Endoderm
Definitions & Key takeaways
Endoderm refers to the innermost layer of cells in a developing embryo. It gives rise to the lining of the gastrointestinal tract, the liver, pancreas, and gallbladder. It also gives rise to the respiratory tract, the thyroid and parathyroid glands, and the lining of the bladder and urethra.
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.
We can put three candles on this cake to help you remember gastrulation happens during week 3. Backing up just a bit - around day 15 of development, two areas of the ectoderm layer- one in the cranial region and one in the caudal region push ventrally and fuse with the endoderm layer, excluding the mesoderm layer entirely, and forming two bilaminar regions in an otherwise trilaminar disc.
The cranial bilaminar region develops into the oropharyngeal membrane which eventually disintegrates in week 4 to form the opening of the mouth.
The caudal bilaminar region develops into the cloacal membrane, which eventually disintegrates in week 7 to form the opening of the anus and genitourinary tracts.
During week 4 of development, the embryo folds in two directions. In the longitudinal plane, there is a cranial and caudal fold, so that it looks less like a pancake and more like a little shrimp.
At this point, the embryo is just beginning to curl into the fetal position. The folding shapes part of the yolk sac into a gut tube, with the remainder of the yolk sac remaining connected not at the cranial or caudal end, but just in the middle.
Now, let’s switch to the transverse plane looking at a spot near the middle of our embryo where you can see the yolk sac.
In the transverse plane, the lateral plates of the mesoderm split into a dorsal layer called the parietal or somatic mesoderm layer and a ventral layer called the visceral or splanchnic mesoderm layer.
More specifically, the endoderm becomes the epithelial cell lining of the gastrointestinal tract while the visceral layer of mesoderm becomes the muscular wall.
The mesoderm also gives rise to serous membranes that become the visceral and parietal pleura. Two layers of the visceral pleura come together to form the mesentery - which was reclassified as a body organ.
The mesentery suspends the gut tube in the abdominal cavity. Meanwhile, the ectoderm and parietal layer of mesoderm fold around with the two sides meeting up and seamlessly coming together in the midline to form the anterior body wall, everywhere except in the middle where the yolk sac still pouches out.
Once that happens, we end up with the gut tube inside the embryo’s body, so in a sense embryonic folding creates a tube inside a tube.
Around week 5, the foregut also gives rise to the trachea and lungs. The midgut remains connected to the yolk sac, and this connection which is called the vitelline duct or yolk stalk, gets thinner and thinner over time.
The endoderm lines the yolk sac which sits outside of the body of the developing embryo, so the vitelline duct passes through the umbilical ring to get to the yolk sac.
Over time the yolk sac gets smaller and smaller, and eventually the vitelline duct completely obliterates. Like the oropharyngeal membrane, the cloacal membrane breaks down as well, except it happens around week 7.
When that happens, the upper two thirds of the anal canal which comes from the hindgut, meets up with the lower one third of the anal canal which comes from the ectoderm.
In the adult, the line where they meet up is called the pectinate line. One way to remember all this, is to think of our cake analogy.
When you take a bite of that delicious strawberry birthday cake you open wide so that it doesn’t touch your mouth, and instead it first touches your pharynx, your esophagus, and goes all the way through the gastrointestinal tract.
But you like it so much that you don’t want to poo it out so it doesn’t give rise to the last bit of the gastrointestinal tract - below the pectinate line of the anus.
The cake can also go down “the wrong way” into your trachea so it also forms the lining of the respiratory tract and lungs.
So in addition to becoming the epithelial lining of the gastrointestinal tract and respiratory tract, the endoderm gives rise to the tonsils, the thyroid, parathyroid glands, thymus, part of the liver, gallbladder, as well as the pancreas.
To remember this, we can build on the analogy. If you want to digest the cake, you need the help of digestive organs like the pancreas, liver, gallbladder so it makes sense that these would be derived from the endoderm.
Moreover, if the cake goes down the respiratory tract, it could get stuck on tissue and organs near the trachea like the tonsils in the throat, the thyroid and parathyroid in the neck, and the thymus in the chest.
Finally, endoderm cells also forms parts of the ear, and the epithelial lining of the urethra and urinary bladder. We can remember this last section by thinking of places where you should never stick cake, like in the ear or in the urethra where it might end up in the bladder!
All right, as a quick recap… When the embryo folds in the longitudinal plane and in the transverse plane, it helps to change the gut tube so that it starts looking like a tube within a tube with a central vitelline duct attached to the yolk sac in the middle.
The endoderm ultimately gives rise to the gastrointestinal tract leading from the pharynx to just above the pectinate line of the anus.
It also gives rise to the liver, pancreas, and gallbladder, the respiratory tract, the tonsils, thyroid, parathyroid glands, thymus, parts of the ear, and the lining of the urethra and bladder.
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