Comparative Electron‐Microscope Studies on the Collagens Extracted from Cartilage, Bone, and Skin

Long spacing segment crystallites, prepared from soluble chick bone and skin collagen with the chain composition α1(I) 2 α2 and soluble chick cartilage collagen with a chain composition α1(II) 3 , were compared in electron microscope studies after staining with phosphotungstic acid and uranyl acetate. Although comparison of bone and skin collagen long‐spacing segment preparations with similar preparations of cartilage collagen revealed several differences in intensities of cross‐striation bands, no differences in the positions of the cross‐striation bands could be observed in the three preparations, indicating that the distribution of polar and apolar regions along the cartilage collagen molecule is markedly similar to that of molecules derived from bone and skin collagen. Comparison of segment long‐spacing crystallites prepared from the various collagens also showed that the cartilage collagen is longer than the bone or skin collagen molecule by about 10–12 nm. The additional length of the cartilage collagen molecule is clearly discerned as an extra band located in the COOH‐terminal region, and is removed by treatment of the collagen with pepsin prior to precipitation as segment long‐spacing crystallites, indicating that this region contains one or more sequences in which the collagen helix cannot form. In studies on fibrils formed by the three collagen preparations, it was noted that bone and skin collagen could be readily precipitated as broad, well‐structured fibrils in which a banding pattern characteristic of native type fibrils could be easily observed. On the other hand, cartilage collagen, precipitated under identical conditions, formed slightly smaller fibrils in which a banding pattern commonly associated with native type collagen fibrils was only faintly visible in certain segments of the fibrils. These results suggest that the relatively large number of bulky carbohydrate prosthetic groups covalently attached to hydroxylysine residues in the α1(II) chains of cartilage collagen sterically hinder the deposition of staining materials and thus prevent the clear manifestation of the banding pattern in fibrils of cartilage collagen.