Type I and Type III Collagen Interactions during Fibrillogenesis

There is some evidence that type I and type III collagens may be present in the same fibril. In order to demonstrate this, double labeling immunofluorescence microscopy and immunoelectron microscopy were performed with antibodies directed against the collagen molecule and the aminopropeptide domains of type I and type III procollagens using embryonic (postabortion) and adult human skin. Double indirect and protein A immunoelectron microscopy were carried out with 5- and 15-nm gold particles. Skin extracts were also studied by immunoblotting. Double immunofluorescence microscopy with antibodies against type I and type III collagen molecules revealed patterns of fluorescence that were identical in both fetal and adult skins. Immunofluorescence microscopy using an antibody directed against the aminopropeptide of type III procollagen labeled the entire dermis in both embryonic and adult skins. In contrast, although the aminopropeptide of type I procollagen was present throughout embryonic dermis, it was markedly reduced in adult dermis, except for the epidermo-dermal junction. Double immunoelectron microscopy of fetal skin revealed labeling of the aminopropeptide of type I and type III procollagens on the same thin (20-30 nm) fibrils. Large type I fibrils (90-100 nm) were coated with type III collagen molecules and their corresponding aminopropeptide but not with the aminopropeptide of type I procollagen. The aminopropeptide of type I procollagen was present on thin fibrils only at the epidermo-dermal junction in adult skin. Immunoblotting of skin extracts revealed the presence of both pN-type III procollagen (collagen plus the aminopropeptide) and pN-type I procollagen in fetal skin, but only pN-type III in adult skin. This study demonstrates that type I and type III collagens coexist within the same fibril and that the aminopropeptide of type III procollagen is present at the surface of type I collagen fibrils that apparently have reached full growth.