Mechanism of retraction of the trailing edge during fibroblast movement.

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Abstract
Retraction of the taut, trailing portion of a moving chick heart fibroblast in vitro is an abrupt dynamic process. Upon retraction, the fibroblast tail always ruptures, leaving a small amount of itself attached to the substratum by focal contacts. Time-lapse cinemicrography shows that retraction produces a sudden, massive movement of both surface and cytoplamic material toward a cluster of focal contacts near the main body of the cell. The appearance of folds on the upper cell surface at this time and the absence of endocytotic vesicles are consistent with this forward movement. Retraction of the trailing edge, either occurring naturally or produced artificially with a microneedle, consists of an initial fast component followed and overlapped by a slow component. Upon artificial detachment in the presence of iodoacetate, dinitrophenol, and sodium fluoride, and at 4.degree. C, the slow component is strongly inhibited and the fast one only slightly inhibited. The tail of glycerinated fibroblasts shortens in the presence of ATP only at a slow rate, which is comparable to that of the slow component observed with the intact cell. Structurally, an extended tail of a fibroblast is highly birefringent, most likely because of the bundles of microfilaments oriented parallel to the long axis of the tail seen in TEM [transmission electron microscopy]. Most of the birefringence is lost during the fast phase and the rest during the slow phase of retraction. The bundles of microfilaments disappear during the fast phase of retraction and are replaced by a microfilament meshwork. All results are consistent with the hypothesis that the initial fast component of retraction is a passive elastic recoil, associated with the oriented bundles of microfilaments, and that the slow component of retraction is an active contraction, associated with a meshwork of microfilaments.