Condensation of collagen fibrils to the direct vicinity of fibroblasts as a cause of gel contraction.

Fibroblasts were cultured within FITC-prelabeled type I collagen. Cells initially round in shape protruded processes, and began to collect the fibrils into their vicinity. Repeated protrusion and withdrawal of cell processes was observed. Consequently, condensed fluorescence was observed on the elongated bipolar cells stained with rhodamine-phalloidin. Concomitantly with these events, the gel began to contract in overall size, with an increase of fluorescence intensity. Scanning electron micrographs of the contracted gel showed a disproportional distribution of collagen fibrils: a highly condensed region surrounding cell bodies and a moderately condensed region. A major portion of condensed fibrils may have been derived from reconstituted collagen fibrils, since fibroblasts within collagen gel synthesized little collagen. When the gel adhered to glass tightly, so that overall contraction was prevented, the fluorescence in a range of scores of micrometers from the cells disappeared owing to depletion of fibrils by the cells. The combined spaces with null fluorescence in total under repressed contraction corresponded well to the reduction in volume due to gel contraction. It seems likely that the fibril condensation onto the cells causes the overall gel contraction.