Fig. 2

The first and second line images presented the histological characteristics of the medial tibial plateau by H&E staining and toluidine blue staining, respectively. The third and fourth line images presented the MMP-13 and TIMP-1 expression in the PCL rupture side and the control side, respectively. a The control side: the surface was continuous and smooth, chondrocytes in normal shape and regularly ranged, basicytes capsule surrounded the cells, clear and dense tissue layers, long axis of the cell parallel to the joint surface, shape were similar to fiber cells. The cells in the middle and deep layers were round, continuous tidemark, no vessel hyperplasia and inflammatory cell formation; even dark blue in toluidine blue staining; barely MMP-13 and TIMP-1 positive-staining cells were detected in chondrocytes. b Fourth week after PCL rupture: smooth and approximately flat surface, superficial layer cells are in spindle shape, long axis of the cell parallel to the joint surface, tissue layers and transitional layer cells were slightly disorderly arranged, no clustering of hypertrophy cells, continuous tidemark; even toluidine blue staining; limited number of MMP-13 and TIMP-1 positive-staining cells were detected. c Eighth week after PCL rupture: slight rough surface, collagen fibers were unevenly thick and arranged disorderly, chondrocytes were round in shape, occasionally fissure and cartilage defects, disorderly arranged cells and tissue layers, common clusters, cell hypertrophy and hyperplasia, vague tidemarks but still continuous; uneven toluidine blue staining, superficial layer light staining; more MMP-13 and TIMP-1 positive-staining cells were observed in the superficial layer and partial middle layer. d Sixteenth week after PCL rupture: extremely rough surface, common depression and deep fissures into the middle layer, superficial and partial radial layer fibrosis, loose tissue and disorderly arrangement of tissue layers, decreased cell numbers and uneven distribution, gathered in clusters, discontinuous tidemarks, occasionally slightly pannus hyperplasia; extremely uneven toluidine blue staining, partial loss of dye in deep cartilage layer; MMP-13 and TIMP-1 positive-staining cells were scattered in cytoplasm of full layer. e Twenty-fourth week after PCL rupture: fracture surface, deep fissures into the radial layer, cartilage thickness and cell number decreased obviously, layers disorderly arranged, large range of cartilage fibrosis, loose tissue, tidemarks disappeared, wide-scope or full-layer loss dye of toluidine blue staining; MMP-13 and TIMP-1 positive-staining cells decreased and located only in superficial layers