Scanning electron microscopy of rat tail tendon collagen

Direct visualization of collagen-bound proteoglycans by tapping-mode atomic force microscopy

Most studies on the interaction of collagen with proteoglycans, two universal components of connective tissues, use technical approaches which substantially modify the shape and size of the proteoglycans themselves. In the present study unfixed, untreated collagen fibrils from rat tail tendon were dehydrated and observed by tapping-mode atomic force microscopy. The surface of collagen fibrils immediately reveals a periodic alternation of gap and overlap zones. A thin, transverse ridge decorates the gap zone, while other filamentous structures run on the fibril surface, either parallel or perpendicular to the fibril axis. These surface structures are much enhanced by Cupromeronic Blue preincubation, while pretreatment with chondroitinase ABC removes them completely, leaving barely detectable transverse ridges. The ridge and filaments are likely to represent, respectively, the core protein and the glycosaminoglycan side chains of proteoglycans, displayed with a far better resolution than with conventional histochemical or immunohistochemical techniques. Our data suggest that proteoglycan molecules are capable of different, multiple interactions with the collagen fibril surface as well as with each other.

Collagen fibril surface: TMAFM, FEG-SEM and freeze-etching observations

Native, unfixed collagen fibrils from rat tail tendon were dehydrated following different procedures and observed under a FEG-SEM and an AFM operated in Tapping Mode (TMAFM). Freeze-etched, untreated fibrils from the same tissue were also observed for comparison. The most notable features of the fibril surface, i.e., the gap/overlap alternation and three prominent intraperiod ridges, were simultaneously visible only in freeze-etched specimens, while under the SEM and the TMAFM their appearance was dependent on both the dehydration procedure and the visualization technique. The different susceptibility of the collagen fibril surface structures to various treatments clearly implies the existence of domains of different composition. Moreover, identical specimens were imaged differently by SEM and TMAFM, highlighting instrument-specific advantages and limitations. The onset of dehydration-dependent, procedure-specific artifacts should be considered in high-resolution studies of connective tissues. As for any biological specimen, the final aspect of collagen fibrils is determined no less by the preliminary treatments than by the visualization approach.