ULTRASTRUCTURAL-LOCALIZATION OF COLLAGEN TYPE-II, TYPE-IX, AND TYPE-XI IN THE GROWTH-PLATE OF HUMAN RIB AND FETAL BOVINE EPIPHYSEAL CARTILAGE - TYPE-XI COLLAGEN IS RESTRICTED TO THIN FIBRILS

The collagen fibrils of hyaline cartilage vary in diameter depending o n developmental stage and location within the tissue, In general, grow th plates and fetal epiphyseal cartilages contain fibrils with diamete rs of less than similar to 25 nm, whereas the permanent cartilage of a dult tissues contains fibrils of similar to 30-200 nm, The interstitia l collagen fibrils of fetal cartilage are complex, having at least thr ee collagen types as integral components, Type XI, a member of the fib rillar collagen class, has been proposed to limit fibril diameter. To test this proposition we sought to determine if Type XI collagen was p referentially associated with fibrils of smaller diameter. We focused our study on human juvenile rib growth plate, which has thin fibrils i n the hypertrophic zone, thick fibrils in the resting zone or permanen t cartilage, and a mixture of thin and thick fibrils in the proliferat ive zone. Tissues were examined by immunoelectron microscopy with anti peptide antibodies to the carboxyl telopeptide and to the amino termin al non-triple-helical domains of alpha 1(XI), These studies showed tha t (a) both epitopes of Type XI collagen were readily accessible to ant ibodies at the fibrillar surface, (b) Type XI collagen was associated predominantly with fibrils <25 nm in diameter, (c) Type XI collagen wa s not found in thick fibrils even after disruption with chaotropic age nts, and (d) collagen Types II and IX were associated with fibrils of all sizes, These studies were extended to human newborn epiphyseal car tilage and to fetal calf cartilage, with the same result.