Cell behaviour during epithelial wound closure in the chick extra-embryonic epiblast.

Light and electron microsope techniques have been used to investigate the normal stage 4-5 epiblast periphery and wound closure at the epithelial margin. The stage 4-5 epiblast is attached to the vitelline membrane by an association of flattened 'edge cells'. Basal 'edge cells' possess lamellipodia oriented in the direction of epiblast expansion. Adjacent cells are connected by junctions and appear to retain their respective positions during the 'gliding' movement of the attached periphery. Fixation at low temperatures produces an alteration in 'edge cell' morphology consistent with a retraction of the epiblast margin. After the excision of approximately 200?m of epiblast periphery, the wound gapes due to tensions within the proximal epithelium. The epiblast cells respond to the trauma of wounding by rounding. Wound closure commences within 1 hour of 'edge cell' excision as epiblast cells at the margins attach to the membrane in distal-proximal sequence. The attached wound margins migrate toward each other across the membrane and close the wound at around 10 hours reincubation. Junctions are not observed at the wound margins and the cells appear to employ a 'rolling and sliding' form of locomotion. These results suggest that 'edge cells' are intrinsically different from those of the proximal epiblast. The normal stage 4-5 epiblast is overlain by a basement membrane. This structure is not observed at the early wound margin. The migrating wound margins deposit extracellular materials on the vitelline membrane and these resemble substances associated with the normal stage 4-5 epiblast basement membrane. Similar materials are also produced by explants and isolated cells of the epiblast periphery when cultured on the vitelline membrane. It is suggested that these materials may represent an attempt to reconstitute a basement membrane during wound closure.