The French group expected that extension in fly embryos would involve a few individual motile cells nosing between other stationary cells. Instead they saw “a global and ordered reorganization,” says Lecuit. Borders between cells that lay anterior and posterior to each other contracted to a point. Perpendicular expansion of this point generated a border between dorsal and ventral cells, thus pushing the anterior cell more anterior and the posterior cell more posterior. Extension was therefore achieved by a geometrical shuffling of the original hexagonal arrangement.
Myosin II was concentrated near the shrinking (anterior–posterior) membranes and reduced near the expanding (dorsal–ventral) membranes. In embryos with less myosin II, junctions froze and intercalation failed. Myosin need only destabilize adhesion proteins at anterior–posterior membranes, as junction proteins are naturally very dynamic.
It is not known how myosin is concentrated preferentially near anterior and posterior membranes. Genes that define anterior–posterior polarity of the fly embryo are needed; they may exert this effect by either local (cell-to-cell) or global (gradient) messages.
These polarity cues result in an intercalation method that, says Lecuit, maintains “the balance between stability and dynamics.” It can account for a nearly twofold extension. If more extension is needed, for a gut tube or an arm, then cell shape changes and oriented cell division may come into play. ▪