Echinoderm collagen fibrils grow by surface-nucleation-and-propagation from both centers and ends

Collagen fibrils from sea cucumber (class Holothuroidea) dermis were previo usly found to grow by coordinated monomer addition at both centers and ends . This analysis of sea urchin (class Echinoidea) collagen fibrils was under taken to compare the growth characteristics of fibrils from two classes of echinoderms, and to determine whether a single growth model could account f or the main features of fibrils from these two taxa. Native collagen fibril s (37-431 mu m long) from the spine ligaments of the sea urchin Eucidaris t ribuloides were studied by scanning transmission electron microscopy and im age analysis. The analyses revealed the mass per unit length, and hence the number of molecules in cross-section, along the entire length of each fibr il. The fibrils were symmetrically spindle shaped. The maximum mass per uni t length occurred in the center of each fibril, where the fibril contains a nti-parallel molecules in equal numbers. The two pointed tips of each fibri l showed similar linear axial mass distributions, indicating that the two t ips retain shape and size similarity throughout growth. The Linear axial ma ss distributions showed that the tips were paraboloidal, similar to those o f vertebrate and sea cucumber fibrils. The computed maximum diameters of th e fibrils increased linearly with fibril length. The overall shapes of the fibrils showed that they retain geometric similarity throughout growth. Com puter modeling showed that the simplest self-assembly mechanism that can ac count for the features of these fibrils, and of the sea cucumber fibrils th at have been described, is one in which the fibril tips produce independent axial growth, while lateral growth takes place through a surface nucleatio n and propagation mechanism. This mechanism produces coordinated growth in length and diameter as well as geometric similarity, characteristic feature s of echinoderm collagen fibrils. (C) 2000 Academic Press.