Collagen fibrillogenesis in situ: fibril segments become long fibrils as the developing tendon matures.

Tissue architecture, stability, and mechanical attributes are all determined by the structure and organization of collagen fibrils. Therefore, the characterization of fibril growth steps and determination of how this growth is regulated is essential to the elucidation of how tissues are assembled. We have proposed that fibril segments are intermediates in the formation of mature fibrils. The purpose of this study was to determine the length and structure of fibrils within a relatively mature tendon. The in situ determination of length performed here was only the second direct determination of fibril length in a vertebrate connective tissue and the first for a relatively mature tissue. The data demonstrate that the fibrils were discontinuous at 18 days of tendon development. However, both ends were not present in any of the analyzed fibrils within the 18-day tendon. Because the data set was 50-60 microm, this indicates a mean fibril length greater than 60 microm. These data are in contrast to data from the 14-day tendon, in which 80% of the fibrils had both ends in a 26-microm data set and the mean segment length was shown to be 10-30 microm. There were equal numbers of alpha and beta ends in the 18-day tendon. The structure of the ends was comparable to that in the less mature tendon. The data also indicate that fibril asymmetry and structure were maintained. The increase in fibril length is interpreted as being the result of a post-depositional, regulated assembly of segments via a lateral association/fusion to form mature fibrils. This hypothesis predicts an increase in diameter at this stage of development. The diameter increases have been documented, but this is the first demonstration of increases in length and maintenance of segment structure during this important stage of tendon development.