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Differentiation of a cultured cell refers generally to a change in the expression of some peculiar functions specific to this cell. Generally differentiation involves only a modification of some biochemical properties of the cell such as synthesis of melanin by melanocytes (KREIDER et al., 1975) or synthesis of collagen by bone fibroblasts (MANNER and KULEBA, 1974) or tendon cells (SCHWARTZ et al., 1976). However, differentiation is sometimes characterized by a net change in the cell morphology which accompanies the biochemical changes as for example with myoblasts.

The biosynthesis of collagen has many similarities to the biosynthesis of other proteins synthesized for “export,” but collagen biosynthesis is distinguished by at least two prominent features: (1) the protein is first synthesized as a precursor form which fulfills several important functions, and (2) the biosynthesis involves several unusual posttranslational modifications which occur after assembly of amino acids into the three polypeptide chains of the molecule and which are essential for some of its critical structural features.