This investigation has sought to determine the significance of the wide range of extractable collagen fractions which appear to exist in growing connective tissues and to determine their position in the process of fibrogenesis.

Carrageenin granulomata were induced in guinea pigs and, after injection of 14C-glycine, this tissue and skin from the same animal were subjected to successive extractions with neutral salt solutions of increasing ionic strength, citrate buffer pH 3.6, and to gelatinization. The specific activity of these fractions was determined at various time intervals. At 8 hours it was found that the specific activity decreased with increasing ionic strength of the neutral salts and was still lower in the citrate extracts and gelatin. At 36 hours the situation was almost completely reversed except that the citrate extract and gelatin still had the lowest activities. The data from skin were more clear cut than that from the granuloma and the reasons for this are discussed.

It is concluded that at any given time in developing connective tissue, there is a continuous spectrum of collagen aggregates of varying degrees of strength of cross-linkage, dependent upon the time that has elapsed since their constituent molecules were synthesized. The various extraction media used remove a particular cross-section of these aggregates depending upon their disaggregating power. These extracts will thus be biologically heterogeneous. The fraction extracted with 0.14 M NaCl will contain the collagen molecules most recently synthesized and in this respect can be considered the earliest form of extracellular collagen.

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