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!

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