The course of events in the solvation and reconstitution of collagen obtained from rat tail tendon is described as seen with the electron microscope. Under the influence of 0.01 per cent acetic acid the collagen fibers swell and dissociate into submicroscopic filaments, the smallest of which are probably beyond the resolution of the electron microscope. These filaments can be made to reconstitute into fibers either by the addition of neutral salts to the acid solution or by raising the pH. The structural form of the resulting fibers is influenced by the concentration of the salt and by the pH of the solution employed. Saline-concentrations around 1 per cent and pH's ranging from 4.8 to 6.8 lead to the formation of needle-shaped crystals, or tactoids, showing the striated structure characteristic of collagen. Saline concentrations outside of this range (0.5 per cent and 5.0 per cent) lead to the formation of long fibrils without evidence of striations. pH's on the alkaline side of 6.8 bring about the formation of long slender fibers. Some of the possible reasons for these different fiber forms are discussed.
Apparently fibers are formed in vitro by the lateral and longitudinal association of the filaments seen in the original solutions. Some of the fibers thus formed may in turn associate laterally and longitudinally to form the larger fibers.
The formation of the needle-shaped crystals appears to be an orderly process since it leads to the formation of a periodicity in the fibers. The striations in the smaller fibers are uniform and regularly spaced at around 210 A. With continued growth of these fibers there is a simultaneous development of a more obvious and precise banding. It is evident that two out of each group of three regularly spaced striae are more prominent. This produces the macroperiod of collagen measuring around 640 A in length.