A light and electron microscopic study of the region of cartilage resorption in the embryonic chick femur.

It has long been known that uncalcified cartilage of embryonic chick long bones is removed to make way for invading marrow. However, no one has clearly established which cells are responsible for this erosion. Using the light and electron microscopes, we have studied the cartilage-marrow interface, which we presume to be the region of resorption. Here, we found two types of mononuclear cells in intimate contact with cartilage matrix. 1. The predominate cell type had a euchromatic nucleus with a nucleolus and a cytoplasm containing extensive profiles of rough endoplasmic reticulum; also, processes extended from these cells into the adjoining cartilage matrix. 2. Macrophages containing many lysosomal vesicles, which often became swollen, were found on or near the surface of cartilage. In addition, a few cells with an intermediate appearance were present. A decrease in the amount of sulfated material in a 25-30 micrometer zone of cartilage in advance of the interface and an alteration in the orientation, and in some cases the integrity, of collagenous fibers were associated with the presence of the above mentioned cells. These alterations in cartilage were not due to the synthesis of sulfated or of collagenous material. The above evidence, although not conclusive, suggests that these mononuclear cells are responsible for cartilage resorption. In this respect, the removal of avian uncalcified cartilage is similar to the resorption of uncalcified articular cartilage which occurs in rheumatoid arthritis.