Reproduction of Characteristics of Extracellular Matrices in Specific Longitudinal Depth Zone Cartilage within Spherical Organoids in Response to Changes in Osmotic Pressure

Articular cartilage is compressed with joint-loading and weight-bearing stresses, followed by a bulging of the tissue during times of off-loading. This loading and off-loading causes changes in water content, and thus alterations in osmotic pressure. Another unique characteristic of articular cartilage is that it has longitudinal depth: surface, middle, and deep zones. Since each zone is composed of unique components of highly negative extracellular matrices, each zone has a different level of osmotic pressure. It was unclear how changes in osmotic pressure affected chondrocyte matrix turnover in specific longitudinal zones. Therefore, we hypothesized that a change in extrinsic osmotic pressure would alter the production of extracellular matrices by zone-specific chondrocytes. We incubated spheroidal cartilage organoids, formed by specific longitudinal depth zone-derived chondrocytes, under different levels of osmotic pressure. We compared the gene expression and the immunohistology of the matrix proteins produced by the zone-specific chondrocytes. We found that high osmotic pressure significantly upregulated the transient expression of aggrecan and collagen type-II by all zone-derived chondrocytes (p < 0.05). At a high osmotic pressure, surface-zone chondrocytes significantly upregulated the expression of collagen type-I (p < 0.05), and middle- and deep-zone chondrocytes significantly upregulated matrix metalloproteinase-13 (p < 0.05). The spheroids, once exposed to high osmotic pressure, accumulated extracellular matrices with empty spaces. Our findings show that chondrocytes have zone-specific turnover of extracellular matrices in response to changes in osmotic pressure.