Table 1 Histological manifestations of osteoarthritis by age (in months) in Dunkin-Hartley guinea pigs

1

Articular surface was smooth and intact, and a small amount of colorless and transparent synovial fluid was seen in the articular cavity. No osteophyte was seen.

No pathologic change. Evenly distributed staining, morphology, and staining of cartilage matrix and chondrocytes.

No MGP expression or only a few light-stained cells.

3

With increasing age, the synovial fluid became turbid and yellow, the articular surface degenerated progressively like getting rougher, duller and thinner, and small areas of cartilage were slightly depressed; No obvious osteophyte was seen.

Tiny cracks were seen in cartilage surface, cell aggregation and cloning were occasionally-seen.

MGP was expressed in a small area of chondrocytes in the superficial layer of the joint with light staining, and with few positive-stained cells.

6

Damaged cartilage surface and cartilage layer structure with significant decreases in cartilage matrix staining and number of chondrocytes as well as more cell clusters and clones.

MGP positive chondrocytes were found in the deep layer of the cartilage.

9

Rough articular surface, disordered structure and irregular arrangement of chondrocytes, necrosis of chondrocytes, and empty cartilage sac.

The whole layer of chondrocytes was significantly positive, and the staining was significantly deeper than in the 6-month-old group.

12

Synovial fluid was obviously reduced, yellow and thick. Rough and dull articular cartilage surface, thin cartilage matrix, many cartilage depressions and losses, and sclerosing osteophytes were obviously seen.

Severely damaged cartilage structure and reduced number of chondrocytes with empty cartilage vesicles; staining of matrix was lost.

The positive cartilage staining was further deepened compared to the 9-month-old group.