Morphology of unaffected and affected sacral vertebrae in the proteoglycan-induced spondylitis (PGISp) mouse. An unaffected sacral vertebral joint from a week-12 mouse shows an intact intervertebral disc (IVD) without inflammation; H&E staining ×40 (A) and ×200 (B). Early inflammation is evident in the IVD of PGISp mice 12 weeks after the first proteoglycan (PG) injection (boxed area); ×40 (C) and ×200 (D). Severely-affected sacral joint in week-12 PGISp show massive mesenchymal cell proliferation, excess matrix formation (black arrowheads) and almost complete IVD destruction (blue arrowheads); ×40 (E), ×100 (F). Inflammation and joint damage is also found in the spine and variable penetrance, a characteristic of AS, is also evident. Severely damaged (black arrowheads) and early-stage inflamed (red arrowhead) joints are found in close proximity to unaffected joints; ×40 (G). Histological scoring of the vertebrae indicates the degree of disease severity in the PGISp mice (H).