Effects of cations on cartilage structure: Swelling of growth plate and degradation of proteoglycans induced by chelators of divalent cations

Summary

Slices of fresh ovine and bovine epiphyseal cartilages swell following extraction in 0.05 M disodium ethylenediaminetetraacetate (EDTA) in Tris buffer, pH 5.8 and 7.4, at 4° and 37°. The swelling is strikingly visible to the unaided eye and is most pronounced in the growth plate region of the epiphysis. Other chelators—ethyleneglycolbis(β-aminoethyl ether)N,N′-tetraacetic acid (EGTA), and citrate buffer—also induce swelling. Swelling is associated with increased degradation of proteoglycans (PG) especially at pH 5.8, however, collagen seems to be unaffected. These effects are prevented by the addition of certain divalent cations (Ca, Mg, Zn) to the extraction media. At higher concentrations, the monovalent cation sodium also prevents swelling. It is concluded that divalent cations are required to maintain structure and function of cartilage. Freezing and thawing the cartilage did not prevent swelling or degradation, which suggests that these phenomena are not dependent on living chondrocytes. Although PG degradation and loss is markedly increased at 37° as compared with 4°, swelling is unaffected. It is concluded therefore that the degradative effects are enzymatic but the swelling is physicochemical. Other cartilages (nasal, manubrium) also swell and show histochemical evidence of PG degradation. These effects are minimal compared with the effects induced in the growth plate. It is inferred that growth plate contains more proteases than other cartilages and has properties that make it more susceptible to swelling. Swelling of the growth plate occurs even when the metaphysis is attached to it albeit to a lesser extent than when it is freed of underlying bone. A hypothesis is offered which attempts to link these phenomena with chondrocyte and matrical imbibition of water (swelling) in the zone of hypetrophy of the growth plate.