ULTRASTRUCTURAL ORGANIZATION OF TYPE-XI COLLAGEN IN FETAL BOVINE EPIPHYSEAL CARTILAGE

Type XI collagen was localized with polyclonal antibodies specific for alpha1(XI) and alpha2(XI) chains in the resting zone of epiphyseal ca rtilage from calf fetuses. The immunofluorescence technique was used o n sections of cartilage, and the immunogold labelling technique for el ectron microscopy on fibrils isolated from cartilage and, for the fist time, in situ on blocks of cartilage fractured in liquid nitrogen. Im munofluorescence showed that without pepsin treatment the staining of type XI collagen was restricted to the pericellular zones; after pepsi n treatment, the staining was co-distributed with that of type II coll agen. Immunoelectron microscopy performed on isolated fibrils and on c artilage blocks showed that after disruption of fibrils with pepsin, t ype XI collagen was labelled on small filaments on the fibrils. When t he fibrils were not disrupted, labelling was observed in situ only at the ends of the fibrils or on cross-sections of some fibrils. These re sults indicate that type XI collagen is located inside type II collage n fibrils in fetal bovine epiphyseal cartilage, as already postulated for embryonic chicken sterna.