Ectoderm

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.

In fact, neural crest cells are responsible for so many of the body’s organs and tissues that you might think of them as the body’s unofficial fourth germ layer.

At this point, like any hollow tube, the neural tube still has openings at both ends: a large opening at the top end called the cranial neuropore, and a smaller opening at the bottom end called the caudal neuropore.

The cranial neuropore seals up around day 25, while the caudal neuropore seals up a few days later, around day 28.