Twisted architectures in cell-free assembled collagen gels: study of collagen substrates used for cultures.

Reprecipitated fibrils from collagen solutions assemble into aggregates often showing a remarkable twisted structure. We first observed these aggregates in collagen gels prepared to facilitate culture of epithelial cells. We verified that these structures form in the absence of cells and correspond to a process of self-assembly. Studies on reconstructed fibrils of collagen are generally based on the examination of thin specimens mounted onto coated grids prepared for electron microscopy. We rather applied the classical methods of fixation, embedding and ultramicrotomy, which allowed us to analyze the structure of these aggregates, several microns in diameter. Our gels were prepared from 2.5 mg/ml tropocollagen solutions usually chosen for cell and organ cultures. The time required to obtain twisted architectures, in these aggregates, depends on temperature and the presence of factors such as fetal calf serum proteins. Twist is observed at two different levels of organization. Microfibrils are gathered into twisted bundles which condense into cross-striated fibrils. These fibrils themselves aggregate and show a mutual twist whose orientation is left-handed as is the twist observed within each microfibril bundle. Several models of these architectures are presented. Planar twist, cylindrical twist and toroidal twist are described and their relation to the structure of certain liquid crystals is considered. Examples of orthogonal packing also have been observed. These structures obtained in vitro are very close to patterns already described in vivo in numerous collagen matrices.