Type-I and -II collagens from bone and cartilage colocalize at the osteochondral cement line

The osteochondral cement line (OCL) plays a key role in joint integrity by attaching articular calcified cartilage (ACC) to underlying subchondral bone (SCB), whose predominant collagens are type 2 (Col-II) and type 1 (Col-I), respectively. Previous studies report contrasting evidence of the presence of collagen fibrils in the OCL, albeit in different species and joints. If present, collagen fibrils might provide a basis for osteochondral bonding in the organic phase. We aimed to study the morphological variations of the osteochondral cement line, to observe whether cartilage and bone collagen fibrils are present in the OCL, and whether they colocalize in a manner that could help explain how ACC attaches to SCB.

We used immunofluorescence, confocal microscopy, and deconvolution to image Col-I and Col-II collagen fibrils and measure their overlap and colocalization, in OCL harvested from equine and bovine femoral head, patella, and proximal and distal metatarsal condyles. Large mammalian species were chosen to have size and pathobiology relevant to human anatomy. Thousands to millions of Col-I/-II colocalizing complexes were observed per mm² of OCL over a tissue depth of 1 to 5 µm. Kruskal-Wallis with Dunn's post-hoc tests and Mann-Whitney U tests were conducted for intra- and interspecies statistical analysis.

The areal volume (µm³/mm²) of Col-I/Col-II complexes was up to ten times greater in equine than bovine OCL (p = 0.016 to 0.029). Similarly, the number of Col-I/-II complexes and mean volume per complex differed significantly (p < 0.001 to 0.032 and p < 0.001 to 0.029, respectively) among anatomical sites between equine and bovine OCL. Gaps or tears near OCL were present uniquely in the bovine patella.

Col-I/Col-II overlap and colocalize at OCL, which could be a critical source of bond strength between cartilage and bone that should be considered when cartilage repair is attempted in clinical settings.