Resorption of uncalcified cartilage in the diaphysis of the chick embryo tibia.

The resorption of the uncalcified cartilage matrix of the middle third of the diaphysis in the chick embryo tibia has been studied using histological, histochemical and electron microscopic techniques. The first stage in the resorption process affects the periosteal bone, which is breached by osteoclasts at one or several points. Capillary vessels and clear, apparently undifferentiated cells penetrate through the holes so formed and reach the cartilage. The loss of acid proteoglycans to a depth of 10--20 micrometer into the matrix is the first sign of cartilage resorption; it is followed by the digestion of collagen fibrils, the opening of cell lacunae, chondrocyte degeneration and fragmentation and, lastly, the complete dissolution of the cartilage. This process is mediated by cells which probably derive from perivascular elements. Most of these cells have an undifferentiated appearance, but they have macrophagic properties, as is shown by phagocytotic activity along their plasma membrane, by the presence of lysosome-like bodies in their cytoplasm, and by their intense acid phosphatase activity. Resorption by giant cells of chondroclastic type only occurs at a late stage.