STEM/TEM studies of collagen fibril assembly

Quantitative scanning transmission electron microscopy (STEM), implemented on a conventional transmission electron microscope with STEM-attachment, ha s been a primary tool in our laboratory for the quantitative analysis of co llagen fibril assembly in vivo and in vitro. Using this technique, a precis e measurement of mass per unit length can be made at regular intervals alon g a fibril to generate an axial mass distribution (AMD). This in turn allow s the number of collagen molecules to be calculated for every transverse se ction of the fibril along its entire length. All fibrils show a near-linear AMD in their tip regions. Only fibrils formed in tissue environments, howe ver, show a characteristic abrupt change in mass slope along their tips. It appears that this tip growth characteristic is common to fibrils from evol utionarily diverse systems including Vertebrate tendon and the mutable tiss ues of the echinoderms. Computer models of collagen fibril assembly have no w been developed based on interpretation of the STEM data. Two alternative models have so far been generated for fibril growth by accretion; one is ba sed on diffusion limited aggregation (DLA) and the other based on an interf ace-limited growth mechanism. Inter-fibrillar fusion can also contribute to the growth of fibrils in vertebrate tissues and STEM data indicates the pr esence of a tight regulation in this process. These models are fundamental for the hypotheses regarding how cells synthesise and spatially organise an extracellular matrix (ECM), rich in collagen fibrils. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.