The IVD is the largest avascular organ in the human body. Nutrients, oxygen and metabolites are diffused through the cartilaginous endplates and the outer annulus with the endplate pathway as the major route [
20]. With the aging and/or other factors, insufficient nutrient supply may play an important role in the degeneration of the IVD. When endplate calcification occurs, diffusion between the NP and blood vessels is compromised. With the fall of oxygen levels from decreased diffusion, anaerobic respiration takes place and the pH value within the disc drops as the lactic acid concentration increases, especially in the mid-layer of the disc [
7,
20] . The dense matrix affects the outgoing lactic acid which leads to an increase in the acidity of the matrix and surrounding cells. Acid hydrolysis of the annulus creates fissures, leaking lactic acid into surrounding tissue. Persistent acid burn leads to chronic inflammation and discogenic pain [
5]. Nachemson [
21] reported a significant correlation between preoperative low back pain and low pH measured by a pH-electrode in lumbar rhizopathy surgery. In addition, NGF and its receptor tropomyosin receptor kinase A receptor (TrkA) also has been found to play a critical role in chronic pain associated with IDD [
12,
13]. In previous studies, Urban et al. showed that a low pH value of IVD caused a series of damage to the IVD [
7]. Increasingly in vitro studies have confirmed that acidic pH conditions mimicking a degenerative IVD may induce a catabolic mechano-response in human NP cells [
22]; and may also impair the survival and biological behavior of mesenchymal stem cells (MSC) which may affect the efficacy of MSC-based IVD regeneration [
23]. Many factors can cause a low pH environment in an IVD [
8,
24]. Lactic acid is the most important factor affecting pH value. The change of lactic acid content in an IVD is closely related to the physiological function of an IVD [
7,
8,
25]. Wu et al. [
26] concluded that high lactate concentration was a pathogenic factor for disc degeneration in rat nucleus pulposus cells, and lactate metabolism may be a new therapeutic target for disc degeneration. However, no study has reported a lactic acid level in a degenerative disc in vivo, and how lactic acid accumulation impacts disc composition.
Pervious study [
5] measured the pH value of IVD by a calibrated micro pH electrode. pH is strongly dependent on lactic acid, however, no study measured the lactic acid in the NP of IVD. Bartels et al. [
27] measured the lactate concentrations in anulus. In this study, we used ELISA to measure the concentration of lactic acid of NP. We believe this is the most accuracy way for evaluate the change of lactic acid. The most important extracellular matrix of NP cells are proteoglycan and type II collagen, which is main components of the ECM which maintains the hydrophilicity and physiological function of the IVD. The loss of proteoglycan results in a decrease in the gelatinous substance of the nucleus pulposus, resulting in gradual fibrosis and decreased elasticity [
28]. In our study, we directly observed anatomical structure changes of IVD. And we used MRI to evaluate changes in the NP signal, which is closely related to the proteoglycan and type II collagen. Therefore, because the results of imaging, histology and molecular biology were compatible. Our study is more accurate and reliable in evaluating the changes of ECM in degenerated IVD as well as corresponding lactic acid concentration. Overexpression of ECM remodelers, such as MMPs are associated with IDD [
29]. The most frequently studied MMPs in disc aging and degeneration are MMP1, MMP3 and MMP13, which degrade different types of collagens [
30‐
33]. Along with MMP3, MMP13 is one of the main proteases described in IDD [
34,
35]. In our study, we found both MMP3 and MMP13 gradually increased in AL group compared with sham group, and MMP13 dramatically increased after 12 weeks, which suggests MMP13 may play an important role in matrix components changes of the NP tissue in AL group. Surgical injury to the IVD is a widely used method of inducing disc degeneration. In this study, we used a superficial fibrous ring injury model to induce IDD that is currently recognized as an appropriate method to simulate the natural degeneration of an IVD [
36].