Procollagen intermediates during tendon fibrillogenesis.

The purpose of this study was to correlate ultrastructural features of tendon collagen fibrils at various stages of development with the presence of procollagen, pN-collagen, pC-collagen, and the free amino propeptides and carboxyl propeptide of type I procollagen. Tendons from 10-, 14-, and 18-day chicken embryos reveal small, well-defined intercellular compartments containing collagen fibrils with diameters showing a unimodal distribution. At 21 days (hatching) and 9 days (post hatching) and at 5 weeks (post hatching), the compartments are larger, less well-defined, and there is multimodal distribution of tendon fibril diameters. Procollagen and the intermediates pN-collagen and pC-collagen are present in tendons up to 18 days. Thereafter there is a marked reduction in procollagen, whereas the intermediates persist throughout all stages of development. Similarly, free amino propeptides and carboxyl propeptides of type I procollagen were found at all stages. The amino propeptide of type III procollagen was restricted to the peritendineum until 7 weeks post hatching. At that time, a network of fibrils containing the amino propeptide of type III procollagen was seen delineating well-circumscribed compartments of collagen fibrils throughout the entire tendon. This study supports the notion that pN- and pC-collagen have an extracellular role and participate in collagen fibrillogenesis.