Eating is an essential activity that requires a mouth. To make a mouth, an embryo must open a hole into itself. This dramatic process requires many steps that are beginning to be understood in molecular detail. In an advanced review published in WIREs Developmental Biology, Chen, Jacox, Saldanha and Sive discuss the evolution of the mouth and insights into the vertebrate mouth opening from the frog Xenopus laevis.
The mouth is essential for eating and has ancient origins. In frog embryos, the mouth opens by using reciprocal signaling between the extreme anterior domain (EAD) and the neural crest (NC). Early in development (early tailbud), the EAD secretes signals that guide the neural crest into the face. Once in the face (late tailbud), the NC signal back to the EAD to begin cellular movements that lead to mouth opening.
Is the mouth a conserved structure? Although its form may differ wildly across species, mouths generally comprise an opening from the outside into the oral cavity and the beginnings of the digestive tract. Critically, all mouths are derived from the same embryonic tissues types, a juxtaposition of ectoderm and endoderm, and express a common gene program. This suggests that the mouth is a conserved structure that evolved once during evolution.
How does the vertebrate mouth open? Using frog embryos, scientists have shown that mouth opening involves communication between two structures—the neural crest (NC) and the extreme anterior domain (EAD). The neural crest is a multipotent cell population that migrates from the edges of the developing brain into the nascent face where it differentiates into the the skeletal elements of the jaws. The EAD is a small group of ectodermal and endodermal cells layered on top of each in the center of the embryonic face that eventually becomes the mouth.
Early in face development, the EAD secretes signaling molecules that guide the movement of the neural crest into the face. Once migration is complete, neural crest cells signal back to the EAD which changes shape, thins, and eventually opens. The reciprocal signaling between the EAD and the neural crest ensures that mouth opening occurs after the neural crest has reached its final destination. Although the signaling activities of the EAD have been demonstrated in frog, future research will determine whether the EAD plays a similar role in other vertebrate species.
Kindly contributed by Justin Chen.
