The balance of bone-forming and bone-resorbing activities is essential to preserve bone homeostasis. Multiple bone cell types, including osteoblasts (OBs), osteoclasts (OCs), and bone marrow mesenchymal stem cells (BMSCs) are involved in the multifaceted process of bone homeostasis (34, 35). OBs and BMSCs are responsible for bone formation, whereas OCs are the primary functional cells involved in bone resorption (36). This interdependent mechanism is referred to as coupling, where OCs break down the organic matrix and dissolve bone minerals, followed by the recruitment of OBs to deposit fresh bone matrix that later mineralizes. An imbalance in the bone remodeling process, leading to excessive bone resorption relative to bone formation, is the principal factor contributing to the greater occurrence of OP (37, 38). The normal bone remodeling equilibrium can be disrupted by various factors, such as hormones, endocrine regulation, cytokines, and others, which interfere with OBs and OCs differentiation and activity (39, 40). Furthermore, the imbalance in bone remodeling also deteriorates the bone microstructure, making it a target for therapeutic approaches to improve bone strength and decrease the risk of fractures (41).

Besides, the regulation of OP initiation and development involves various signaling pathways, incorporating genetic elements, and modulatory molecules, like TGF-β, RANKL, as well as BMP, which have been studied in great detail over the past few decades (42). These signal transduction pathways are implicated in regulating the differentiation and growth of OBs, OCs, or BMSCs. For example, the Wnt/β-catenin signaling pathway plays a crucial role in the differentiation of BMSCs into OBs by determining their directionality, whereas knocking down β-catenin expression significantly elevates OCs numbers and accelerates bone resorption rates, resulting in the development of OP (42, 43). In addition, TGF-β signaling not only inhibits OBs maturation, mineralization, and transition into osteocytes, but also inhibits OCs differentiation by decreasing RANKL/OPG secretion ratio (44, 45). Other signaling pathways, including Notch, and BMP2/SMADs, also participate in bone homeostasis and are highly correlated to the differentiation, growth, and maturation of OBs, OCs, and BMSCs (46–48). In general, dysregulation of signaling pathways mentioned above is associated with the risk of OP.

The existence of blood vessels in the bone microenvironment is necessary for appropriate bone development and growth, post-fracture healing, and maintaining healthy bones. Angiogenesis in bone tissues involves the proliferation, migration, and tube formation of endothelial cells, which establishes blood flow conduits that deliver essential nourishment, vital oxygen, crucial growth-promoting substances, and hormones to bone cells (31). On the contrary, disruption in angiogenesis in bone tissues can disturb the equilibrium within the bone niche, which is intimately linked to the pathological progression of OP (49, 50). Emerging clinical evidence reveals that individuals with OP have comparatively decreased blood supply to the bone than those with normal bone density, suggesting a significant correlation between bone mineral density and bone vascularization (51). Thus, a significant hallmark of OP is a reduction in the number of sinusoidal and arterial capillaries located in the bone marrow, resulting in decreased delivery of blood to the bone tissue (52). Moreover, the reduction in blood perfusion within the bone marrow is also linked to a decline in BMD and an increased incidence of fractures in elderly women (53). Similarly, age and ovariectomized (OVX) mice show decreased levels of PDGF-B in serum and bone marrow, along with a marked decrease of type H vessels in long bones (54, 55). All these findings indicate a cause-and-effect connection between the number of type H vessels and OP, given the type H vessels’ capability to promote angiogenesis and bone formation, and maintain the delicate equilibrium between bone resorption and formation (54, 56). Therefore, activating angiogenesis might be a vital therapy strategy in the prevention of OP.

Bone surrounds the bone marrow, which serves as an essential center for the immune system’s maturation and hematopoiesis. It is well-documented that there is a significant connection between the immune system and the skeletal system. T cells and B cells are the two types of lymphocytes responsible for adaptive immunity. They and their cytokines can impact the functions and activities of both OBs and OCs, with additional indirect implications on OCs due to an increase in RANKL expression via OBs proliferation (57–59). Moreover, these two systems share several regulatory molecules, including cytokines and signaling molecules. The innate immune cells are major producers of pro-inflammatory mediators, with several noteworthy cytokines such as IL-6, TNF-α, IL-1β, as well as ROS that have a crucial impact on OP (60). For instance, TNF-α can promote OCs formation by directly interacting with both OCs along with their precursors, synergistically with RANKL (61). Noteworthy, the regulation of bone homeostasis involves the contribution of multiple immune factors, such as T cells, B cells, NK cells, macrophages, and many other cytokines (37, 62). Different subsets of T cells, namely CD4⁺ T cells and CD8⁺ T cells, all exert crucial impacts in regulating bone health. Of these, Th17 cells stand out as a significant cause of bone loss since they produce significantly more RANKL and TNF-α (63, 64). Besides, the presence of B cells and their production of RANKL could be involved in the pathogenesis of ovariectomy-induced bone loss (65). Likewise, M1 macrophages exert a significant impact on promoting osteoclastogenesis by producing abundant ROS and pro-osteoclastogenic cytokines such as TNF-α and IL-1β (66). In contrast, unlike M1 macrophages which promote osteoclastogenesis, M2 macrophages contribute to bone mineralization by promoting the differentiation of MSCs and precursor OBs into mature OBs, thus supporting bone health (67).

Accumulating evidence reveals that the development of OP is also affected by various other factors, such as disease complications, estrogen levels, and gut microbiota (68). Among them, endocrine systemic diseases, hematological diseases, or other metabolic diseases, have been shown to be closely related to the occurrence and development of OP (68). It has been reported that diabetic patients may suffer reduced bone turnover and bone loss due to deleterious impacts on bone metabolism and degradation of the extracellular matrix. And the development of brittle bone in diabetes is controlled by many contributing elements, such as obesity, insulin resistance, dysglycemia, myopathy, and the use of certain medications. In parallel, certain comorbidities, including thyroid dysfunction, gonadal disorders, and maldigestion, may disrupt bone metabolism and contribute to an imbalance in affected individuals (69, 70). Given the close relationships between bone and bone marrow, hematological diseases exert a significant influence on the secondary forms of OP; there is an increased rate of bone resorption among patients with hemophilia (71). Additionally, estrogen is considered to act directly on bone cells in preventing a decline of bone mass (72, 73), and it directly or indirectly influences immune cells to impact OBs and OCs via additional intermediaries like OPG/RANKL and chemokines such as TNF-α and IL-1β (74). Moreover, growing evidence indicates there is a connection between a balanced microbiome and stable bone health, and that an imbalance in gut bacteria might worsen the function of OCs, thus resulting in OP (68).

Furthermore, clinical evidence reveals that long-term and high-dose glucocorticoid treatment is another significant contributor to OP (75). Accumulating studies have demonstrated that glucocorticoids can inhibit OBs proliferation and increase rates of apoptosis of OBs, inhibit calcium uptake in the gastrointestinal tract, act as an antagonist to vitamin D, upregulate RANK ligand, and suppress OPG (76, 77). Importantly, glucocorticoid therapy also has been shown to adversely affect muscle mass and muscle strength, increasing the risk for OP fractures (78, 79).

In general, many TCM formulas have the effect of improving OP phenotypes to some extent by promoting bone homeostasis, and angiogenesis, and regulating the immune system (Table 1). In terms of TCM formulas for the treatment of OP, LWDHP increases the BMD of femurs and improves the biomechanical capability of the vertebral body in OVX rats, mechanically, it could upregulate Runx2 and Osx expression and augment the number of calcified nodules in OBs by activating canonical Wnt/β-catenin cascade through the increase of Lrp-5 and β-catenin expressions (89). Bioinformatics analysis also provides potential therapeutic targets of LWDHP against OP such as AKT1, ATF2, and FBXW7 from a systemic perspective (90, 96). Qing’e Pill (QEP) inhibits OB ferroptosis and increases the transcription of osteogenesis-related genes, such as BMP2, Runx2 to increase the number of trabecula and trabecular connections by activating PI3K/AKT pathway and inhibiting ATM Serine/Threonine Kinase expression, thereby exerting a therapeutic role in OVX rats (97). In parallel, in vitro studies of OBs cultured with Danggui Buxue Decoction (DGBXD) led to a significant improvement in mitochondrial function, antioxidation, and anti-inflammatory, leading to increased OBs activity (91, 92).

As another part of bone homeostasis, OCs are also extensively studied in treated with TCM formulas. TCM formulas, BSHXD, and Jianpi Qingchang Bushen decoction (JQBD) can alleviate bone loss, decrease bone volume, and limit osteoclastogenesis by reducing the RANKL-stimulated NF-κB, ERK, JNK signal transduction pathways in OP model mice (93, 98). Bushen Tongluo decoction (BSTLD) and You Gui Yin (YGY) can promote bone generation and attenuate OC activity and bone resorption in the OVX-induced OP mouse model by reducing RANKL/OPG ratio (53, 99).

Meanwhile, numerous studies have demonstrated that TCM formulas can promote the BMSCs’ growth and migration to alleviate OP symptoms by enhancing the ability of BMSCs to differentiate into OBs. Duhuo Jisheng Decoction (DHJSD) was found to promote the activity of BMSCs, leading to upregulation of the expression of BMP2 and Runx2 genes, and OBs differentiation through activating SMAD1/5/8 and ERK signaling pathway (94). Taohong Siwu Decoction (THSWD) enhances the proliferation, and migration, together with bone-forming potentiality by upregulating VEGF expression and the phosphorylation of FAK and Src (100). Zuogui Pill (ZGP), another classical TCM prescription, could up-regulate let-7f and Runx2 mRNA expression and down-regulate Beclin-1, ATG12, ATG5, LC3, and CTSK mRNA expression in BMSCs to promote osteogenic differentiation of BMSCs, thus improving the OP progression (95), while another study indicated it can also slow down the senescence of BMSCs through modulating Wnt/β-catenin signaling pathway (101).

Moreover, EXD, which has been extensively used for the treatment of OP (102), can improve the bone microstructure and deterioration of bone loss in the OVX-induced OP model through lipid metabolism and the IGF1/PI3K/AKT pathway (103). BSTLD treatment results in a significant promotion in angiogenesis in OVX-induced OP rats, which could be associated with the promotion of HIF-1α/VEGF-regulated angiogenesis signal transduction and inhibition of the RANKL/OPG ratio (53). In a clinical trial, LWDHP has been found to promote the expression of immune-related cardiotrophin-like cytokine factor 1 (CLCF1) in the peripheral blood of postmenopausal osteoporosis patients with kidney Yin deficiency by activating the JAK/STAT signaling pathway, thereby improving immunocompetence, which provides a promising therapeutic approach for postmenopausal OP (104). Other classic Chinese herbal medicine like Xianling Gubao Capsule, Zhuanggu Zhitong Capsule, and Guilu Erxian Glue were also demonstrated to provide potential therapeutic benefits for the treatment of OP through in vivo and in vitro research (105–108).

Extensive in vivo studies have demonstrated the anti-OP effects of various Chinese herbs and their extracts, including Rhizoma Drynariae homogenous polysaccharide, mixed extracts of Fructus Corni and Radix Achyranthis Bidentatae (the main components of LWDHP and ZGP), Rhizoma Drynariae. These herbs have been shown to ameliorate the OP phenotype by increasing BMD and bone mineral content, which is closely related to the activity of OBs (109–111). Resveratrol, derived from Reynoutria japonica, has been found to exhibit significant antioxidant capacity in OBs by triggering the NRF2/SIRT1/FoxO1 signaling pathway and improving excessive iron-induced oxidative stress (112–114). Puerarin, another bioactive ingredient extracted from the root of Pueraria lobata (Willd.) Ohwi, has been found to significantly upregulate the expression of ALP and OPG mRNA levels while inhibiting OBs apoptosis (115, 116). Similarly, Icariin, a compound rich in TCM formulas like EXD, can also promote OBs proliferation, differentiation, and mineralization, as demonstrated by increased expression of ALP and Col I, and bone nodule formation but inhibit their apoptosis by activating ERK and JNK signaling but not p38 signaling (117). Intriguingly, in another study, MC3T3-E1 subclone 14 cell line administered with Icaritin, a hydrolytic product of Icariin, increased mRNA levels and protein expression of ALP, COL1, OC, OPN, and RUNX2 to enhance preosteoblastic cell differentiation, and upregulated the bone nodule formation and collagen synthesis to improve mineralization through the activation of ERK and p38 signalings but not the JNK signaling (118). These discrepancies in reported signaling activation can be attributed to several factors, including differences in the compounds used, varying concentrations, and inconsistent timing of signal detection between the studies (119).

OCs and a major osteoclastogenic molecule, RANKL, have also been studied in relation to the effective monomer and active ingredients from TCM formulas. For instance, it has been suggested that dendrobium, a traditional medicinal plant, exhibits potent suppression of OCs by inhibiting the increase in ROS, the expression of c-fos, and NFATC1, which are mediated through RANKL, and significantly blocking Mmp9 RNA transcription to reduce LPS-stimulated inflammatory bone loss (120). Moreover, Schisandrin A, which is rich in BSTLD, can inhibit the advantage of ROS induced by RANKL on OCs to improve bone resorption in OVX-induced OP mice via stimulating Nrf2 activity (121). Ellagic acid inhibits the expression of genes and proteins exclusive to OCs and hinders OCs differentiation, which limits bone loss, by blocking RANKL-RANK ligation, along with RANKL-conducted osteoclastogenesis (122, 123). In parallel, Aconitine, Berberine, and dihydro-artemisinin can inhibit RANKL-mediated osteoclastogenesis and bone resorption activity, leading to the promotion of osteogenic recovery (124–126).

Moreover, some TCM has been suggested to have dual effects. For example, Icariin can alleviate mitochondrial membrane potential dysfunction and oxygen-free radical production, promote Runx2, ALP, and OPN expression in OBs caused by excessive iron accumulation, and additionally inhibit the cellular differentiation and activity of OCs (127).

As for BMSCs, Artemisinin exerts antioxidant effects on BMSCs by inhibiting hydrogen peroxide-induced Caspase 3 activation and apoptosis of BMSCs (128). Likewise, Leonurine attenuates oxidative stress-induced COX2 and NOX4 mRNA expression in BMSCs and up-regulates mitochondrial membrane potential levels through the PI3K/Akt/mTOR signaling cascade to enhance the growth and differentiation capability of rat BMSCs (129, 130). Salvianolic acid B, extracted from Salvia miltiorrhiza, has been investigated in human BMSCs that can increase the expression of ALP, OPN, and Runx2 while promoting the formation of bone minerals (131). In addition, Panax notoginseng saponins dose-dependently increased ALP activity and ALP, Cbfa1, OC, and BSP mRNA levels of BMSCs, and decreased the mRNA and protein expression of PPARγ2, so as to accelerate proliferation and osteogenic differentiation of BMSCs (132). Meanwhile, it can also downregulate the number of OC precursor cells to inhibit bone resorption, together with enhancing BMSCs differentiation activity in the direction of osteogenesis (133). Oral administration of Ginkgolide has been identified as increasing the OPG-to-RANKL ratio and thus improving bone mass in three animal OP models, including the aging-, OVX-, and glucocorticoid-induced OP models (134, 135).

The destruction of bone microstructure is one of the typical characteristics of OP, and thus, the repair of bone microstructure is a standard to measure medicinal effectiveness. α-asarone, which is extracted from a TCM herb Acorus tatarinowii, is capable of inhibiting osteoclastogenesis and strengthening bone microstructure in OP models induced by estrogen deficiency (136). Cuscuta extract, which is from TCM formulas such as EXD and BSHXD, indicates an anti-OP effect by increasing bone density and bone microstructure via the c-fos/c-Src kinase/NFATC1 signaling pathway in OVX-induced OP mice (137). As mentioned above, the Wnt/β-catenin signaling pathway exerts a key role in the development of OP. Studies have shown that Ginsenoside Rc and Rhizoma drynariae total flavonoids noticeably hinder the decline of bone mass and bone volume, and promote osteogenesis and bone formation via the Wnt/β-catenin signaling pathway (138, 139). Moreover, Bionic Tiger-Bone Powder promotes osteogenesis, inhibits OCs, and increases collagen content in OVX-induced OP mice, improving bone microstructure and biomechanical strength. In line with the in vivo study, it also increases BMD in postmenopausal patients with OP in clinical (140). In different animal OP models, current evidence also suggests that TCM has preventive effects on OP by improving bone microstructure. Ganoderma lucidum contained in EXD can prevent extra bone loss and improve bone microstructure in OVX-induced OP mice by regulating bone and adipose tissue homeostasis, which can prevent bone loss caused by estrogen deficiency (141). Radix Achyranthis Bidentatae with Eucommiae Cortex herbal pair ameliorates glucocorticoid-induced OP by modulating the expressions of Runx2, OP-1, and β-catenin (142). In terms of the disuse-induced OP model, Scutellaria extract can significantly improve bone density and mechanical strength to prevent OP in the hindlimb unloading tail-suspended rat model (143). And oral administration of total flavonoid extracts from epimedium increase maximum BMD in normal-growing rats (144).

There is increasing evidence that angiogenesis is intimately associated with bone generation and remodeling to ensure adequate bone homeostasis. Thus, the promotion of angiogenesis-dependent osteogenesis and the reduction of bone loss are essential for preventing and managing OP (54). Studies have shown that Naringin derived from the traditional Chinese medicinal plant Drynaria regulated the function of endothelial cells and promoted angiogenesis, thereby exhibiting an anti-osteoporotic effect in postmenopausal osteoporotic rat models (145). As an active component found in many TCM herbs, vitexin modulates both osteogenesis and angiogenesis in an OVX-induced OP rat model through vitamin D receptor and endothelial NO synthase pathway (146). It is well-recognized that diabetes mellitus raises the risk of OP, and TCM also contributes to treating diabetic OP via angiogenesis. Curcumin prevents bone loss and promotes angiogenesis in diabetic OP mice by suppressing the hyper glucose-activated NF-κB pathway to restore osteogenic and angiogenic coupling of BMSC in hyperglycemia (147). In addition, Ginsenoside Rg1 facilitates the secretion of VEGF, triggers the Noggin/Notch signaling pathway, and enhances the interaction between H-type blood vessels and bone formation, leading to an improvement in bone structure in a mouse model of diabetic OP (148). Besides, Salvianolic acid B and Salidroside were also reported to prevent bone loss by enhancing angiogenesis in Glucocorticoid-induced and OVX-induced OP model, respectively, (149, 150).

The term “osteoimmunology” was coined in a commentary in Nature to highlight the connection point between osteology and immunology (151). In recent decades, mounting evidence suggests that TCM herbs and their extracts can regulate the immune system, which is essential for effective OP treatment. Ganoderma lucidum mycelium extract Ling-Zhi-8, an immunomodulatory protein, can effectively alleviate glucocorticoids-induced OP phenotypes in mice, as indicated by the increase in the OPG/RANKL ratio, suppression of osteoclastogenesis, and improvement in serum mineral metabolism, bone formation, and absorption markers expression, as well as expressions of hormone molecules such as estradiol and parathyroid hormone (152). Osthole, a type of coumarin compound present in various medicinal plants, including Cnidium monnieri, can enhance the immunosuppressive capacity of osteoporotic BMSCs to improve the therapeutic effect of BMSCs transplantation on colitis as well as OP (153). Likewise, Cissus quadrangularis markedly promotes the production of immune cells that counteract osteoclastogenesis, including Th1, Th2, and Tregs, in the bone marrow, while simultaneously decreasing the number of OC-promoting Th17 cells. This effectively inhibits RANKL’s ability to generate OCs and hinders the capacity of OCs in bone resorption (154).

Given the importance of proinflammatory cytokines, including TNF-α, IL-1β, as well as IL-17 in osteoimmunology, there is accruing evidence from recent studies suggesting that TCM herbs and their active ingredients such as Sinomenine (155), and Galanin (156) have therapeutical effects on OP by regulating the inflammatory reaction and inflammation-related signaling pathways. We have summarized the chemically active constituents in TCM herbs for treating OP in Table 2.