Muscles, bones, and mesoderm

Today in class, someone asked a really good question about the derivatives of mesoderm. The question stemmed from the following two slides in the embryology lecture that seem to have conflicting information:

Slide 24, which summarizes the fate of the three germ cell layers:

Slide 33, which shows somites and somatomeres:

The confusion arose, I think, from this text on slide 24:

The way it’s written, it sounds like paraxial mesoderm gives rise to the bones and muscles of the body…but not the bones and muscles of the head. And that contradicts slide 33, which says that somatomeres (which are derived from paraxial mesoderm) turn into the muscles of the head! In this slide, I was trying to summarize things as much as possible, so there wouldn’t be so many separate things to memorize – but in doing so, I made the paraxial mesoderm more confusing (not less!).

So here are the correct derivations for the bones and muscles of the head and body:

  • The bones of the head are derived from neural crest.
  • The bones of the rest of the body (everything except the head) are derived from paraxial mesoderm.
  • The muscles of the entire body (including the head and the rest of the body) are derived from paraxial mesoderm.

I edited slide 24 to make this more clear.  Here’s the new slide (the updated text is circled in red):

I hope this makes sense now! I updated the embryology ppt and pdfs on our lecture page so they all contain the new slide. Let me know if you have questions!

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.