Georgia Tech mechanical engineers studied the mechanisms of quail embryo epiboly, a developmental process involving mass movement of cells as a sheet, which is linked with medical conditions that include wound healing and cancer.  Avian embryos could join the list of model organisms used to study a specific type of cell migration called epiboly, thanks to the results of a study published this month in the journal Developmental Dynamics. The new study provides insights into the mechanisms of epiboly, a developmental process involving mass movement of cells as a sheet, which is linked with medical conditions that include wound healing and cancer.  The study, published online on March 15, explains how epithelial cells expand as a sheet and migrate to engulf the entire avian egg yolk as it grows. It also reveals the presence of certain molecules during this process that have not been previously reported in other major developmental models, including Xenopus frogs and zebrafish.

“These molecules and mechanisms of early development in the avian embryo may demonstrate evolutionary differences across species in the collective movement of epithelial cells and motivate additional studies of avian embryo development,” said Evan Zamir, an assistant professor in the George W. Woodruff School of Mechanical Engineering at Georgia Tech.  Matt Futterman, who worked on the project as a graduate student at Georgia Tech, and mechanical engineering professor Andrés García also contributed to this study. The research was funded by Zamir’s new faculty support from Georgia Tech and by a grant to García from the National Institutes of Health.

In the study, the researchers conducted immunofluorescence and high-resolution confocal microscopy experiments to examine the spatial distribution and expression of five proteins — vimentin, cytokeratin, β-catenin, E-cadherin and laminin — as cells moved to wrap the yolk sac of quail embryos during development.  The results showed that during this process, four of the proteins — vimentin, cytokeratin, β-catenin and E-cadherin — appeared in the cells located at the free edge of the migrating cell sheet. Finding dense interconnected networks of both vimentin and cytokeratin in the edge cells surprised the researchers.

This finding provides evidence that epithelial cells normally attached to a membrane surface underwent biochemical changes that enabled them to assume a mesenchymal cell phenotype, which enhanced their migratory capacity. This process, called partial epithelial-to-mesenchymal transition, has many similarities to the initiation of tumor cell metastasis and wound healing.  In addition to detailing protein expression in the quail embryo during development, the researchers also determined the origin of the new cells required at the migrating edge to cover the growing yolk. During development, the radius of the quail yolk doubles every day for the first few days, representing a hundreds-fold increase in the egg yolk surface area.

“These experiments confirm that edge cell proliferation is not the primary mechanism for expansion of the edge cell population,” noted Zamir. “And our observation of epithelial-to-mesenchymal transition in the edge cells explains how these epithelial cells might be changing phenotype to become migratory in this rapidly expanding sheet.”