Ectoderm

Definitions & Key takeaways

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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When the embryo is one week old, it has two layers of cells: a dorsal or outer epiblast layer and a ventral or inner 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 like a strawberry birthday cake with the ectoderm as the candles, the mesoderm as the lime frosting, and the delicious cake as the endoderm.
We can put three candles on this cake to help you remember gastrulation happens during week 3. During gastrulation, some mesoderm cells start to differentiate and they form a structure called the notochord - a plate of cartilage that’s a precursor to cartilage at the center of the intervertebral disc called the nucleus pulposus.
The notochord starts a process called neurulation where it stimulates the cells in the ectoderm layer causing it to thicken forming the neural plate, the neural plate starts to fold, and it dips down forming a neural groove with edges that are called neural folds.
As the groove continues to grow ventral to the ectoderm layer, the neural folds comes together and they 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 forms 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, part 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. Once the cranial and caudal neuropores have closed, the neural tube is formed.
Meanwhile, ectoderm, or surface ectoderm now that the neural tube has closed, starts to form the ears, and eyes. Two bumps, called the otic and lens placode, form near the cranial end of the ectoderm.
Over time, the otic placodes will form otic vesicles, and they develop into the cochlea and the inner ear. At the same time, the lens placodes develop into the lense and cornea of the eyes.
Finally, many of the ectoderm cells go on to form sensory epitheliums like those in the nose and mouth. Others will go on to form the epidermis layer of the fetal skin, as well as structures associated with the skin like hair, nail, sweat glands, and mammary glands.
All right, as a quick recap…the ectoderm is the dorsal most germ layer, and through a process called neurulation forms the neural tube.
From the neural tube, neural crest cells migrate to help form the central nervous system, the peripheral nervous system, the sensory epithelium of the ear, nose and eye, and mouth, as well as glands such as sweat glands, mammary glands, and the pituitary gland.
Here’s a story to help you remember the important structures derived from the ectoderm: A man touches the ectoderm candle on the strawberry cake.
His skin (epidermis) and nails gets burned and turn black (melanocytes) and he feels a sharp pain (central and peripheral nervous system).
He pulls his burning finger back, but pokes himself in the face, setting his head on fire. His eyes, ears, nose, hair, and even teeth get set on fire.
In a last ditch effort to put the fire out, his body starts sweating and lactating (sweat and mammary glands) and even tearing up (pitui-teary gland).