Table 2.

Summary of studies investigating associations of hypertrophy (COL10) and osteogenic differentiation (OPN) with ectopic calcifications and endochondral ossification in IDD.

Reference	Species	Summary
(Yeh et al., 2016)	Human AF cells (surgery)	Endochondral ossification in IDD was induced by resident progenitor cells in the annulus fibrosus. Osteogenic differentiation was induced in normal and degenerated AF cells with osteogenic medium. Mineralisation, increased gene expression of Runx2, alkaline phosphatase and osteocalcin was more prominent in degenerated AF cells. Overexpression of microRNA-221 decreased the osteogenic potential of degenerated AF cells, indicating that endochondral ossification can be regulated by microRNA-221.
(Marfia et al., 2015)	Human IVD-MSC (surgery)	OPN gene expression was significantly increased in MSCs from degenerated IVDs compared to MSCs from herniated IVDs. OPN protein was only detected in MSCs from degenerated IVDs, suggesting that OPN plays an important role in IDD.
(Hristova et al., 2011)	Human IVD (scoliosis surgery) (autopsy)	COL10 positive cells, ectopic calcifications and endochondral ossifications were increased in degenerated and scoliotic IVDs, suggesting that IDD is associated with ongoing mineral deposition and might be related to degenerative processes.
(Bian et al., 2011)	Rat IVD (in vivo)	Prolonged upright posture induced cartilage end-plate calcification and hypertrophy in rat lumbar spine as detected by μCT, histology, immunohistochemistry and gene expression for COL10, VEGF and TGFβ3.
(Rutges et al., 2010)	Human IVD (autopsy)	Degenerated NP tissue was more positive for COL10, OPG and Runx2. Osteoprotegerin levels correlated significantly with degeneration grade and number of microscopic calcifications measured on μCT and von Kossa staining. Results indicated that hypertrophic differentiation occurs during IDD.
(Melrose et al., 2009)	Ovine IVD	Ectopic calcification in aged ovine IVDs likely precedes or predisposes IVD degeneration and occurred independently from COL10 and OPN expression.
(Roberts et al., 1998)	Human IVD (fusion surgery)	COL10 positive enlarged hypertrophic chondrocyte-like cells were most commonly found in NP tissue of degenerated IVDs, in areas of disorganised architecture, such as clefts, and in areas of CEP that calcified ectopically, suggestive of IDD.
(Boos et al., 1997)	Human IVD (autopsy)	COL10 was localised in degenerated IVDs in the pericellular region of the ECM and increased with aging and degeneration.
(Moskowitz et al., 1990)	Rat IVD (motion segment)	Aging diabetic rats exhibited IDD including IVD ossification, ligamentous calcifications, subchondral endplate sclerosis as well as IVD-space narrowing, chondrocyte replication and IVD necrosis.