The epiboly of the Caenorhabditis elegans hypodermis involves the bilateral spreading of a thin epithelial sheet from the dorsal side around the embryo to meet at the ventral midline in a process known as ventral enclosure. We present evidence that ventral enclosure occurs in two major steps. The initial migration of the hypodermis is led by a quartet of cells, which exhibit protrusive activity at their medial tips and are required to pull the hypodermis around the equator of the embryo. These cells display actin-rich filopodia and treatment with cytochalasin D immediately halts ventral enclosure, as does laser inactivation of all four cells. Once the quartet of cells has migrated around the equator of the embryo and approaches the ventral midline, the remainder of the leading edge becomes visible on the ventral surface and exhibits a localization of actin microfilaments along the free edges of the cells, forming an actin ring. Cytochalasin D and laser inactivation block ventral enclosure at this later stage as well and, based upon phalloidin staining, we propose that the second half of enclosure is dependent upon a purse string mechanism, in which the actin ring contracts and pulls together the edges of the hypodermal sheet at the ventral midline. The ventral cells then form junctions with their contralateral neighbors to complete ventral enclosure.

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