Three-Dimensional Organization of Extracellular Matrix in Elastic Cartilage as Viewed by Quick Freeze, Deep Etch Electron Microscopy

Abstract

Freeze-etch electron microscopy was used to reexamine the ultrastructure of extracellular matrix in elastic cartilage. This revealed that extremely delicate, approximately 4 nm diameter fibrils join end-to-end and sometimes side-to-side to form a tightly woven mesh that extends continuously from the cell membrane throughout the intercellular space. Within this meshwork were found large, irregularly contoured and densely packed elastin fibers as well as long, thin (20 nm) fibers with the appearance of type II collagen. By comparison, type I collagen fibers found in the skin surrounding the cartilage appeared much thicker (30 nm) and displayed the usual periodic banding pattern. Freeze-etching the latter fibers displayed a helicoidal arrangement of subfibrils within. In both cartilage and type I collagen-rich extracellular matrix, some of the approximately 4 nm filaments in the matrix could be seen to contact collagen fibers orthogonally, apparently connecting adjacent fibers at regular intervals. The organization of these fine filaments and others composing the matrix has several features that suggest a different organization for cartilage than currently thought. Specifically, the distance between branch or contact points of fibrils in the matrix is seldom more than 35 nm, substantially less than the length of one extended proteoglycan monomer. This suggests that other proteoglycans, or other unidentified components of the matrix, bind along the proteoglycan core protein at intermediate binding sites in order to form a finely partitioned structure.