Growth of sea cucumber collagen fibrils occurs at the tips and centers in a coordinated manner

Collagen fibrils are the principle source of mechanical strength in the mut able dermis of the sea cucumber Cucumaria frondosa. To obtain information a bout the mechanism by which collagen molecules self-assemble into fibrils, we have isolated single intact fibrils with lengths in the range 14-444 mu m These fibrils have been studied by scanning transmission electron microsc opy, yielding data that show how cross-sectional mass, and hence the number of molecules in the cross-section, depend on axial location. In an individ ual fibril, the two ends always display similar mass distributions. The two tips of each fibril must therefore maintain identity in shape and size thr oughout growth. The linear relationship between cross-sectional mass and di stance from the adjacent end shows that a growing tip is (like the tip of a vertebrate collagen fibril) paraboloidal in shape. Comparison of data from many different fibrils, ever a wide range of lengths, however, revealed th at the paraboloidal tip becomes blunter as the fibril grows in length. In c ontrast to vertebrate fibrils, those from C. frondosa do not have a central shaft region of constant cross-sectional mass. Rather, the cross-sectional mass increases to a maximum in the center of each fibril. The maximum cros s-sectional mass of the fibrils increases exponentially with increasing fib ril length. The centrosymmetry, the paraboloidal shape of the tips, and the hyperbolic increase in maximum cross-sectional mass with fibril length, is evidence for a cc-ordinated regulation of length and diameter, which diffe rs from the kind of regulation that gives rise to collagen fibrils in verte brates (chickens and mice). (C) 1998 Academic Press.