Parallel studies of in vitro native collagen fibrillogenesis: Cold start, warm start and dialysis assembly pathways revealed by atomic force microscopy

Collagen structures are involved in nearly every system and organ in the body. Astonishingly, when in vitro conditions are made to approximate physiological pH, temperature and ionic strength, the collagen monomer will undergo self-assembly to produce the same native fibrils found in vivo. Here, type I collagen fibrillogenesis is examined using the atomic force microscope in an attempt to resolve the long-standing debate regarding the mechanism by which collagen assembles. It is demonstrated that fibrillogenesis proceeds via hierarchical assembly through a series of intermediates rather than by simple nucleation and growth for all three commonly studied in vitro pathways: cold start, warm start and dialysis. In each case, early axial association into microfibrils is followed by lateral organization into mature fibrils. Furthermore, the first direct evidence is presented that the removal of telopeptide regions on the ends of the collagen monomer does not alter the mechanism of fibrillogenesis.