Morphology of fibroblasts in collagen gels: a study using 400 keV electron microscopy and computer graphics

We have used 400 kilovolt intermediate voltage electron microscopy (IVEM) to examine thick sections of fibroblasts cultured in collagen gels. In these 3D collagen lattices, the long, narrow pseudopodial extensions that extend out and make contact with the collagen matrix exhibit a complex topography not seen in the processes put out by cells moving on planar substrata. For this reason, sections 1 to 2 microns thick that enclose a whole cell process are more informative of the overall morphology of the interaction between cells and the collagen than are thin sections. To aid the discrimination of topography of cell processes in stereo views of micrographs, some cells were labeled with antibodies and protein A-colloidal gold conjugates. The gold particles provided clear 3D reference points for computer-aided reconstructions of membrane topography from tilt series of IVEM images. Our results confirm that cells that move through collagen lattices lack the well-spread morphology of their counterparts moving on glass. They are generally rather spindly with several long branching anterior pseudopodia. We found that the cell bodies and major pseudopodial processes were cylindrical, as one might expect of cells in a 3D environment, but at the leading edge of advancing pseudopodia there are small flat extensions similar to those seen in cells on glass. This similarity suggests that the lamellipodium is a basic type of protrusive structure used by fibroblasts during locomotion on all types of substratum. The flattened shape of lamellipodia may be part of the mechanism by which cells sense the orientation of fibrillar extracellular matrices during embryonic morphogenesis.