Epithelial fusion is mediated by a partial epithelial-mesenchymal transition
Epithelial fusion is a fundamental morphogenetic process critical for the closure and compartmentalisation of developing organs. While widely studied in systems such as neural tube and palatal closure, the cellular transitions that enable fusion remain poorly understood. Here, we investigate epithelial fusion during chick otic vesicle (OV) closure and identify a transient population of cells at the epithelial interface that mediate this process. These otic edge (OE) cells exhibit distinct morphology, reduced apicobasal polarity, and dynamic junctional remodelling, including altered distribution of ZO-1, CDH1, and RAC1. Notably, OE cells lack basal contact and display high sphericity, consistent with a partial epithelial-to-mesenchymal transition (EMT) phenotype. Transcriptomic profiling of microdissected tissues reveals that OE cells constitute a transcriptionally distinct population, enriched for EMT regulators, ECM remodelling genes, and WNT pathway components. Among these, the transcription factors Grhl2 and Sp8 were specifically expressed at the OE and exhibited opposing roles in epithelial identity. CRISPR-Cas9 mediated knockdown of either gene led to disrupted CDH1 localisation, loss of OE cell morphology, and failure in epithelial segregation. These results suggest that epithelial fusion requires a regulated, hybrid EMT state that balances junctional plasticity with tissue cohesion. Our findings demonstrate that fusion-competent epithelial cells are not merely passive participants but actively modulate their shape, polarity, adhesion, and genetic identity to enable morphogenesis. Overall design: Transcriptome signature of cells involved in epithelial fusion in the otic vesicle: The opening of the otic vesicle (the otic edge) constitutes the border between the otic cells and the surface ectoderm. These cells undergo epithelial fusion to close the otic vesicle. RNA-seq was done on cells from the otic edge, the otic vesicle without the edge, and the surface ectoderm caudal to the otic vesicle. Differential gene expression analysis between the otic edge and otic vesicle and otic edge and the surface ectoderm will reveal genes enriched in the otic edge cells, that may have a role to play in epithelial fusion.