Fig. 1. Electron micrograph of segment-long-spacing aggregates of procollagen type I (top) and a model of procollagen type I molecule (bottom). B = Amino-terminal propeptide extension; C = minor triple helix of the amino-terminal protease cleavage site; D = amino-terminal telopeptide; E = triple helix; F = carboxy-terminal telopeptide; G = carboxy-terminal propeptide extension.
The regulation of collagen deposition and turnover in tissue is complex (fig. 2). Firstly, the amount of procollagen precursors secreted by the cells is controlled at the level of transcription  and by regulating intracellular degradation . Secondly, extracellular conversion in collagen molecules and deposition are controlled by proteolytic processing and is counteracted by degradation via collagenolytic enzymes.
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