| Because much of morphogenesis involves cells moving while attached to other cells, the effects of intercellular attachments on motility were examined using movement of clusters of fish epidermal cells as a model system. Time-lapse cinemicrography in situ and in vitro reveals that these clusters move by extending broad Iamellar protrusions, by which clusters adhere to the substratum, as shown by micromanipulation and interference-reflection microscopy. The surfaces of these protrusions display certripetally moving undulations observed by correlated light and scanning electron microscopy. The spreading of lamellae in opposing direction creates tension within the cluster. Nonetheless, clusters translocate up to 4 (mu)m/min and, once moving, tend to continue in the same direction, due to imbalances in the distribution of lamellae around the cluster's margin. A correlation exists between cell shape and spreading, with elongate cells lacking lamellae. This may reflect an antagonism between tension and spreading, since pulling on clusters with a microneedle supresses spreading perpendicular to the tensile force.;To understand the ultrastructural basis for this, particular clusters, whose precise motile behavior was recorded cinemicrographically, were examined by immunofluorescence and by transmission electron microscopy. Microfilaments appear disorganized in spreading lamellae, but are aligned perpendicular to the cell's edge in stationary or retracting lamellae. In alamellar regions under tension, microfilaments lie parallel to the cell surface in bundles. They also align at intercellular contacts as if they were connected across the cell surface. Thus, the effects of tension on motility may be transmitted within and between cells through a microfilamentous network. In contrast, intermediate filaments show no specific orientation relative to locomotion, are not readily reoriented by tension, and display no long-range orientation across cell boundaries. They do follow the contours of the cell, suggesting involvement in maintaining cell shape. They may also give epithelial sheets their tensile strength. |
