Collagen fibrils forming in developing tendon show an early and abrupt limitation in diameter at the growing tips.

The formation of very long and near-uniform diameter collagen fibrils is fundamental to the assembly of the extracellular matrix of animals. However, how growth in length and diameter is regulated, and how fibrils increase in diameter during development, are poorly understood. The approach in this study was to examine the tips and central shaft regions of fibrils from 12 and 18-day embryonic chick metatarsal tendon using quantitative mass mapping electron microscopy. We found that the fibrils had smoothly tapered C and N-terminal tips, which had linear axial mass distributions and were consequently parabolic in shape. An invariant feature of all tips (N and C) was an abrupt stop in lateral growth leading to a local plateau in diameter. The distance from the end of the fibril to the abrupt stop occurred at multiples of five D-periods (where D=67 nm). This implies that D-periods at the ends of fibrils are not equivalent sites for accretion, and that diameter regulation relies on surface structural features, which repeat every 5D. Mass mapping of entire fibrils at day 12 showed that, on average, the coarseness of the fibril tips was independent of fibril length, consistent with individual fibrils growing at constant tip shape. Comparison of diameters in the plateau (close to the tips) and shaft regions of the fibril showed that fibrils in day 12 tendons grow in length at constant diameter. Analysis of tendons from day 18 embryos showed that the increase in diameter at this stage of development was the result of both increases in the coarseness of the tips and continued lateral accretion of mass onto the central shafts at distances away from the growing tips. Regulated tip growth provides an attractive explanation for how cells are able to synthesise very long fibrils during the organisation of the extracellular matrix.