USMLE question - Ossification

[Phloston]

USMLE Rx says that "in addition to long bones, a variety of other bones form from endochondral ossification: sphenoid, ethmoid, incus, stapes, malleus, limb girdles, vertebrae, sternum and ribs. Bones that form via intramembranous ossification include the frontal, parietal, maxilla, zygomatic, palatine and mandible."

Now that confuses me a bit because in First Aid 2012, on the bottom of p. 413, it says for membranous ossification: "Flat bone growth (skull, facial bones, and axial skeleton)."

The incus, stapes and malleus are together the ossicles of the ear. Those, combined with the vertebrae, sternum, ribs, hyoid bone and skull, comprise the axial skeleton.

Do you see a discrepancy here?

It appears as though FA may be over-generalizing by saying that the facial bones ossify via the membranous route, because clearly the ethmoid and sphenoid bones demonstrate a cartilaginous model during their development.

At the same time, that doesn't explain the fact that Rx says that the limb girdles, vertebrae, sternum and ribs undergo endochondral ossification. Are not the latter three part of the axial skeleton, and therefore undergo membranous ossification?

Could anyone help out here? I recognize this isn't the most straightforward question, so if anyone could offer up input and/or sources I would be very appreciative!

Thanks,

~Phloston

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[ray656712]

Hey @Phloston what did you come up with on this one? Ive cross referenced several sources and the consensus seems to be that ribs and pelvis are formed via endochondrial ossification, but UWorld (and I believe pathoma as well) says they are formed via membranous ossification

 

[Phloston]

Hey @Phloston what did you come up with on this one? Ive cross referenced several sources and the consensus seems to be that ribs and pelvis are formed via endochondrial ossification, but UWorld (and I believe pathoma as well) says they are formed via membranous ossification

Wow, look at this throwback thread. It's like a time machine, and I say, "did I really write this?" Thanks for linking me back here.

I would suspect bones such as the ribs and pelvis would be membranous ossification, particularly the ribs due to their "flat" nature. But apparently there's part of the rib that's also endochondral.

Sorry I'm not more of a help here. The USMLE, if they asked this stuff, would focus more on the process rather than a pedantic detail anyway.

Know achondroplasia in and out.

Know that if cartilage is involved or mentioned, it's endochondral.

 

[ray656712]

Wow, look at this throwback thread. It's like a time machine, and I say, "did I really write this?" Thanks for linking me back here.

I would suspect bones such as the ribs and pelvis would be membranous ossification, particularly the ribs due to their "flat" nature. But apparently there's part of the rib that's also endochondral.

Sorry I'm not more of a help here. The USMLE, if they asked this stuff, would focus more on the process rather than a pedantic detail anyway.

Know achondroplasia in and out.

Know that if cartilage is involved or mentioned, it's endochondral.

Great thanks, and achondroplasia is exactly why I was asking. From what I've read, the ribs are uneffected in achondroplasia, leading me to assume they are formed by intramembranous ossification, which contradicts what FA says. I'm sure if asked that the question will have enough info to deduce what they want thought!