COLLAGEN FIBRILLOGENESIS IN-SITU - FIBRIL SEGMENTS BECOME LONG FIBRILS AS THE DEVELOPING TENDON MATURES

Tissue architecture, stability, and mechanical attributes are all dete rmined by the structure and organization of collagen fibrils. Therefor e, the characterization of fibril growth steps and determination of ho w this growth is regulated is essential to the elucidation of how tiss ues are assembled, We have proposed that fibril segments are intermedi ates 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 da ta demonstrate that the fibrils were discontinuous at 18 days of tendo n development, However, both ends were not present in any of the analy zed fibrils within the 18-day tendon. Because the data set was 50-60 m u m, this indicates a mean fibril length greater than 60 mu m, These d ata are in contrast to data from the 14-day tendon, in which 80% of th e fibrils had both ends in a 26-mu m data set and the mean segment len gth was shown to be 10-30 mu m, There were equal numbers of alpha and beta ends in the 18-day tendon, The structure of the ends was comparab le to that in the less mature tendon, The data also indicate that fibr il asymmetry and structure were maintained, The increase in fibril len gth is interpreted as being the result of a post-depositional, regulat ed assembly of segments via a lateral association/fusion to form matur e fibrils. This hypothesis predicts an increase in diameter at this st age of development. The diameter increases have been documented, but t his is the first demonstration of increases in length and maintenance of segment structure during this important stage of tendon development . (C) 1997 Wiley-Liss, Inc.