The movement of cells in small groups, or clusters, was studied in vitro using epithelioid cells from Gordon-Kosswig melanomas (from poecelid fish) and time-lapse cinemicrography. Tumour explants cultured on glass yield cell sheets from which groups of cells separate and become independently motile clusters. These clusters typically contain 3–30 cells, but may have as many as 50. They propel themselves at speeds of 0.2-4.0 micrometer/min by means of broad hyaline lamellae. The distribution of lamellae around the perimeter of each cluster correlates with both direction and speed of cluster movement, i.e. a cluster moves with its most lamellar region at its leading edge, and the greater the extent of the leading lamellar region the greater the speed. Also, a cluster tends to keep moving in the same direction. This persistence is due to a relatively constant distribution of lamellae. Cells on the trailing edge usually lack lamellae and most are very elongate and oriented perpendicular to the direction of cluster movement. In general, whenever a cell elongates, there is a loss of lamellar activity along its taut edges, parallel to the axis of elongation. Thus, any region with less lamellar activity would tend to be elongated by the outward pull of the more active regions to either side and would, in consequence, suffer a further reduction in lamellar activity. In this way, the distribution of regions of lamellar activity is self-reinforcing and the result is persistence of movement in a particular direction. This phenomenon could play an important role in giving directionality to certain morphogenetic movements, such as neural crest cell migration.

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