The EFs arise because channels shuttle ions between the tear fluid on the outside of the eye and the extracellular fluid bathing the underlying tissue layers. Before the surface of an eye is damaged, channels actively pump out Cl− ions and pump in Na+ and K+ ions. That gradient is destroyed by wounding, leaving an EF in the extracellular fluid that runs from positive (far from the wound) to neutral or less positive (at the wound site). The authors directly measured the decline of this EF near wound sites. Various drugs changed the EF by activating or inhibiting the ion pumps.
The authors then provided the first in vivo correlation of EF magnitude with cellular behavior. Proximity to the wound and some of the drugs led to both higher EFs and greater orientation of mitotic spindles parallel to the EF. This orientation may help cells feed into the wound area—a process that is also promoted by EF-directed cell migration. McCaig and others have shown in earlier work that EGF receptors, which have charged extracellular domains, redistribute in an EF and are necessary for directed cell migration.
The high level of migration may suppress cell division nearest the wound, but further from the wound the authors again found a correlation: this time between a higher EF and increased cell division. This may also be driven by the clustering of EGF receptors on the cell surface nearest the wound.
Similar EFs may operate during neural development, so should prospective mothers be worried about the effects of high voltage power lines? McCaig says that the high electrical resistance of skin would render such voltages “vanishingly small” for an embryo, and that the power lines generate AC rather than DC electrical fields. But individuals recovering from laser eye surgery might think twice before basking in the sun under a major electrical supply. ▪