoblast apoptosis; Macrophage; Efferocytosis

Funding: Heilongjiang Natural Science Foundation (LH2021H092); Heilongjiang Provincial Special Project for Research on Popularization of Traditional Chinese Medicine Classics (ZYW2023-046)

Citation: YANG Y, GAO X. Research progress of paradoxical bone formation in osteoporosis [J]. Chinese General Practice, 2025. DOI: 10.12114/j.issn.1007-9572.2024.0675. [Epub ahead of print]

Osteoporosis (OP) is an extremely common skeletal disease in clinical practice, characterized primarily by lower back and lower limb joint pain that worsens with activity, accompanied by height reduction and spinal deformity. In early stages, OP often goes unnoticed, typically discovered only when fractures and pain manifest. In severe cases, thoracic cage deformity may impair respiration, potentially leading to disability or death. OP features reduced bone mass, compromised bone microarchitecture, and increased bone fragility. The pathogenesis involves disruption of bone homeostasis when bone resorption exceeds bone formation, resulting in decreased bone mass. Osteoclasts (OC) are the sole cells involved in bone resorption, while osteoblasts (OB) are the main cells participating in bone formation. Studies have found that OP prevalence reaches 18.3% globally among individuals aged 15-105 years, with particularly high rates of 23.1% in postmenopausal women due to estrogen deficiency, and 11.7% in men over 50. Aging is the most important cause of primary OP, as advancing age brings estrogen deficiency, declining testosterone, and gut microbiome dysbiosis, which act on T cells in the immune environment to increase expression of pro-inflammatory factors such as tumor necrosis factor α (TNF-α) and interleukin (IL)-1. These changes promote expression of receptor activator of nuclear factor κB ligand (RANKL), osteoprotegerin (OPG), and macrophage colony-stimulating factor (M-CSF). Other inflammatory factors can increase prostaglandin E2 (PGE2) expression, which enhances RANKL expression. TNF-α also regulates Runx2 degradation, and TNF receptor-associated factor 3 (TRAF3) activates the nuclear factor κB (NF-κB) signaling pathway. In secondary OP, glucocorticoids represent a major causative factor, inducing bone marrow-derived mesenchymal stem cell (BMSC) apoptosis while reducing bone morphogenetic protein 2 (BMP-2) expression and disrupting Wnt signaling. The persistent inflammatory state in OP patients also affects macrophage (Mø) polarization related to bone immunity. These abnormal factors and pathways ultimately lead to aberrant osteoclast activation and suppressed osteoblast differentiation, causing bone loss and OP development.

Previous research suggested that normal bone formation occurs through promoting osteoblast differentiation and inhibiting osteoblast apoptosis. However, deeper investigation revealed that approximately 50% of osteoblasts undergo apoptosis in normal bone remodeling sites. Paradoxical bone formation, described by BATOON et al., refers to the phenomenon where promoting apoptosis in less than 50% of osteoblasts increases vertebral bone mass in mice. These experiments also observed increased bone surface macrophages and BMSCs, with a positive correlation between macrophage and BMSC numbers, yet no increase in osteoclast numbers. This contradicts the conventional theory of inhibiting osteoblast apoptosis to promote bone formation, hence the term "paradoxical bone formation." Since osteoblast bone-forming capacity declines with age, senescent osteoblasts produce less bone than fresh osteoblasts. Promoting apoptosis in senescent osteoblasts not only stimulates BMSC differentiation into osteoblasts but also allows fresh osteoblasts to replace senescent ones, ultimately increasing bone mass. Based on this theory, paradoxical bone formation may represent a novel therapeutic approach for OP.

1 Overview of Paradoxical Bone Formation

Paradoxical bone formation is a process that regulates bone formation and increases bone mass by modulating partial osteoblast apoptosis, affecting macrophage efferocytosis, and promoting fresh osteoblast differentiation. Historically, osteoblast apoptosis was considered detrimental to bone homeostasis. However, recent studies demonstrate that approximately 50% of osteoblasts undergo apoptosis at bone remodeling sites under steady-state conditions, suggesting that osteoblast apoptosis may be an important mechanism for promoting bone formation. Regulating programmed osteoblast death has become a hot research topic for OP treatment. BATOON et al. conducted pro-apoptosis experiments on less than 50% of osteoblasts, observing increased vertebral bone mass in mice. These studies also found increased bone surface macrophages and BMSCs, with a positive correlation between macrophage and BMSC numbers, but no effect on osteoclast numbers. This phenomenon, opposite to the conventional theory of inhibiting osteoblast apoptosis to promote bone formation, was termed "paradoxical bone formation." Since osteoblast bone-forming capacity gradually weakens with age, senescent osteoblasts produce less bone than fresh osteoblasts. When senescent osteoblasts undergo apoptosis, BMSC differentiation is promoted while fresh osteoblasts replace senescent ones, ultimately achieving increased bone mass. Based on this theory, paradoxical bone formation holds promise as a new treatment method for OP.

2 Paradoxical Bone Formation and Bone Homeostasis

Bone is a dynamic tissue structure composed of osteocytes, osteoblasts, osteoclasts, and other elements. Postnatally, bone exists in a constant "use it or lose it" remodeling state to adapt to biomechanical changes. This remodeling process involves bone resorption and bone formation phases, with the balance depending on dynamic equilibrium between osteoblasts and osteoclasts. Osteoblast-mediated bone formation and osteoclast-mediated bone resorption interact precisely, with osteocytes serving as crucial mediators for communication between these cell types, which is essential for maintaining bone mass. Postnatal bone homeostasis is regulated by multiple mechanisms acting primarily on osteoblasts and osteoclasts, including bone immunology-related regulation. Paradoxical bone formation is closely related to osteoblast-mediated bone formation and osteoclast-mediated bone resorption, and is also regulated by macrophages in bone immunology.

2.1 Paradoxical Bone Formation and Osteoblasts

Osteoblasts are crucial cells involved in bone formation, differentiating from BMSCs, which are important regulators of postnatal bone homeostasis and can be considered osteoblast progenitor cells. Promoting bone formation can be achieved by regulating BMSC differentiation into osteoblasts. BMSC-to-osteoblast differentiation is complex, involving STAT3 signaling pathway, Runx2, Smad proteins, BMP-2, transforming growth factor β (TGF-β) superfamily, and Wnt signaling pathway. STAT3 activation initiates transcription, representing the first step in BMSC differentiation. Runx2 can inhibit differentiation of BMSCs into other lineages, directing them specifically toward osteoblasts, while BMP-2 and Wnt signaling promote Runx2 expression. When the STAT3 pathway activates in BMSCs, downstream factors like Runx2 are activated, leading to Smad effectors and non-Smad-dependent pathways such as p38 mitogen-activated protein kinase (MAPK) activation, enhanced TGF-β/BMPs signaling, increased osterix expression, Akt activation, and Wnt pathway regulation. The canonical Wnt pathway works synergistically with Runx2 and Osterix to promote osteoblast differentiation and maturation, while also exerting complex stimulatory and inhibitory effects on TGF-β activity. Akt can induce enhanced expression of Runx2-related cells, providing positive regulation for osteoblast differentiation and maturation. BMP-2 activation is also influenced by parathyroid hormone (PTH), which activates downstream effector proteins to promote BMP-2 expression and osteoblast differentiation while inhibiting osteoblast apoptosis. Additionally, BMP transmission is affected by Hedgehog (Hh) signaling, which has five signaling factors that can receive BMP signals and upregulate PTH expression. Sonic hedgehog (Shh) cooperates with BMP-2 in osteoblast differentiation and upregulates Osterix expression, while Indian hedgehog (Ihh) acts directly on BMSCs to promote osteoblast differentiation and interacts with the Runx2/Smad pathway. Notch-related pathways are also important for postnatal bone homeostasis regulation and osteoblast differentiation, promoting expansion of immature osteoblasts. Insulin-like growth factor (IGF) can also promote BMSC-to-osteoblast differentiation by cooperating with Wnt, BMP, TGF-β, and Hh/PTH signaling pathways.

[FIGURE:1] Mechanisms involved in the differentiation of BMSC into OB

2.2 Paradoxical Bone Formation and Osteoclasts

Osteoclasts are the only cells involved in bone resorption. When osteocytes undergo apoptosis, osteoclasts can be recruited by apoptotic osteocytes to clear them. Osteocytes differentiate from osteoblasts after bone formation completion. Osteoclasts originate from mononuclear-macrophage cells, with differentiation and activation primarily regulated by M-CSF, RANK, and RANKL. RANKL belongs to the TNF superfamily. M-CSF is essential for osteoclast precursor proliferation, survival, differentiation, and for osteoclast survival and cytoskeletal rearrangement required for bone resorption. Under M-CSF stimulation, monocytes differentiate into bone macrophages, which can further differentiate into osteoclasts with RANKL cooperation. Inhibiting RANKL levels under normal M-CSF conditions can suppress osteoclast differentiation. Although osteoblasts were previously thought to secrete RANKL, recent studies show osteocytes are the primary RANK source. When osteoblasts undergo apoptosis, ATP drives surrounding viable osteocytes to produce RANKL, recruiting osteoclasts and promoting their production. RANKL binding to its receptor in the presence of M-CSF promotes TRAF recruitment and activation of osteoclast-related signaling pathways including NF-κB, Akt, c-Jun N-terminal kinase (JNK), extracellular signal-regulated kinase (ERK), and MAPK. Osteoblasts remain important for osteoclast generation, producing RANKL receptor activator and promoting M-CSF synthesis. Osteoprotegerin (OPG) inhibits osteoclast differentiation and activation by binding RANKL and blocking its interaction with RANK, making the RANKL-RANK-OPG pathway crucial for bone homeostasis regulation.

[FIGURE:2] OC differentiation-related mechanisms

2.3 Paradoxical Bone Formation and Macrophages

Beyond the classic roles of osteoblasts and osteoclasts, bone homeostasis is also regulated by bone immunology, with increasing research on macrophage regulation of bone homeostasis. Macrophages play important roles in bone homeostasis regulation, primarily affecting osteoblast differentiation and maturation to promote bone formation and osteoclast differentiation to promote bone resorption. Both macrophages and osteoclasts originate from monocytes, which differentiate into macrophages under M-CSF stimulation and into osteoclasts with RANKL cooperation. Macrophages exist in two polarization states: classically activated macrophages (M1) and alternatively activated macrophages (M2). The M1 phenotype is considered pro-inflammatory, polarizing under Toll-like receptor (TLR) and interferon γ (IFN-γ) signaling to release IL-1α, IL-1β, IL-6, IL-12, IL-23, TNF-α, and NO for pathogen defense and clearance. The M2 phenotype is considered anti-inflammatory, polarizing under IL-4 and IL-13 stimulation to release vascular endothelial growth factor A (VEGFA), BMP-2, IL-10, and TGF-β to suppress inflammatory responses and participate in inflammation resolution and tissue repair. Macrophage polarization is a complex process regulated by multiple pathways.

Studies confirm that M2 macrophages positively influence bone formation by promoting BMSC-to-osteoblast differentiation. When BMSCs are co-cultured with macrophages, macrophages polarize toward the M2 phenotype, which is considered a polarization state that promotes bone formation and inhibits bone resorption. M2 polarization upregulates BMP-2, TGF-β, and Osterix expression—critical factors for osteoblast differentiation. Additionally, M2 macrophages enhance VEGFA expression, which inhibits the PI3K/Akt pathway that promotes osteoclast differentiation, thereby suppressing bone resorption. The IL-20 cytokine in the IL-10 family can inhibit NF-κB and RANKL pathways, suppress NFAT expression, inhibit IL-1β-mediated MAPK upregulation, and increase OPG and M-CSF expression, ultimately promoting osteoblast generation and inhibiting osteoclast differentiation for bone homeostasis regulation.

When macrophages polarize to the M1 phenotype, they secrete various inflammatory factors, with IL-1 being most critical for bone homeostasis regulation, particularly in promoting osteoclast generation and bone resorption. IL-1 can directly downregulate OPG and upregulate RANKL expression, promote osteoclast precursor development under MAPK regulation in RANKL-rich environments, and activate NF-κB to upregulate RANKL expression. IL-1 also induces release of other inflammatory cytokines like TNF-α and IL-6 that affect osteoclast generation. TNF-α activates NF-κB to promote osteoclast formation, while IL-6 activates JAK/STAT, MAPK/ERK, PI3K-Akt, and NF-κB pathways—key pathways for osteoclast generation. When M1 macrophages produce pro-inflammatory signals, they can promote anti-inflammatory effects through the COX-2-PGE2 pathway, ultimately polarizing toward the M2 phenotype.

[FIGURE:3] Mechanisms associated with macrophage polarization involvement in OB and OC differentiation

Macrophage recruitment and M2 polarization represent the central link in paradoxical bone formation. In OP patients, inflammatory changes promote M1 polarization, creating a feedback loop that perpetuates the inflammatory state. Although macrophages can actively polarize toward M2 for anti-inflammatory effects, this is often insufficient to control the dominant inflammatory state. Actively inducing paradoxical bone formation could enhance M2 polarization through multiple mechanisms, shifting the M1/M2 balance. This would not only control M1 polarization and its pro-inflammatory effects but also enhance M2 anti-inflammatory actions to control the inflammatory environment, increase M2-mediated regulation of osteoblast differentiation, inhibit osteoclast generation, enhance bone formation, suppress bone resorption, and increase bone mass—potentially reversing the bone homeostasis imbalance and bone loss in OP.

3 Paradoxical Bone Formation and Apoptosis

The key difference between paradoxical and normal bone formation lies in its mediation of osteoblast apoptosis to enhance bone formation. During this process, macrophage polarization to the M2 phenotype increases bone formation. However, the precise mechanisms by which paradoxical bone formation controls macrophage M2 polarization remain under investigation, likely involving macrophage efferocytosis of apoptotic osteoblasts.

3.1 Effects of Apoptosis on Macrophages

During paradoxical bone formation, a subset of osteoblasts undergoes apoptosis. Macrophages are then recruited to engulf these apoptotic cells through efferocytosis, which polarizes macrophages toward the M2 phenotype and ultimately increases osteoblast differentiation. Apoptosis is a multi-gene controlled, autonomous cell death mechanism crucial for maintaining internal homeostasis. Upon apoptotic signaling, cells undergo sequential changes, first releasing "Find me" signals to recruit macrophages, followed by "Eat me" signals to initiate recognition and clearance. This macrophage-mediated clearance of apoptotic cells is termed "efferocytosis." Without efferocytosis, release of apoptotic cell contents could trigger inflammation, making efferocytosis anti-inflammatory. During efferocytosis, macrophages block NF-κB and TLR pathways that generate TNF-α, IL-6, and other cytokines, inhibiting M1 polarization. Apoptotic body formation during efferocytosis activates receptors like peroxisome proliferator-activated receptors (PPAR) that promote IL-10 and TGF-β generation, facilitating M2 polarization. Studies confirm that apoptosis-induced efferocytosis does not trigger pro-inflammatory macrophage (M1) proliferation, instead regulating macrophage polarization toward the M2 phenotype with anti-inflammatory effects.

3.2 Effects of Osteoblast Apoptosis on Bone Formation

Osteoblast apoptosis is widely recognized as the final fate of osteoblasts after bone formation completion. When osteoblasts undergo apoptosis, macrophages are rapidly recruited to perform efferocytosis. Bone tissue contains resident macrophages that maintain homeostasis, clear debris, and repair tissue—known as bone macrophages—which form coronal structures around osteoblasts, enabling rapid recruitment upon osteoblast apoptosis. Under efferocytosis, bone macrophages polarize to the M2 phenotype, ultimately affecting bone formation. As discussed, M2 macrophages and their associated cytokines promote bone formation by upregulating BMP-2, TGF-β, and Osterix expression to enhance BMSC-to-osteoblast differentiation, while suppressing NFAT expression, MAPK and NF-κB pathways, enhancing VEGFA expression, and upregulating osteoblast-derived OPG to inhibit osteoclast production. Theoretically, osteoblast apoptosis promotes macrophage recruitment and M2 polarization, stimulating BMSC-to-osteoblast differentiation while inhibiting osteoclast production, thereby positively affecting bone formation and suppressing bone resorption.

4 Therapeutic Potential of Paradoxical Bone Formation in Osteoporosis

Current OP treatments primarily aim to increase bone density and mass by inhibiting osteoblast apoptosis and promoting osteoblast generation to enhance bone formation, while suppressing osteoclast differentiation to reduce bone resorption. Paradoxical bone formation can ultimately increase bone formation, inhibit bone resorption, and increase bone mass, representing a potential new therapeutic direction. During OP, the body exists in a chronic inflammatory microenvironment with increased expression of TNF-α, IL-1, M-CSF, and PGE2, which promote macrophage M1 polarization and activate osteoclast differentiation pathways, increasing bone resorption. Although M1-derived pro-inflammatory signals can promote anti-inflammatory effects through the COX-2-PGE2 pathway, polarizing macrophages toward M2 and secreting anti-inflammatory factors like IL-4, IL-10, and IL-13 while increasing VEGF, BMP-2, TGF-β, and Osterix expression, the persistent inflammatory microenvironment and strong inflammatory factor expression drive predominant M1 polarization that cannot actively control OP progression.

Based on the observation that 50% of osteoblasts undergo apoptosis at bone remodeling sites under normal conditions, actively promoting paradoxical bone formation could potentially reverse OP. By inducing partial osteoblast apoptosis, macrophages are recruited and clear apoptotic osteoblasts through efferocytosis, polarizing to the M2 phenotype. M2 macrophages promote BMP-2, TGF-β, and Osterix upregulation, suppress NFAT expression, MAPK and NF-κB pathways, enhance VEGFA expression, and increase OPG production, ultimately inhibiting further osteoclast production and controlling bone resorption while promoting BMSC-to-osteoblast differentiation. Fresh osteoblasts rapidly replace senescent osteoblasts, enhancing bone formation and increasing bone mass, thereby correcting the imbalanced bone homeostasis and bone loss in OP. Experimental evidence confirms that pro-apoptosis experiments on less than 50% of osteoblasts increase vertebral bone mass, bone surface macrophages, and BMSCs, with positive correlation between macrophage and BMSC numbers, while osteoclast numbers remain unchanged, validating the feasibility of paradoxical bone formation as a novel OP treatment method.

[FIGURE:4] Osteoporosis-related mechanisms of occurrence

OP remains a clinically challenging disease. While previously common in individuals over 50 and postmenopausal women, unhealthy lifestyles and adverse effects of other disease treatments now cause OP across all age groups, maintaining it as a research hotspot. Current treatments increase bone density and mass by inhibiting osteoblast apoptosis, promoting osteoblast generation, and suppressing osteoclast differentiation. However, deeper research reveals that osteoblast apoptosis is crucial for bone homeostasis regulation, leading to the paradoxical bone formation concept. This approach promotes partial osteoblast apoptosis, recruits macrophages for efferocytosis and M2 polarization, generates factors promoting osteoblast differentiation while inhibiting osteoclast generation, and replaces senescent with fresh osteoblasts that have higher bone-forming capacity, thereby increasing bone mass. This method may become a new therapeutic avenue for OP.

However, limited experimental validation exists for paradoxical bone formation's ability to increase bone mass and improve OP. Currently identified apoptosis pathways include membrane receptor (Fas/Fas), mitochondrial cytochrome C release, and endoplasmic reticulum stress (ERS) pathways, which ultimately activate caspase activity through cascade reactions. Precisely controlling apoptosis in a specific proportion of osteoblasts without harming other cells remains experimental. Since different tissues exhibit varying apoptotic responses to the same stimulus, achieving uniform apoptosis across all osteoblasts in OP patients presents considerable difficulty requiring extensive experimental exploration and validation. Therefore, "paradoxical bone formation for OP treatment" remains theoretical. Nevertheless, future research will gradually elucidate the specific mechanisms of paradoxical bone formation in OP treatment, potentially providing additional therapeutic options.

Author Contributions: YANG Yang and GAO Xi conceived and designed the article. YANG Yang collected and organized literature, and drafted the manuscript. GAO Xi conducted feasibility analysis, revised the manuscript, and was responsible for quality control, review, and overall supervision.

Conflict of Interest: The authors have no conflicts of interest to declare.

References:
[1] Chinese Medical Association Physical Medicine and Rehabilitation Branch. Guidelines for Osteoporosis Rehabilitation Treatment (2024 Edition) [J]. Chinese Journal of Evidence-Based Medicine, 2024, 24(6): 626-
[2] REDLICH K, SMOLEN J S. Inflammatory bone loss: pathogenesis and therapeutic intervention [J]. Nat Rev Drug Discov, 2012, 11(3): 234-250. DOI: 10.1038/nrd3669.
[3] SALARI N, DARVISHI N, BARTINA Y, et al. Global prevalence of osteoporosis among the world older adults: a comprehensive systematic review and meta-analysis [J]. J Orthop Surg Res, 2021, 16(1): 669. DOI: 10.1186/s13018-021-02821-8.
[4] WU D, CLINE-SMITH A, SHASHKOVA E, et al. T-cell mediated inflammation in postmenopausal osteoporosis [J]. Front Immunol, 2021, 12: 687551. DOI: 10.3389/fimmu.2021.687551.
[5] DRAKE M T, KHOSLA S. Hormonal and systemic regulation of sclerostin [J]. Bone, 2017, 96: 8-17. DOI: 10.1016/j.bone.2016.12.004.
[6] STEIN E, SHANE E. Secondary osteoporosis [J]. Endocrinol Metab Clin North Am, 2003, 32(1): 115-34, vii. DOI: 10.1016/s0889-8529(02)00062-2.
[7] Chinese Medical Association Osteoporosis and Bone Mineral Diseases Branch. Guidelines for Diagnosis and Treatment of Primary Osteoporosis (2022) [J]. Chinese Journal of Osteoporosis and Bone Mineral Diseases, 2022, 15(6):
[8] BATOON L, KOH A J, MILLARD S M, et al. Induction of osteoblast apoptosis stimulates macrophage efferocytosis and paradoxical bone formation [J]. Bone Res, 2024, 12(1): 43. DOI: 10.1038/s41413-024-00341-9.
[9] LIANG S L, CHENG W X, ZHANG P, et al. Research progress on programmed death of osteoblasts [J]. Chinese Journal of Osteoporosis, 2024, 30(3): 385-390.
[10] ZHANG S, LIU Y, YU W F, et al. Research trends and hotspots on osteoporosis: a decade-long bibliometric and visualization analysis from 2014 to 2023 [J]. Front Med, 2024, 11: 1436486. DOI: 10.3389/fmed.2024.1436486.
[11] TONG A L, CHEN L L, DING G Z. Research progress on osteoblast bone formation mechanisms [J]. Chinese Journal of Osteoporosis, 1999, 5(3): 60-64. DOI: 10.3969/j.issn.1006-7108.1999.03.020.
[12] CHEN Y Q, DAI K R, QIU S J. Cellular basis of bone remodeling [J]. Foreign Medical Sciences: Trauma and Surgery, 1989, 10(2): 85-89.
[13] ANSARI N, SIMS N A. The cells of bone and their interactions [J]. Handb Exp Pharmacol, 2020, 262: 1-25. DOI: 10.1007/164_2019_343.
[14] MATSUO K. Cross-talk among bone cells [J]. Curr Opin Nephrol Hypertens, 2009, 18(4): 292-297. DOI: 10.1097/mnh.0b013e32832b75f1.
[15] MA Q Y, LIANG M M, WU Y T, et al. Mature osteoclast-derived apoptotic bodies promote osteogenic differentiation via RANKL-mediated reverse signaling [J]. J Biol Chem, 2019, 294(29): 11240-11247. DOI: 10.1074/jbc.RA119.007625.
[16] YAMADA T, FUKASAWA K, HORIE T, et al. The role of CDK8 in mesenchymal stem cells in controlling osteoclastogenesis and bone homeostasis [J]. Stem Cell Reports, 2022, 17(7): 1576-1588. DOI: 10.1016/j.stemcr.2022.06.001.
[17] HUANG Y K, YIN Y, GU Y Z, et al. Characterization and immunogenicity of bone marrow-derived mesenchymal stem cells under osteoporotic conditions [J]. Sci China Life Sci, 2020, 63(3): 429-442. DOI: 10.1007/s11427-019-1555-9.
[18] STEGEN S, CARMELIET G. Metabolic regulation of skeletal cell fate and function [J]. Nat Rev Endocrinol, 2024, 20(7): 399-413. DOI: 10.1038/s41574-024-00969-x.
[19] SANPAOLO E R, ROTONDO C, CICI D, et al. JAK/STAT pathway and molecular mechanism in bone remodeling [J]. Mol Biol Rep, 2020, 47(11): 9087-9096. DOI: 10.1007/s11033-020-05873-7.
[20] DATTA H K, NG W F, WALKER J A, et al. The cell biology of bone metabolism [J]. J Clin Pathol, 2008, 61(5): 577-587. DOI: 10.1136/jcp.2007.048868.
[21] JASCHKE N, SIPOS W, HOFBAUER L C, et al. Skeletal endocrinology: where evolutionary advantage meets disease [J]. Bone Res, 2021, 9(1): 28. DOI: 10.1038/s41413-021-00149-x.
[22] BORDUKALO-NIKŠIĆ T, KUFNER V, VUKIČEVIĆ S. The role of BMPs in the regulation of osteoclasts resorption and bone remodeling: from experimental models to clinical applications [J]. Int J Mol Sci, 2020, 21(24): 9707. DOI: 10.3390/ijms21249707.
[23] ZHANG Z D, ZHANG X Z, ZHAO D W, et al. TGF-β1 promotes the osteoinduction of human osteoblasts via the PI3K/AKT/mTOR/S6K1 signalling pathway [J]. Mol Med Rep, 2019, 19(5): 3505-3518. DOI: 10.3892/mmr.2019.10051.
[24] YANG J W, UEHARU H, MISHINA Y. Energy metabolism: a newly emerging target of BMP signaling in bone homeostasis [J]. Bone, 2020, 138: 115467. DOI: 10.1016/j.bone.2020.115467.
[25] MUKHERJEE A, ROTWEIN P. Selective signaling by Akt1 controls osteoblast differentiation and osteoblast-mediated osteoclast development [J]. Mol Cell Biol, 2012, 32(2): 490-500. DOI: 10.1128/MCB.06361-11.
[26] WU M R, WU S L, CHEN W, et al. The roles and regulatory mechanisms of TGF-β and BMP signaling in bone and cartilage development, homeostasis and disease [J]. Cell Res, 2024, 34(2): 101-123. DOI: 10.1038/s41422-023-00918-9.
[27] LIU Q, LI M, WANG S Y, et al. Recent advances of osterix transcription factor in osteoblast differentiation and bone formation [J]. Front Cell Dev Biol, 2020, 8: 601224. DOI: 10.3389/fcell.2020.601224.
[28] ENGELMANN J, ZARRER J, GENSCH V, et al. Regulation of bone homeostasis by MERTK and TYRO3 [J]. Nat Commun, 2022, 13(1): 7689. DOI: 10.1038/s41467-022-33938-x.
[29] CHEN G Q, DENG C X, LI Y P. TGF-β and BMP signaling in osteoblast differentiation and bone formation [J]. Int J Biol Sci, 2012, 8(2): 272-288. DOI: 10.7150/ijbs.2929.
[30] FLORENCIO-SILVA R, SASSO G R, SASSO-CERRI E, et al. Biology of bone tissue: structure, function, and factors that influence bone cells [J]. Biomed Res Int, 2015, 2015: 421746. DOI: 10.1155/2015/421746.
[31] NICOLAIDOU V, WONG M M, REDPATH A N, et al. Monocytes induce STAT3 activation in human mesenchymal stem cells to promote osteoblast formation [J]. PLoS One, 2012, 7(7): e39871. DOI: 10.1371/journal.pone.0039871.
[32] MOLLAZADEH S, FAZLY BAZZAZ B S, KERACHIAN M A. Role of apoptosis in pathogenesis and treatment of bone-related diseases [J]. J Orthop Surg Res, 2015, 10: 15. DOI: 10.1186/s13018-015-0152-5.
[33] ZHOU H, ZHANG L, CHEN Y, et al. Research progress on the hedgehog signalling pathway in regulating bone formation and homeostasis [J]. Cell Prolif, 2022, 55(1): e13162. DOI: 10.1111/cpr.13162.
[34] ZIEBA J T, CHEN Y T, LEE B H, et al. Notch signaling in skeletal development, homeostasis and pathogenesis [J]. Biomolecules, 2020, 10(11): 1493. DOI: 10.3390/biom10111493.
[35] BALLHAUS T M, JIANG S, BARANOWSKY A, et al. Relevance of Notch signaling for bone metabolism and regeneration [J]. Int J Mol Sci, 2021, 22(3): 1325. DOI: 10.3390/ijms22031325.
[36] RUAN X Y, JIN X H, SUN F J, et al. IGF signaling pathway in bone and cartilage development, homeostasis, and disease [J]. Front Endocrinol, 2022, 13: 872549. DOI: 10.3389/fendo.2022.872549.
[37] YAO Y, CAI X Y, REN F J, et al. The macrophage-osteoclast axis in osteoimmunity and osteo-related diseases [J]. Front Immunol, 2021, 12: 664871. DOI: 10.3389/fimmu.2021.664871.
[38] CLARKE B. Normal bone anatomy and physiology [J]. Clin J Am Soc Nephrol, 2008, 3(Suppl 3): S131-S139. DOI: 10.2215/CJN.04151206.
[39] WANG X M, WANG X, CAO L Z, et al. Mechanism of RANK/RANKL/OPG signaling axis in the correlation between osteoporosis and inflammatory bowel disease [J]. Chinese Journal of Osteoporosis and Bone Mineral Diseases, 2017, 10(5): 499-504. DOI: 10.3969/j.issn.1674-2591.2017.05.012.
[40] YIN Z Y, GONG G, LIU X H, et al. Mechanism of regulating macrophages/osteoclasts in attenuating wear particle-induced aseptic osteolysis [J]. Front Immunol, 2023, 14: 1274679. DOI: 10.3389/fimmu.2023.1274679.
[41] YE Z, WANG Y T, XIANG B Q, et al. Roles of the Siglec family in bone and bone homeostasis [J]. Biomed Pharmacother, 2023, 165: 115064. DOI: 10.1016/j.biopha.2023.115064.
[42] MCCUTCHEON S, MAJESKA R J, SPRAY D C, et al. Apoptotic osteocytes induce RANKL production in bystanders via purinergic signaling and activation of pannexin channels [J]. J Bone Miner Res, 2020, 35(5): 966-977. DOI: 10.1002/jbmr.3954.
[43] KENNEDY O D, LAUDIER D M, MAJESKA R J, et al. Osteocyte apoptosis is required for production of osteoclastogenic signals following bone fatigue in vivo [J]. Bone, 2014, 64: 132-137. DOI: 10.1016/j.bone.2014.03.049.
[44] CHEN X, WANG Z Q, DUAN N, et al. Osteoblast-osteoclast interactions [J]. Connect Tissue Res, 2018, 59(2): 99-107. DOI: 10.1080/03008207.2017.1290085.
[45] IGLESIAS-VELAZQUEZ O, TRESGUERRES F G, TRESGUERRES I, et al. OsteoMac: a new player on the bone biology scene [J]. Ann Anat, 2024, 254: 152244. DOI: 10.1016/j.aanat.2024.152244.
[46] CHANG M K, RAGGATT L J, ALEXANDER K A, et al. Osteal tissue macrophages are intercalated throughout human and mouse bone lining tissues and regulate osteoblast function in vitro and in vivo [J]. J Immunol, 2008, 181(2): 1232-1244. DOI: 10.4049/jimmunol.181.2.1232.
[47] MUÑOZ J, AKHAVAN N S, MULLINS A P, et al. Macrophage polarization and osteoporosis: a review [J]. Nutrients, 2020, 12(10): 2999. DOI: 10.3390/nu12102999.
[48] HU K Y, SHANG Z Y, YANG X R, et al. Macrophage polarization and the regulation of bone immunity in bone homeostasis [J]. J Inflamm Res, 2023, 16: 3563-3580. DOI: 10.2147/JIR.S423819.
[49] CHEN K X, JIAO Y R, LIU L, et al. Communications between bone marrow macrophages and bone cells in bone remodeling [J]. Front Cell Dev Biol, 2020, 8: 598263. DOI: 10.3389/fcell.2020.598263.
[50] MENG B W, WU D L, CHENG Y F, et al. Interleukin-20 differentially regulates bone mesenchymal stem cell activities in RANKL-induced osteoclastogenesis through the OPG/RANKL/RANK axis and the NF-κB, MAPK and AKT signalling pathways [J]. Scand J Immunol, 2020, 91(5): e12874. DOI: 10.1111/sji.12874.
[51] MAHAJAN A, BHATTACHARYYA S. Immunomodulation by mesenchymal stem cells during osteogenic differentiation: Clinical implications during bone regeneration [J]. Mol Immunol, 2023, 164: 143-152. DOI: 10.1016/j.molimm.2023.11.006.
[52] LI L, LIU Y J, QIAN X S, et al. Modulating the phenotype and function of bone marrow-derived macrophages via mandible and femur osteoblasts [J]. Int Immunopharmacol, 2024, 132: 112000. DOI: 10.1016/j.intimp.2024.112000.
[53] LU Y, FAN D D, WANG W, et al. The protein 4.1R downregulates VEGFA in M2 macrophages to inhibit colon cancer metastasis [J]. Exp Cell Res, 2021, 409(1): 112896. DOI: 10.1016/j.yexcr.2021.112896.
[54] SUN Y, LI J B, XIE X P, et al. Macrophage-osteoclast associations: origin, polarization, and subgroups [J]. Front Immunol, 2021, 12: 778078. DOI: 10.3389/fimmu.2021.778078.
[55] ZENG J N, HU X F. Research progress on apoptotic bodies [J]. Biology Teaching, 2024, 49(10): 5-8. DOI: 10.3969/j.issn.1004-7549.2024.10.002.
[56] GERLACH B D, AMPOMAH P B, YURDAGUL A Jr, et al. Efferocytosis induces macrophage proliferation to help resolve tissue injury [J]. Cell Metab, 2021, 33(12): 2445-2463.e8. DOI: 10.1016/j.cmet.2021.10.015.
[57] HOU G S, WANG X Y, WANG A H, et al. The role of secreted proteins in efferocytosis [J]. Front Cell Dev Biol, 2024, 11: 1332482. DOI: 10.3389/fcell.2023.1332482.
[58] GENG R, LU J. Research progress on the relationship between macrophage efferocytosis and inflammatory diseases [J]. Journal of Southeast University: Medical Science Edition, 2023, 42(3): 466-474.
[59] JILKA R L, WEINSTEIN R S, BELLIDO T, et al. Demonstration of programmed cell death (apoptosis) in osteoblasts: Evidence for modulation of this process by growth factors and cytokines [J]. J Bone Miner Res, 1996, 11: P213. DOI: 10.1096/fasebj.13.8.793.
[60] LI M, LIN J. Apoptosis pathways and their mechanisms [J]. International Journal of Obstetrics and Gynecology, 2014, 41(2): 103-107.
[61] GRUVER-YATES A L, CIDLOWSKI J A. Tissue-specific actions of glucocorticoids on apoptosis: a double-edged sword [J]. Cells, 2013, 2(2): 202-223. DOI: 10.3390/cells2020202.

(Received: December 10, 2024; Revised: April 6, 2025)
(Editor: JIA Meng-meng)