Mitosis of Chondrocytes Induced in the Knee Joint Articular Cartilage of Adult Rabbits *

Since Leidy's experiment," numerous investigations have been made of the changes occurring in mature articular cartilage experimentally traumatized by incision.' Although one of the frequently described changes was an increase in the number of chondrocytes occurring at or near the site of injury, only two investigators"'8 reported finding chondrocytes undergoing mitotic division. These results differ from those of Elliott's study' on normal, mature articular cartilage, for he concluded that although mitosis is the method of cell division in growing articular cartilage, after growth ceases, amitosis becomes the rare but sole normal method of chondrocyte division. Recently, changes similar to those of degenerative arthritis were induced by compression in the knee joint cartilage of the adult monkey, rabbit,' and rat,' however, no chondrocyte proliferation was described. The present experiment was designed to investigate the occurrence of chondrocyte mitosis in mature articular cartilage traumatized by sustained pressure. Colchicine was employed to detect the occurrence of chondrocyte mitosis, as in previous studies of the interpubic joint cartilage of pregnant mice.' The articular hyaline cartilage of the right knee of ten adult rabbits was subjected to pressure in the following manner. Two stainless steel pins were passed transversely through the distal metaphysis of the femur and two through the proximal metaphysis of the tibia in the coronal plane. The two sets of pins were interconnected on both the medial and lateral aspects of the knee by a bolt and nut mechanism which, when tightened, rendered the knee completely immobile in a position midway between full extension and full flexion. The clamp maintained the weight-bearing surfaces of the right femoral and tibial condyles in direct contact under sustained pressure for 16 days. Eight hours before each rabbit was sacrificed, each received by intraperitoneal injection, 5 mg. of colchicine to arrest at metaphase any mitosis occurring during that time. The localized areas of articular cartilage which had been in direct contact under pressure in the right knees and comparable areas of articular cartilage in the untreated left knees were excised and sectioned serially at 8,. The sectioned cartilage from both the femoral and the tibial condyles of the clamped knees of three rabbits was in the following initial stages of reaction to the pressure: the matrix at the free surface of the cartilage was increased in amount, the free surface of the cartilage was roughened, the