Microfibrils purified from wild-type (A), mgN/+ (C), or mgN/mgN (B and D) neonatal fibroblast cultures are compared after rotary shadowing and electron microscopy. Wild-type and mgN/+ microfibrils contain the normal and half the normal amount of fibrillin-1, respectively, and significantly less fibrillin-2. Instead, mgN/mgN microfibrils are composed exclusively of fibrillin-2. A density just to one side of each globular domain was seen only in the fibrillin-2 microfibrils and best visualized after negative staining of the fibrillin-2 microfibrils (D). Similar electron densities were occasionally observed in microfibrils from mgN/+ fibroblast cultures (C) Arrows point to the globular beads, and arrowheads mark some of the densities. Scale bar, 100 nm.