Proteoglycan:collagen interactions and subfibrillar structure in collagen fibrils. Implications in the development and ageing of connective tissues.

Evidence is presented confirming that collagen fibrils are aggregates of subfibrils. Rat tail tendon fibrils swollen in acetate buffer, pH 5.8, (with or without MgCl2 or Cupromeronic blue, a proteoglycan (PG) precipitant used to locate PGs by electron histochemistry) showed two characteristic populations of subfibrils, of 10-15 nm, and approximately 25 nm thickness respectively. The smaller protofibrils, helically orientated, were derived from, or coalesced into, the larger subfibrils. The results prove that cross-links are not regularly distributed throughout the tissue but are limited to intra-protofibrillar collagen. Cupromeronic blue-stained PGs were seen in thick fibrils from cartilage and tendons, axially orientated in bands across the fibrils in longitudinal sections, or chordally in fibril cross-sections. Based on the intrafibrillar location of PGs vis-à-vis the fibril a-e banding pattern, the relative orientation of PGs and protofibrils and PG distribution in very young, as compared with mature or old, tissue, it is suggested that intrafibrillar PG is originally associated with protofibrils, which coalesce to larger fibrils, e.g. during development. The function of the large amount of PG and hyaluronan in young tissues is seen to keep the collagen protofibrils and subfibrils from coalescing. Disaggregation to protofibrils was demonstrated in conditions not very different from certain in vivo situations. It is suggested that this phenomenon is potentially important during tissue modelling and remodelling, allowing rapid access of reactants to the interior of thick fibrils and recycling of component protofibrils.