## 1 Introduction  
Low back pain (LBP) is a leading cause of disability, affecting more than 600 million people worldwide [1]. Although it is well established that the causes of LBP are multifactorial and can include mechanical injury [2], genetic predisposition [3] or even lifestyle activities [4], a significant proportion of the cases are associated with intervertebral disc (IVD) degeneration [5, 6]. The IVD is an avascular organ that interfaces the superior and inferior vertebral bodies via the cartilaginous endplates (CEPs), regulating the nutrient and waste metabolites path into and out of the IVD, respectively [7]. The microenvironment of a degenerative IVD is considered a hostile niche characterised by large concentration gradients of nutrients and metabolites across its domain, with significantly lower glucose and oxygen concentrations in the centre of the disc, the nucleus pulposus (NP), compared to the periphery, the annulus fibrosus (AF) [8]. Due to the low levels of oxygen in the centre of the IVD, cells residing in the NP rely mainly on anaerobic respiration to create energy, producing lactate as a by-product of glycolysis, and thereby causing acidification of the local microenvironment. With the onset of disc degeneration CEPs undergo physical changes such as thinning and calcification [9; 10], thereby reducing the bidirectional flow of nutrients and metabolites to and from the NP region causing local lactate accumulation and a consequent decrease in pH [11]. It has been shown that the pH of the disc changes with the degree of degeneration, ranging from pH 7.1 to pH 6.5 in healthy to severely degenerated conditions respectively [12; 13] and that the decrease in the pH in the organ is directly correlated with matrix catabolism [14; 15] and reduced cell viability [16]. The exact mechanisms that trigger degeneration of the IVD are unknown, although it is believed to be mediated by the abnormal production of pro-inflammatory cytokines by NP cells [17; 18]. These cytokines are believed to trigger a series of cellular responses that promote cell senescence, autophagy and apoptosis [19; 20; 21]. While a number of pro-inflammatory cytokines have been identified in degenerated discs, tumour necrosis factor-\(\alpha\) (TNF-\(\alpha\)) and interleukin-1\(\beta\) (IL-1\(\beta\)) are believed to play central roles [21; 22; 23]. Both IL-1\(\beta\) and TNF-\(\alpha\) have been identified to being involved in disc herniation [24], nerve ingrowth [25] and in the upregulation of genes encoding matrix-degrading enzymes [26; 27; 28]. Although the correlation of IL-1\(\beta\) and TNF-\(\alpha\) with disc degeneration is well established in vivo, the response to their in vitro supplementation in 2D and 3D cultures has been diverse and often conflicting [19; 23]. One explanation for the differential response observed from in vitro experimentation could be the concentration of supplemented cytokines being supraphysiological. In a recent study, [PERSON] et al. measured the concentration of IL-1\(\beta\) and TNF-\(\alpha\) in degenerated human discs and found their levels to be notably increased in patients affected by acute IVD degeneration, with concentrations in the order of pg/mL [29]. Nevertheless, the concentration of inflammatory cytokines used in the majority of experiments investigating their effects on cells in vitro is usually found to be in the order of ng/mL, and significantly higher compared to in vivo [30; 31; 32]. The role of inflammatory cytokines and environmental pH in IVD degeneration can be linked to many detrimental effects, however it is difficult to determine which of the two has a primary role in disc disease. To fully appreciate the effectiveness of any proposed cell therapy for disc regeneration for clinical use, the regenerative capacity of the cellular component must be examined under physiologically relevant culture conditions.