Analysis of the role of microfilaments and microtubules in acquisition of bipolarity and elongation of fibroblasts in hydrated collagen gels.

Abstract

In this study of embryonic avian corneal fibroblasts grown in collagen gels, the steps in the acquisition of the elongate shape are described; the effect of cytoskeleton-disrupting drugs on filopodial activity are analyzed as is an assumption of bipolarity, and cell elongation within extracellular matrix. Previously immunofluorescence has shown that filopodia contain actin but not myosin, and are free of organelles. The cell cortex is rich in actin and the cytosol in myosin. By using antitubulin, it is seen that microtubules are aligned along the long axis of the bipolar cell body. The 1st step in assumption of the elongate shape is extension of filopodia by the round cells suspended in collagen, and this is not significantly affected by the drugs used: taxol to stabilize microtubules; nocodazole to disassemble microtubules; and cytochalasin D to disrupt microfilaments. The 2nd step, movement of filopodia to opposite ends of the cell, is disrupted by cytochalasin, but not by taxol or nocodazole. The 3rd step, extension of pseudopodia and acquisition of bipolarity similarly requires intact actin, but not microtubules. If fibroblasts are allowed to become bipolar before drug treatment, moreover, they remain so in the presence of the drugs. To complete the 4th step, extensive elongation of the cell, both intact actin and microtubules are required. Retraction of the already elongated cell occurs on microtubule disruption, but retraction requires an intact actin cytoskeleton. The cell interacts with surrounding collagen fibrils via its actin cytoskeleton to become bipolar in shape, and microtubules interact with the actin cortex to bring about the final elongation of the fibroblast.