FIG. 2.
Oxygen, HIF-1α, transcription factor, and ECM gradients in OC tissue. Oxygen pressure is low in native cartilage, but high in the subchondral bone. The cellular response to these varying oxygen pressures, which are mediated by HIF-1α, plays a role in regulating transcription factors important in progenitor cell differentiation toward chondrogenic and osteogenic phenotypes and in promoting the production of proteins that are abundant in cartilage compared with bone. (a) A histological section of the OC tissue from a rabbit metacarpophalangeal joint stained with hematoxylin, Safranin O, and fast green. Cartilage appears red, calcified cartilage appears dark green, and bone appears blue. Image is adapted from106 (CC-BY 2.0). (b) A schematic of OC tissue showing expression gradients of HIF-1α, transcription factors, proteins, and enzymes. Factors in blue are upregulated or promoted in response to low oxygen pressures, while those in red are downregulated.2,107–112 col II, collagen type II; acan, aggrecan; col I, collagen type I; col X, collagen type X. Color images are available online.