The present study demonstrates for the first time that human intervertebral disc cells stain for the SA-β-Gal biomarker. This suggests that, like all other cell types investigated, they are subject to senescence. Replicative senescence is believed to be a protective mechanism for the individual against tumourigenesis [14
]. However, because senescent cells have an altered, and often more catabolic metabolism, the trade off for this anti-tumour protection appears to be “ageing”. This has been suggested as one of the reasons why “Dolly”, the cloned sheep (using cells which have undergone many replications), and her siblings had premature arthritis and early ageing [18
]. Replicative senescence can be clearly demonstrated in cells in vitro, with cell types having a typical number of cell divisions that they will undergo (alternatively know as the “Hayflick number”) [8
] before ceasing replication and entering growth arrest in the transition phase G1–S of the cell cycle [14
]. Human fibroblasts will undergo approximately 60 and chondrocytes 35 population doublings, respectively.
Martin and Buckwalter [12
] have reported cell senescence in chondrocytes from articular cartilage; they found that the incidence of SA-β-Gal positivity in these cells in culture increased from <0.1% of cells from donors younger than 10 years old to >0.4% from those over 70 years of age. They conclude that chondrocyte senescence contributes to an increased susceptibility of osteoarthritis with age. There is a similar incidence in tissue sections of cadaveric discs in the present study with the younger individuals having the lowest incidence (0 and 0.5% senescent cells in discs of 21 and 28 year olds, respectively, and 2% in a 92 year old). The age dependence was not significant but the number of normal samples examined in this study was very limited. However, herniated discs have almost 10× as many senescent cells as discs obtained at autopsy or anterior surgery.
There was a much greater incidence of SA-β-Gal positive cells in cell clusters than in the population of single or doublet cells. Since we have demonstrated previously that these cell clusters form by cell replication [9
], this would seem to be a plausible explanation for this higher incidence on first inspection. However, when the incidence of clusters is examined in herniated discs in comparison, it becomes clear that other factors must also be involved. Other types of cells have been shown in some circumstances to become senescent prematurely. For example, high levels of oxygen or loading can lead to stress induced premature senescence (SIPS) of chondrocytes [11
] and fibroblasts [16
]. The adult intervertebral disc, being virtually avascular, has lower oxygen content than most places elsewhere in the body. It would appear, however, that disc cells are well adapted to existing in such an environment, being capable of surviving at 0% oxygen [2
]. When discs herniate, particularly if they become extruded or sequestrated, the cells are likely to experience greatly increased access to oxygen, either via vascularisation within adjacent tissues or of the herniation itself (a common occurrence) [13
]. In addition, the prolapsed disc is likely to experience a different loading pattern from normal and this in turn has been shown to lead to an increase in reactive oxygen species and hence to increased cell senescence in articular cartilage [11
Within the anterior fusion samples one stood out as being very different from the rest, with 88% of cells within clusters being SA-β-Gal positive. On close examination of the clinical and radiographic history of this patient it became obvious that, although surgery had been carried out for discogenic back pain, the magnetic resonance image interestingly demonstrated a small protrusion of this disc. This was not the case for any other such samples. Although it is dangerous to read too much into one sample, this does support the general results seen with other herniated discs in this study.
The relevance of the findings of this study is twofold. Firstly this demonstration of cellular senescence within disc cells suggest that they could, via their decreased anabolism and increased catabolism, be important in the pathogenesis of disc degeneration. Secondly, it could be very important to the current interest in biological therapies, both tissue engineering and genetic engineering. These techniques depend for their success on metabolically active cells. Hence if autologous intervertebral disc cells are used to implement such treatments, they may not function optimally, particularly if taken from herniated discs [7
]. This reiterates the suggestion by Alini et al. [1
] that these biological approaches are not currently appropriate to treat disc degeneration, but they may be in the future when we have a better understanding of disc cell biology.