High‐voltage electron microscopy of extracellular fibrillogenesis

Abstract

High-voltage electron microscopy was employed to observe developing extracellular connective tissue elements in the cervical perinotochordal and perivertebral regions in the chick embryo from 2 through 15 days' incubation. During days 2 and 3, small (10 nm) and large (18-20 nm) microfibrils surrounded the notochord, becoming evident around fibroblast-like cells in day 4. Amorphous material, globular granules and microfibrillar bundles were present at this time. Microfibrillar length increased as did the total population of microfibrils. At four days microfibrils 3-5 nm in diameter arose in all directions from globular granules. During day 9 and thereafter to day 15, microfibrillar diameters increased. This growth formed unit collagenous fibrils 30 nm in diameter or greater. Axial periodicity became evident at day 14. Small microfibrils appear to be composed largely of glycoproteins and do not contain a significant amount of collagen. The globular granules and associated filaments are probably proteoglycans. The amorphous material is believed to provide molecular collagen to developing fibrils. Large microfibrils and unit collagenous fibrils contain significant amounts of molecular collagen.

The nature of the microfibrillar glycoproteins of elastic fibres. A biosynthetic study

“…There have been several reports of a close spatial association between 10-20nm-diameter microfibrils and collagen fibres (Frederickson et al, 1977;Serafini-Fracassini et al, 1977;Jones et al, 1980). Such an association might be promoted if there were a structural relationship between the microfibrillar glycoproteins and collagen.…”

“…Several different criteria were used to establish that glycoproteins MFP I and MFP II and fibronectin are three distinct molecular entities. Suggestions that microfibrils might be related to the collagens have come largely from electron-microscopic studies of extracellular fibre development during embryogenesis (Frederickson et al, 1977). Our own ultrastructural studies revealed the frequent juxtaposition of microfibrils and collagen fibres in the extracellular space in vitro , and prompted a search for 'collagenous' characteristics in glycoproteins MFP I and MFP II.…”

Development of the connective tissue network in the neonatal hamster heart

“…The 20-nm microfibrils increased in length, number, and diameter to form unit collagen fibrils, whereas the 10-nm microfibrils remained the same size. Electron-dense granules, 30 nm in diameter, were associated with fibrillogenesis (Frederickson et al, 1977). Using enzyme digestion, .they showed that the 20-nm microfibrils were collagenous, the 10-nm microfibrils were probably glycoprotein, and the electron-dense granules were proteoglycans.…”

“…HVEM studies of collagen fibrillogenesis by Frederickson et al (1977) showed that there was a continuum in the formation of the unit collagen fibrils. Fibrillogenesis was associated with microfibrils and nonfibrillar components of the extracellular matrix.…”

“…In vivo collagen fibrillogenesis has been studied primarily in embryological models (Porter and Pappas, 1959;Frederickson et al, 1977;Trelstad and Hayashi, 1979;Revel and ,Hay, 1963), whereas our studies are in tissues that have completed differentiation. The function of the myocyte-associated strut in the hamster may be a good model for the study of collagen fibrillogenesis associated with the myocyte.…”