Tendon collagen fibrillogenesis is a multistep assembly process as revealed by quick-freezing and freeze-substitution

The ultrastructure of chick embryo tendons has been examined after quick-freezing by liquid helium and freeze-substitution. Several stages of collagen assemblies were observed: intracellular packing of SLS-like aggregates surrounded by membrane containing areas with a clathrin coat; fine non cross-striated filaments connecting the cell membrane at 1 pole of the cells and collagen fibrils; tufts of filaments directly linked to collagen fibrils. This study reveals that some stages are more constant and abundant than supposed (the intracellular SLS-like aggregates) and that other extracellular assemblies that were hypothesized but usually badly preserved by conventional electron microscopy are clearly captured by the method.