Ultrastructural identification of extension aminopropeptides of type I and III collagens in human skin.

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Abstract
Human skin was labeled with purified antibodies against type I and III collagens and against their extension aminopropeptides by using the ferritin technique. Both aminopropeptides were visualized using immunoelectron microscopy mainly along thin collagenous fibrils (diameter, 20-40 nm) and rarely in nonfibrillar regions of the skin. The labeling showed a periodicity of 60-65 nm resembling the D (67 nm) stagger of collagen molecules. Blocking of antibodies with aminopropeptides and treatment of tissues with procollagen NH2-terminal protease abolished labeling. Antibodies against type I collagen uniformly labeled .apprxeq. 80% of the fibrils (diameter, 20-80 nm), while reaction with antibodies against type III collagen was restricted to thin fibrils. It is currently thought that the aminopropeptides of procollagen molecules are cleaved after they are released from the cell and before fibril formation. Data indicates that aminopropeptides are removed at the fibrillar level and that fibril growth which is disturbed in some skin diseases can be regulated by extracellular procollagen processing.