ABSTRACT
Explants of pig articular cartilage including invading marrow and subchondral bone (together='invasion zone') were cultivated for 10 or 14 days in complement-sufficient rabbit antiserum to pig erythrocytes (AS+C'), and then transferred to heat-inactivated normal rabbit serum (NRS) for a period of recovery. In AS+C' the cartilage matrix lost first proteoglycan and then collagen, but the cells remained viable. The degradation of collagen was accompanied by a fibroblastic transformation of the chodrocytes, first seen in the region immediately above the invasion zone. Immunohistochemical studies showed that after cultivation in AS+C', IgG antibodies entered areas in which the matrix was depleted of proteoglycan and reacted strongly with the majority of chondrocytes; those that had under-gone fibroblastic transformation exhibited little or no reaction. The degree of recovery in NRS depended on the extent to which the matrix had broken down in AS+C'. If degradation of collagen was confined to the region immediately above the invasion zone, and elsewhere only proteoglycan had been lost, new metachromatic material was regenerated in the non-calcified cartilage, and the fibroblast-like chondrocytes resumed their normal appearance and regained their reactivity with the IgG antibodies of AS; new cartilage and chondroid tissue appeared in the cavities of the invasion zone. If degradation of collagen and fibroblastic transformation of chondrocytes had spread throughout the cartilage, breakdown continued in NRS and cartilage disappeared completely above the invasion zone; new cartilage waw sometimes formed in the cavities of the invasione zone.
