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USMLE® Step 1 questions

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USMLE® Step 1 style questions USMLE

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A researcher is studying the development of the peripheral nervous system in a human embryo. Which of the following embryological structures are primarily responsible for forming the Schwann cells that myelinate peripheral neurons?  

External References

First Aid









branchial clefts p. 643

derivatives of p. 637


When the embryo is a week old, it has two layers of cells: a dorsal epiblast layer and a ventral hypoblast layer.

During week 3 of development the embryo undergoes gastrulation where the cells in the epiblast layer form a three-layered trilaminar disc with an ectoderm, mesoderm and endoderm layer.

So, imagine the embryo is like a strawberry birthday cake with the ectoderm as the candles, the mesoderm as the lime frosting, and the delicious sponge cake as the endoderm.

We can even put three candles on this cake to help you remember that gastrulation happens during week 3.

During gastrulation, some mesodermal cells start to differentiate and they form a structure called the notochord, a rod of cells that release different genetic transcription factors that help embryonic cells develop into the body’s various organs and structures.

The notochord also kickstarts a process called neurulation, stimulating the cells in the nearby ectoderm layer to thicken and form a layer of cells called the neural plate.

As it forms, the neural plate starts to fold, and it dips down to form a neural groove with edges called neural folds.

As the groove continues to deepen, ventral to the ectoderm layer, the neural folds comes together and pinch off from the surface of the ectoderm layer, forming the neural tube.

The neural tube now sits between the mesoderm and the ectoderm.

On the dorsal side of the neural tube where the neural folds fuse, there are special cells called neural crest cells that migrate out and form a new layer of cells between the ectoderm and neural tube.

Neural crest cells are like little explorers: they migrate throughout the developing fetus to form a variety of tissues including the peripheral nervous system, melanocytes in the skin, specific parts of the facial bones, chromaffin cells of the adrenal glands, and parafollicular C cells in the thyroid.


The ectoderm is the outermost germ layer in animals. It gives rise to the skin, nervous system, and sense organs. In the early embryo, it is the first layer to form from a fertilized egg. The ectoderm cells are constantly moving and changing as they develop into different tissues.


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