Stabilization of fluid cholesteric phases of collagen to ordered gelated matrices

Liquid crystalline assemblies occur spontaneously in highly concentrated solutions of type I acid-soluble calf skin collagen. The degree of order, identified by optical microscopy in polarized light, varies from a random distribution of molecules at low concentrations to highly organized structures as the concentration increases up to 80 mg/ml. Ultrastructural studies using classical techniques of chemical fixation are inappropriate for liquid crystalline phases due to the absence of stable links maintaining their three-dimensional order. In order to analyse the collagen liquid crystalline phases by electron microscopy the viscous preparations were stabilized under ammonia vapour. Observations of the gels in polarized light indicated that the liquid crystalline order, established at acidic pH in a sol state, persists at neutral pH in a gel state. Transmission electron microscopic observations allow us to validate the geometrical model interpreted from observations in polarizing microscopy, that is continuously twisting orientations in cholesteric phases characterized by typical series of arced patterns when viewed in oblique sections. A significant result is that the ultrastructure of the stabilized liquid crystalline collagen faithfully mimics fibrillar patterns described in vivo in extracellular matrices. This strongly supports the hypothesis that liquid crystalline properties are involved in the morphogenesis of collagen matrices.