The tight junction component Claudin E is required for zebrafish epiboly.

Zebrafish epiboly results in the thinning and spreading of the blastoderm to cover the yolk cell and close the blastopore. The extra-embryonic yolk syncytial layer (YSL) tows the blastoderm vegetally during epiboly by means of its tight junction attachments to the enveloping layer (EVL). Claudins are the major transmembrane protein components of tight junctions. Here, we focus on the function of Claudin E (Cldne), which is expressed specifically in the EVL. Morpholino knock-down of cldne produced a highly penetrant epiboly delay. Our analysis suggested that the EVL margin, which is attached to the YSL, was under reduced tension in morphant embryos. We propose that local variation in the strength of EVL-YSL attachment in morphant embryos resulted in slow and uneven advancement of the EVL and blastoderm. Our work is the first to demonstrate that Claudins are important for zebrafish epiboly.