Getting Above the Tongue: Palatal Shelf Elevation

Compare two stages. Before (about human week 7, mouse about E12.5), the shelves are vertical, hanging down on each side of the tongue, with the tongue high and filling the space. After (human week 8, mouse about E13.5 to E14), the shelves are horizontal, lying flat above the tongue and nearly meeting in the midline; the tongue has dropped down and forward, and the head has grown wider and taller. Between these snapshots each shelf has rotated roughly 90 degrees, a fast change called elevation.

Now a natural experiment. Sometimes a baby is born with a much smaller-than-normal lower jaw. The tongue then sits higher and farther back, and in about 85 percent of these babies the palate is cleft. The reason is mechanical: the high tongue physically blocks the shelves from flipping up, so they cannot elevate in time and never meet. This pattern is the Pierre Robin sequence (small jaw, tongue back, cleft palate). Importantly, the palate genes can be perfectly fine; the shelves are blocked by a neighbor, the tongue, so the cleft is a downstream consequence of something else.