While simple aggregation in solution typically leads to ramified objec
ts, some polymers and proteins form filamentous structures. The aggreg
ation of collagen, a structural protein, from monomers to fibrils was
studied by light scattering and atomic force microscopy. The presence
of clear, stable intermediates indicates that the assembly proceeds in
a modular or hierarchical fashion. Temperature and concentration depe
ndence studies show that the detailed assembly mechanism may change un
der different conditions.