How to visualize folding

Q.  How much detail should we know about the folding of the embryo for the exam?

A. The head-tail folding is pretty simple: the head and tail fold toward the ventral (front/abdominal) surface of the body. The folding of the head leads to the formation of the stomatodeum (primitive mouth).

The lateral folding process looks complicated in diagrams – but really, all that is happening is that the lateral surfaces of the trilaminar disk fold toward the ventral (front/abdominal) surface of the body, meeting in the front. The endoderm meets endoderm (forming the gut tube), and ectoderm meets ectoderm (forming the abdominal wall). The mesenchyme splits in two – part of it follows the endoderm and surrounds the gut tube, and the other part follows the ectoderm and becomes the abdominal (and other) muscles. Also: the amniotic cavity follows along for the ride, and ends up surrounding the embryo.

Here are some good videos that might help you visualize the folding process:

  • The best one I can find is the one I suggested at the beginning of class (the one by the guy who draws and uses play-doh). It also covers the stuff that happens before folding, which is nice.
  • Here’s another good one. It goes into more detail than we did, which isn’t necessarily a bad thing – just watch it to get an overall sense of the process (and try to ignore the extra details).
  • I posted a few other videos on embryology topics on our resources page, if you want to take a look.

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