The NP is the colloidal part that is located at the center of the IVD. The NP contains more than 70% water, which reduces the effects of external pressure and prevents internal stress concentration (Raj, 2008). The remaining components of the NP are mainly proteoglycan and collagen. Aggrecan is the most abundant type of proteoglycan in NP, though other forms such as biglycan and versican are also present (Melrose et al., 2001). Proteoglycans are essential for maintaining the high-water content of NP. There is type Ⅰ and type II collagen. The NP has a higher content of type II collagen than other IVD tissues (Raj, 2008). The NP cells are present in a colloid-like matrix that is composed of proteoglycan and collagen. In general, the cell density in the NP tissue is relatively low, only 2–5*10⁶ cells/ml, and it further decreases with age (Maroudas et al., 1975; Stairmand et al., 1991). In the early stage of IVD development, the NP has vascular distribution and notochord-derived cells (Roberts et al., 2006; de Bree et al., 2018), both of which are not found in adult NPs (Colombini et al., 2008). The presence of notochord-derived cells has been linked to self-repair abilities, while the lack of vascular distribution means that NP cells are in a relatively anoxic environment, so nutrition or drugs are difficult to transport through the blood (Risbud and Shapiro, 2011; Boubriak et al., 2013). All these suggest that NP has a fragile ability to repair itself.

As mentioned above, proteoglycans and high osmotic pressure are essential for resisting axial compressive forces and spinal pressure during daily motion and activities. The range of mechanical loads required by the NP is enormous. When lying prone, the pressure load on the NP is only 0.1 MPa, but when bending down to lift heavy objects, the pressure load will rise to 2.3 MPa (Schultz et al., 1982; Adams et al., 1996). Abnormal mechanical loads have been proved to be an important factor that contributes to IDD (Vergroesen et al., 2015). The main pathological changes in the NP are proteoglycan loss, type II collagen and osmolality reduction. In spine magnetic resonance imaging (MRI), these changes manifest as decreased disc and appear darkened to black on the T2 signal (Antoniou et al., 1996; Brinjikji et al., 2015).

The AF is the fibrocartilage that surrounds the NP, and can be further divided into outer and inner AF (Hsieh and Twomey, 2010). The main function of the AF is to stabilize and encapsulate the NP. In the presence of an intact AF structure, the NP tissue can maintain a high hydrostatic pressure, and the entire disc can resist the high intensity pressure that emanates from daily motion such as flexion-extension and lateral bending. The AF is a lamellar structure that is predominantly composed of type Ⅰ collagen in a highly oriented manner, with about 15–25 layers (Marchand and Ahmed, 1990). As the AF approaches the NP, the proportion of type II collagen and water content gradually increases (Feng et al., 2006). The cell density in the AF is higher than that in the NP, with a range of about 5–10*10⁶ cells/ml (Bowles and Setton, 2017). These cells are arranged in concentric lamellae that interleave each other and are offset by 30–60° from the vertical spinal axis (Walker and Anderson, 2004; Raj, 2008). During the early stages of IVD development, the boundary between the NP and AF is not obvious but as the development approaches maturity, clear morphological boundary begins to appear (Roberts et al., 2006).

When the AF structure is damaged or fractured, the uneven distribution of stress may lead to the herniation or displacement of the IVD tissue, which may further compress the surrounding blood vessels and nerves, thereby causing obvious clinical symptoms. Despite the possibility of adequate decompression by surgical treatment, the AF structure is not repaired, and the abnormal biomechanical structure is not effectively corrected. Although AF and NP are significantly different in composition and structure, they are closely related in maintaining normal physiological functions. Moreover, any tiny damage on any single part will lead to the disorder of the overall function.

The CEP is composed of the hyaline cartilage and lies between the soft tissue of the IVD and the bony structure of the vertebral body (Luo et al., 2021). The hyaline cartilage is mainly composed of water, proteoglycans, and collagen. The CEPs are critical for maintaining the mechanical integrity, as well as the vascular and avascular separation of the IVDs. Due to the avascular structure of the IVD, the essential substances such as glucose and oxygen are mainly supplied by the capillaries that are around the IVD through CEP, to maintain cell activity (Urban et al., 2004; Malandrino et al., 2014; Bowles and Setton, 2017). The CEP in adults is thinner and less vascularized than the endplates in the early development stage (Harmon et al., 2020). Comparable to osteoarthritis, cartilage damage is also one of the pathological features of IDD (Francisco et al., 2022). Thinning and mineralized CEP not only makes it difficult to maintain spinal biomechanical stability, but also prevent nutrients from entering the NP and AF through the CEP (Huang et al., 2014). All these factors will further aggravate the rate of intervertebral disc degeneration. In addition to these, the increase in nerve fibers in the CEP of the degenerative segment is also thought to be related to the pain that is caused by IDD. In recent years, promoting CEP repair in a bid to enhance the nutrient supply to the IVD has become an alternative way to interfere with the progression of IDD. Having said this, stem cells have also achieved promising application prospects in promoting cartilage regeneration (Yang et al., 2018; Kangari et al., 2020).

Most tissues and cells in the human body are undergoing constant renewal, and sufficient cells plays a crucial role in maintaining normal tissue function and metabolism. This rule also holds true for the IVD tissue. Again, it is worth noting that the low cell density, lack of a powerful source of regenerative cells, and the harsh microenvironment after IDD make the endogenous self-cell recruitment of IVD difficult. In degenerated IVD, cell loss was reflected by higher rates of senescence, apoptosis, and pyroptosis. Direct replacement of the missing cells seems to be the most straightforward and effective method, though there are still many limitations, such as the survival rate of the implanted cells and unnecessary cell dissipation. Stem cells combined with biomaterials offer new hope for stem cell-based endogenous repair.

Many experiments have shown that various stem cells can differentiate into functional cells of IVD if they are exposed to appropriate stimulation. Although conditions such as hypoxia are often considered to be unfavorable for cell survival, appropriate hypoxia is indeed a stimulating condition for further cell differentiation. Han et al. (Han et al., 2015), co-cultured human ligamentum derived stem cells and NP cells in vitro under hypoxia conditions and successfully differentiated ligamentum derived stem cells into NP-like cells. Strassburg et al. (Strassburg et al., 2010), reported varying stimulating effects of different NP cells. BMSCs were co-cultured with NP cells from degenerated or non -degenerated discs for 7 days. Co-culturing with NP cells derived from degenerated discs enhanced the expression of the transforming growth factor (TGF) -β and cartilage-derived morphogenetic protein -1, which can regulate NP cell metabolism and promote the production of ECM (Thompson et al., 1991). In an in vivo study using rabbit model, BMSCs were transplanted into degenerated IVD and resulted in significant improvement in cell numbers, cell survival rate, and disc water content after 24 weeks (Sakai et al., 2006). Similar findings have been reported in other types of stem cells. Zhao et al. (Zhao et al., 2020), isolated NP cells from patients with severe IDD and induced apoptosis by in vitro compression. After co-culturing with UCMSCs, the apoptosis was significantly reversed. Lu et al. (Lu et al., 2008), co-cultured ADMSCs with NP cells in a hydrogel containing collagen Ⅱ. The results showed that the number of cells that were producing type II collagen was significantly increased and the aggrecan-related genes were upregulated. In another in vivo study, when ESCs were induced for differentiation by TGF-β and ascorbic acid, they were implanted into the IDD model by needle puncture (Sheikh et al., 2009). The results revealed the presence of notochord-derived cells and higher proteoglycan content was observed. In recent years, notochord-derived cells have been identified as key points in IVD regeneration (Harfe, 2022). He et al. (He et al., 2018), also reported that co-culturing cartilage endplate stem cells (CESCs) with NP cells promoted the proliferation of NP cells.

Although the growth and differentiation of stem cells are necessary for the stem cell-based therapy, the differentiation of abnormal location and direction caused by stem cell leakage often reduces the therapeutic effect, sometimes leading to serious consequences such as tumorigenesis. Therefore, the introduction of biomaterials cannot only prevent stem cell leakage but also provide suitable spatial structure for stem cells. In addition, the combination of drugs can provide further support for stem cells. Bertolo et al. (Bertolo et al., 2015), developed an injectable microcarrier with a size between 100 and 1,500 µm based on collagen, which achieved significant cell proliferation under 5% hypoxia. Andrea et al. (Friedmann et al., 2021), implanted collagen-based scaffolds loaded with ADMSCs into a sheep IDD model and made observations for up to 1 year. The researchers noted a stabilization of disc degeneration but did not observe recovery of disc height (Figure 2). Similarly, Zhang et al. (Zhang et al., 2014), used chitosan and alginate to fabricate an injectable 3D scaffold and achieved good growth and differentiation of ADMSCs under 2% hypoxia. Daisuke et al. (Ukeba et al., 2020), combined ultra-pure alginate-based gel with BMSCs. Compared with the gel-free control group, stem cells that were cultured in vitro showed higher expression of growth factors and ECM-related genes, and the stem cells combined with the gel group effectively induced IVD regeneration in vivo. Upon further comparison of four kinds of commercial scaffolds approved for medical use, Alessandro et al. (Bertolo et al., 2012), indicated that collagen and gelation-based scaffolds had better cell survival rate and aggrecan expression. Peroglio et al. (Peroglio et al., 2013), developed a hyaluronic acid-based thermoreversible hydrogel, which showed better differentiation induction than the alginate in vitro culture. The hydrogel maintained 90% of MSCs viability in nucleotomized IVD for at least 1 week. To compensate for the lack of cell binding sites in alginate-based materials and the unsatisfactory mechanical properties of collagen-based materials, Guillaume et al. (Guillaume et al., 2015), developed an alginate-collagen composite scaffold with the property of shape memory. In addition, the composite scaffold showed better cell fixation ability than alginate only scaffold and maintained the viability of BMSCs for 5 weeks.

The high pressure in human IVD may cause the displacement of liquid cell carrier, which has higher requirements on the mechanical properties of the material. Zeng et al. (Zeng et al., 2015), loaded the alginate precursor encapsulated with ADMSCs into Poly (ethylene glycol) diacrylate (PEGDA). This thermos-responsive hydrogel improved cell survival rate while preventing cell leakage. Moreover, a better reduction in degeneration was observed than in other treatment groups in an IDD dog model after 6 months. In 2017, Diba et al. (Diba et al., 2017), assembled silica and gelatin nanoparticles to form a colloidal gel with excellent mechanical properties and impressive self-healing ability upon shear failure. In their subsequent study, they used this colloidal gel as the carrier to deliver BMSCs and injected it into rabbit IDD models. The gel showed excellent biocompatibility and degradability in vivo. Besides, the colloidal gel effectively prevented cell leakage, in addition to providing a favorable environment for the growth and differentiation of the BMSCs. Apart from the characteristics of the material itself, external physical stimulation can also affect stem cells. Elsaadany et al. (Elsaadany et al., 2017), evaluated the effects of different equiaxial mechanical strains and frequencies on the survival of ADMSCs in the scaffold and indicated that under suitable loading, ECM protein secretion and AD marker gene expression were significantly increased. Similarly, Li et al. (Li et al., 2021), used poly-caprolactone and nano-hydroxyapatite to fabricate scaffolds to load BMSCs, prior to treating these scaffolds with a sinusoidal electromagnetic field. The results indicated that stimulated BMSCs exhibited excellent osteogenic potential (Figure 3). Artificial polymer materials are easy to work with so they can be used to determine the relationship between material properties and stem cells. Tasi et al. (Tsai et al., 2014), used poly-l-lactic acid and poly-caprolactone to construct a fibrous scaffold mimicking the AF structure. Zhu et al. (Zhu et al., 2016a), used a biodegradable poly (ether carbonate urethane) urea material to achieve adjustable scaffold elasticity. AFSCs exhibited significant differences in protein expression on different elastic scaffolds. Chu et al. (Chu et al., 2019), further used this scaffold to demonstrate that the mechanical properties, topography, and geometric characteristics of the material affect the growth and differentiation of AFSCs.

Adding cytokines or drugs to the material offers more possibilities to control stem cell growth and differentiation in the material. TGF is an important signal factor for IVD development and repair, considering that it can stimulate ECM anabolism, induce stem cell differentiation, and inhibit inflammatory response and ECM catabolism (Watabe and Miyazono, 2009; Yang et al., 2015; Chen et al., 2019). Liang et al. (Liang et al., 2013), reported a cell carrier by adding nanoparticles containing dexamethasone and TGF-β3 to poly (lactide-co-glycolide) microspheres loaded with ADMSCs. At week 24 post-transplantation, the mice models showed significant disc height recovery and proteoglycan accumulation. Similar results were reported by Kim et al., (Kim M. J. et al., 2020), who engineered porous particles with leaf-stack structures to simultaneously load BMSCs and TGF-β3; such particles released TGF-β3 continuously for up to 18 days. As a result, significant disc regeneration was observed in dog IDD models. To further overcome the defects of rapid clearance and short half-life of TGF in vivo, Shen et al. (Shen et al., 2020), used graphene oxide nanosheets to achieve slow release of TGF. Whether cultured in vitro or implanted subcutaneously, the addition of this material caused BMSCs in the hydrogel to express more cartilage matrix. In addition, the initial compression strength of the hydrogel was also enhanced by graphene oxide nanosheets. Furthermore, the repair stimulatory effect of TGF on the NP and AF cells has also been demonstrated (Guillaume et al., 2014; Ligorio et al., 2021). Platelet-rich plasma (PRP) obtained by autologous whole blood centrifugation can provide a greater variety of bioactive factors than a single TGF component, and its application in regeneration medicine has received extensive attention in recent years (Everts et al., 2020; Everts et al., 2021). Chen et al. (Chen et al., 2009), used the culture system containing PRP to culture MSCs and observed the upregulation of genes that are related to type II collagen, aggrecan, and an increased cartilage matrix deposition in vivo. Wang et al. (Wang et al., 2018), further compared the effects of different doses of PRP and the presence or absence of leukocytes in PRP on NP stem cells, and at a 10% dose, PRP without leukocytes exerted the best differentiation effects on NP stem cells. Russo et al. (Russo et al., 2021), further developed a hydrogel composed of PRP and hyaluronic acid as the carrier to deliver the BMSCs, and this could integrate well with the surrounding tissue while maintaining cell viability (Figure 4).

New tissue engineering technologies allow people to design and manipulate material properties more accurately. In recent years, an emerging technology called 3D bioprinting has attracted much attention in tissue engineering, a scenario that improves the control of the spatial distribution of materials to a new level (Shapira and Dvir, 2021; Zhu et al., 2016b; D'Este et al., 2018). Serra et al. (Serra et al., 2016), used 3D bioprinting to design an anatomical lumbar spine skeleton with similar mechanical properties to trabecular bone and demonstrated ideal biocompatibility in preliminary cell culture. Tarafder et al. (Tarafder et al., 2016), achieved precise spatial control of growth factor release by exploiting the differences in the melting points of various materials, in addition to interchanging dispensing cartridges during a single printing process. Electrospinning is a technology that can be used to produce ultrathin fibers. It makes it possible to reduce the diameter of ultrathin fibers to nanometer levels (Hong et al., 2019; Xue et al., 2019). Liu et al. (Liu et al., 2015a), fabricated an aligned fiber-polyurethane scaffold carrying AF stem cells using the electrospinning technology, and the AFSCs on this kind of scaffold were more aligned and produced more collagen Ⅰ and aggrecan (Figure 5). Wang et al. (Wang et al., 2020), compared the effects of static culture, intermittent centrifugation culture, and dynamic bioreactor on the infiltration ability of BMSCs in low-porosity electrospinning scaffolds. Zhu et al. (Zhu et al., 2021), combined 3D printing and electrospinning technology to construct a composite scaffold, which simulate the structure and properties of the NP and AF at the same time. The BMSCs that are distributed in the scaffold maintain ideal cell viability. In an in vivo experiment, new ECM production was observed and IVD height was maintained.

Considering the unavoidable ethical and cell survival problems of stem cell therapy, extracellular vesicles based on stem cells have attracted attention as a new treatment method for IDD in recent years. Extracellular vesicles can be roughly divided into exosomes, microvesicles, and apoptotic bodies, according to their size. Exosomes are the smallest category, with a size of about 30–150 nm (Vader et al., 2016; Li et al., 2019). Apoptotic bodies are the largest, with a size of between 50 nm and 5 μm. Microvescicles are in the middle with a size of about 50–100 nm. Of all the extracellular vesicles, exosomes have been the most extensively studied. Almost all cells will produce exosomes, the outer layer is the lipid double layer, which contains a variety of substances like cytokines, proteins, lipids, mRNA, among others (DiStefano et al., 2022a; Bhujel et al., 2022). Due to their cell origin, exosomes have extremely low immunogenicity. As a carrier of intercellular information, exosomes can act on a variety of signal transduction pathways, such as fusion, endocytosis, and soluble signaling pathways (Zhang et al., 2019). A remarkable number of studies confirmed that stem cell-derived exosomes interfere with the pathological development of IDD through various mechanisms. Liao et al. (Liao et al., 2019), demonstrated that exosome-derived BMSCs could ameliorate excessive apoptosis in IDD, through endoplasmic reticulum stress. Similarly, Xiang et al. (Xiang et al., 2020), indicated that urine-derived stem cells could also regulate endoplasmic reticulum stress. Zhu et al. (Guo et al., 2021), further demonstrated that exosomes from urine-derived stem cells have the effect of promoting cell proliferation and ECM synthesis by regulating TGF-β. In an in vivo experiment using a rabbit IDD model, exosomes derived from MSCs also exhibited anti-oxidative and anti-inflammatory effects and prevented further degeneration. The combination of exosomes and biomaterials also has a broad application prospect. Luo et al. (Luo et al., 2022), designed a cartilage ECM hydrogel that was loaded with CEP stem cells and injected the hydrogel near the CEPs in mouse models. They observed that the exosomes produced by the stem cells penetrated the AFs to reach the NP cells and attenuated the development of IDD. Xing et al. (Xing et al., 2021), designed a thermosensitive hydrogel loaded with ADSC exosomes. The hydrogel provides a suitable environment for the growth of NP cells, and the exosomes regulate the anabolism and catabolism of the ECM by regulating metalloproteinases (Figure 6). DiStefano et al. (DiStefano et al., 2022b), loaded exosomes produced by MSCs under different oxygen concentrations into poly (lactic-co-glycolic acid) microspheres and showed that AF cells were more sensitive to the production of exosomes under hypoxic conditions.

Generally, the exosome strategy avoids many problems that are difficult for stem cells to circumvent while retaining most of the advantages of the stem cell strategy. Although exosomes brought a new direction to regenerative medicine, their application in IDD is still in its infancy. Its formal application in clinical practice still requires extensive evaluation. The metabolism, distribution, appropriate dosage, and administration frequency of exosomes in the human body still need a lot of research. In addition, the differences in the expression of genetic information between different individuals may lead to significant variations in the therapeutic effects of exosomes. Therefore, a comprehensive and multi-level evaluation system is necessary. From the point of view of drug production, their large-scale preparation is still difficult, not to talk of a lack of unified standards. The storage and transportation of exosome also need to be considered.