The mechanism could be epigenetic signaling. Recently scientists have been able to control and influence cell-differentiation via long-lifetime electric fields. (Although the electric potential was created chemically via ion pumps rather than externally).

Still they showed pretty crazy results like being able to create a worm with heads on both ends.

It's looks like this kind of electrical signaling is likely the primary mechanism by which your cells know what to become and in which direction to grow.

A super interesting paper that I'm surprised isn't more famous: https://www.sciencedaily.com/releases/2017/05/170523124219.h...