Immunoelectron microscopic localization of laminin, type IV collagen, and type III pN-collagen in reticular fibers of human lymph nodes.

We studied the ultrastructural distribution of laminin, type IV collagen, and the amino terminal pro-peptide of type III collagen (type III pN-collagen) in normal human lymph nodes. After fixation with freshly prepared 4% paraformaldehyde mixed with 0.1% glutaraldehyde, cryoultramicrotomy proved to preserve the antigenicity of these proteins better than embedding in Lowicryl K4M. Sections were treated with rabbit antibodies against the 7S domain of human type IV collagen, the fragment P1 of human laminin, and the amino terminal pro-peptide of human type III pro-collagen, followed by anti-rabbit IgG conjugated to 10-nm colloidal gold. Laminin and type IV collagen were seen in the basement membrane structures of the blood vessels and in the walls of sinuses. The amorphous material between the collagenous fibers in locations corresponding to reticular fibers also contained laminin and type IV collagen. The amino terminal pro-peptide of type III pro-collagen was present in the collagenous fibers in reticular fibers and in the walls of blood vessels and sinuses. Therefore, a significant number of the type III collagen molecules in these fibers must have retained their amino terminal pro-peptide. These results indicate that the basement membrane proteins laminin and type IV collagen are genuine components of reticular fibers, as suggested earlier by immunohistochemical studies at the light microscopic level.