Ultrastructure of irregular collagen fibrils of shark mandible

Sawada T. and Inoue S. 2011. Ultrastructure of irregular collagen fibrils of shark mandible. —Acta Zoologica (Stockholm) 92: 62–66. Collagen fibrillogenesis was investigated in developing fibrous connective tissue (tooth band) in shark mandible by transmission electron microscopy. Fibrils varied considerably in shape and size. Both thin and thick fibrils 40–200 and 400–500 nm in width, respectively, were observed, with the latter showing irregular contours. Examination of both transverse and longitudinal sections of fibril suggested that the irregular, thick fibrils were formed by fusion of the thin fibrils. This was in agreement with a previously proposed mechanism of collagen fibrillogenesis in a variety of tissues, in which formation of thin fibrils is followed by their coalescence into thicker fibrils. Detailed high resolution ultrastructural examination revealed decorin-like, 4.5- to 5.5-nm-wide polygonal frames and 3-nm-wide ribbon-like structures previously identified as chondroitin sulfate proteoglycan ‘double tracks’ localized within the interfibrillar spaces. These structures may be closely involved in collagen fibrillogenesis.