Molecular and Cellular Characteristics of Epithelial Tissue Fusion of the Edge in the Chick Embryo

During the first 4 days of incubation, the chick blastoderm expands and surrounds the yolk by epibolic movement. This expansion is driven by ‘edge cells’, located at the most margin of the embryo. Unlike the expansion, how edge cells behave when the opposing ends of the embryo meet at day 4 and whether there are changes in gene expression during the tissue fusion have not been explored. A major obstacle to studying the final stages of chick epiboly is the technical difficulty in monitoring embryos at those developmental stages over time. In this study, we mimic the final stage of the epiboly by confronting the two isolated embryonic halves from an early embryo during <i>ex-ovo</i> culture. The results show changes in the expression of the markers that are known to be expressed strongly in the edge cells, such as <i>DACT2</i>, <i>RNH1</i>, and <i>SNAI2</i>, upon the tissue fusion. All the markers are also newly expressed in the wound edges made by cutting the embryo in half, suggesting a possibility of similar cellular mechanisms between developmental tissue fusion and wound healing process. Also, to investigate cell migration across the border between the opposing tissue after fusion, one of the isolated embryonic halves is stained with a lipophilic dye, DiI. As a result, no prominent cell migration from one side to the other side is observed. Our results show distinct molecular and cellular characteristics in tissue edges of different situations.