all right in this video today we're going to talk about the ectoderm before we get started in this video please hit that like button comment down the comment section and please subscribe also down the description box we'll have links to all our social media platforms for you guys to communicate and support us if you'd like to all right let's get started Alright Ningeners, so before we start talking about what the ectoderm starts deriving into, let's talk about the development of the ectoderm. It's going to be a brief review of this, right? So first thing you guys need to know is we're gonna start off here with a bilaminar disc, right? So your bilaminar disc.
What does that mean? That means that we have two layers here. You have an a top layer, which is called your epiblast layer. So this is called your epiblast layer. Then you have the layer on the bottom here, this green layer, and that's called your hypoblast layer.
Now, just to give you some orientation, this little blue dot here, that's called your precordal plate. That's towards the head end of the embryo or the cranial end of the embryo, right? What happens is the bileminar disc, right, starts having some of this tissue here start kind of like melting or degrading away.
And it forms this little like structure here that's really important. You see this little tube here, which is kind of some of these epiblast cells kind of like this broke down and actually moved away and create this little tube. That little tube here is called the primitive streak.
What happens here is that some of these epiblast cells, guess what they start doing? Some of these epiblast cells, right, they start becoming mobile. And when they start becoming mobile, guess what they start doing?
They start moving, right, from this epiblast layer down through this primitive streak. When they start moving down through this primitive streak, they start moving in between the layers, right, between the epiblast and the hypoblast layers. And also when they move down through this primitive streak, they also move towards the cranial end. When they start doing that, guess what starts happening? As these epiblast cells migrate through the primitive streak, start sandwiching in between these layers and moving towards the cranial end, it then converts the bilimiter disc into the tri-laminar disc.
So that's an important thing to know. So what happens here is very important. We started off with those epiblast cells.
They move through the primitive streak. And what did we say? Well, the first thing we said here is that it starts kind of migrating between the epiblast and hypoblast layers.
Well, actually what really happens is that some of these cells start actually invading the hypoblastic layer. When some of those... epiblast cells invade the hypoblast layer, they convert the hypoblast layer into a new type of layer.
What is that new type of layer here? You see how the color changed? This is called your endoderm. It's called your endoderm.
Then what happens is, again, some of those epiblast cells, when they move through that primitive streak, they move between the epiblast layer and this new endoderm layer. What is that layer that they form between them? That's now in red. That's the mesodermal layer.
So this is your mesodermal layer. And then what happens is at the end, some of those epiblast cells start actually differentiating themselves and turning this whole new blue layer into a different type of layer here. Another name, if you will, after these epiblast cells differentiate, they become what's called the ectoderm.
So the ectoderm is actually basically... Differentiated epiblast cells is what they are. And how do we get the ectoderm?
We have to go from a bilimiter disc to a trilimiter disc. What is that process called? Gastrulation.
That's an important term to remember. So in order to get from a bilimiter disc to a trilimiter disc, what had to happen? The gastrulation process. And we've talked about this in great detail in our other video in the embryology playlist if you guys want to go watch that more.
Now, We formed our trilimiter disc, which consists of our ectoderm. Now what starts happening? Something next is going to happen.
We're going to start forming the notochord. That's the next process. We need to make the notochord.
So how do we do that? From the trilimiter disc, which consists of your ectoderm, right? Your mesoderm and your endoderm.
Then what you got to do is you got to make your notochord. So something really interesting happens. Again, remember how we said all those epiblast cells and the bilimiter disc were moving through the primitive streak? We didn't talk about this other little divot here called the primitive.
Node so now some of those ectodermal cells will start moving through that primitive node now when they do it It's really weird the way they do it when they move through the primitive node. They move in a tubular process towards the pre-chordal plate or the cranial end when they move towards that direction of the pre-chordal plate and make this kind of like little tubular process Eventually they form an entire cord or tube underneath the ectoderm and in between the mesoderm. What is that little tubular process that they form from the ectodermal cells moving through the primitive node?
It is called the nodochord. So we're going to form the nodochord next. Okay.
So we got bilimiter disc, trilimiter disc, and then we made our nodochord. Why is this important? Here's why.
The nodochord starts secreting certain types of growth factors. and proteins that start stimulating these ectodermal cells to proliferate and thicken. When that happens, guess what it does to the ectoderm? It really thickens up and causes proliferation of these ectodermal cells here. You see how this is kind of like a thickened area on the ectoderm?
And then again this would be our what would be here our pre-cordial plate. This here whenever that notochord starts secreting those substances that stimulates the ectodermal cells to proliferate, It forms a very specialized thickened structure here called the neural plate. So then we're going to form what's called the neural plate.
Now what happens is, is in the central portion of the neural plate, it's going to form a divot. So it's going to form a divot and what will that look like? Look over here.
Then as we have that neural plate and we form like a divot in it, it forms like a little groove. You see this like little groove here? What is that called?
It's called the neural groove. So then you form what's called the neural groove after you form that neural plate. Now as you form the neural groove, you'll notice something.
What are these little things here coming and trying to approach each other? These two ends, they're trying to kiss one another or touch one another. What is that called?
Those are called your neural folds. That's very important. You want to know why? Because some of the cells in the neural folds start to differentiate. You know what those are called?
Neural crest cells. So some of these cells in that neural folds will start to differentiate and make what's called neural crest cells. Now that is very important and we'll talk about how that becomes very apparent in a second. Now watch what happens here next.
We take the plate, form a divot, get a groove, have the folds there approaching one another. Some of the neural folds differentiate into neural crest cells and eventually those folds touch one another, fuse, and then bud underneath the ectoderm, sandwiched in between the mesoderm. And what do we result out of that? Once that happens, look what you get here. You get your neural tube.
You have your neural tube here. And what's following on the side of the neural tube that gushed over the edges when the neural folds fused together? Your neural crest cells.
Beautiful. So what we need to now talk about is what in the heck does that neural tube become is what's really important. So what I want to do is pull this neural tube out and the neural crest cells that are gushed on the side of it, pull that out and talk about what it becomes. All right, so we're going to yank that neural tube out with the neural crest cells kind of along the edge, right? So here's what I want you to imagine here, just so we have some orientation here.
Here's our precordal plate, right? I want you to imagine that the precordal plate would be here at the top. Right?
So if that's the case, then whenever this neural tube is developing, you see how it's kind of still a circle. It hasn't closed off. Technically, that's kind of like a little opening there. And that's called a pore called the anterior neuropore, right?
So it's called anterior neuropore. Now, generally, that pore doesn't close unless you have not lots of folate in the blood, but it doesn't close until around day 24, day 25, right? So around these days, eventually, that tube will close and form like a nice little structure like that. Same thing here towards the cloacal membrane.
There's going to be a little pore. Okay. And that's called your posterior neuropore. And this is also important because again, in order for this pore to close, you need lots of folate. Whenever it does close, it's usually around days 26 and 28. Okay.
So once this eventually closes, we avoid any types of kind of disorders like spina. bifida and encephaly meningomyelitis all that stuff that's why it's important to know this what happens is this neural tube it starts undergoing vesiculation which we talk about in future videos and it becomes eventually what your central nervous system what is your central nervous system made up of Well, it's made up of the brain and the spinal cord, right? Well, what is all that brain and spinal cord stuff made up of?
Well, you need to have neurons, right? Obviously. You need to have glial cells.
What are the glial cells of the central nervous system? Primarily the astrocytes and the oligodendrocytes. What else?
You also have to consider that the posterior pituitary, which kind of comes off of the what? Comes off of the hypothalamus. also a consistent of the central nervous system.
Here's two other areas. Another one is the pineal gland. Don't forget this one.
It's kind of an interesting one, but don't forget that one. And one more that you commonly wouldn't think of is the retina. Okay? Alright, beautiful.
So that covers the derivations from the neural tube. Now we got to talk about those neural crest cells. We pulled the neural tube out.
Here it is. All these crest cells are on the sides of it that we formed from the neural folds differentiating. What do these neurocrest cells become? This is a son of a gun, so I'm going to give you a mnemonic to remember this.
Alright, so the neurocrest cells, they make so many different things, they're derived into so many different things. So let's use a mnemonic to help us remember this. And so I helped to come up with this mnemonic for what the crest cells derive into, and thankfully, it is crest cell.
So, what does this crest cell mnemonic help us to figure out? Neurocrest cells can become what's called chromophin. Cells.
You know, chromaffin cells are basically located within the adrenal medulla. That's one thing. This one's a little bit of a stretch, but it was the best I could come up with, which is rostral tissues. So rostral means towards the head, right?
So it's all the connective tissue, the bones, and even some of the muscles of the head and neck. A little bit of a stretch, but It gets the job done. So rostral tissues, all the connective tissue, bone, muscles of the head and neck. E is for the enteric nervous system.
Okay, for the enteric nervous system. S is a double. It's for the satellite cells and the Schwann cells, which are the glial cells of the peripheral nervous system.
So the Schwann cells and the satellite cells. The other aspect of the peripheral nervous system is the neural tissue of the peripheral nervous system. So don't forget the peripheral nervous system.
What aspects? All your spinal nerves and their ganglia. All of that stuff as well. Then come down to the cardiac area.
You see these little like blue dots coming around the carotids? What is this? This is called our carotid bodies.
The carotid bodies. And these are important because they measure concentrations of oxygen and CO2. Then, you know there's tissues here that make the valves, right? And the heart. This is called our endocardial cushions.
They help with kind of forming the septum and some of the valves. Then, another little bit of a stretch, but light skin and dark skin. What gives skin its kind of color? melanocytes.
So don't forget that one. And then last but not least is your leptomeninges. What is your leptomeninges of the brain?
This is your pia matter and your arachnoid matter. Okay? That covers our crest cells. Okay now that we've covered the derivations of the crest cells based upon this mnemonic now let's talk about the other aspect other than the neural tube and the neural crest cells what the ectoderm derives into. Alright so the next thing we got to do is we talked about how the ectoderms arrives into the neural tube and the neural crest cells but there's other parts of the ectoderm that we didn't talk about what that derives into.
So what I want us to do is the best way to look at these next things is to look at the top of the embryo where you only see the ectoderm. So you know how like for example I'm just gonna draw like a little thing here We had, we were drawing like the trilemma or disc here, right? And we were looking at it from this view, kind of like in this way.
I'm going to be looking down from here. And this is what I'm going to see. But the question is, is what stage in the development of the embryo are we actually at where I'm looking at the top of the embryo at the ectoderm? We're at the level.
the neural plate. So whenever that embryo was forming that neural plate, this is what we're looking at the top of the ectoderm. That's what we're seeing.
So here's that neural plate that we're forming around the primitive streak and the kind of like the primitive node area, right? Well what happens is you see these thickened areas here, all these like thickened areas of tissue that's forming there, these are called placodes and there's special types of placodes that we give names for. These ones here that are in black at the top, this is called your olfactory placode.
The one that's kind of just a little bit lateral to that is going to be the lens placode. And then the one that's most lateral and kind of inferior more specifically to that is going to be the otic placode. Now there's another set of placodes that sit over here. They're, I'm not super worried you guys know them.
I'm just going to mention them, but we're not going to go over all of them in detail. There's these little placodes that sit kind of right here. And these have a couple, there's three of them. One is called a no-dose placode. The other one is called the trigeminal placode.
And the last one is called the geniculate placode. These actually derive all into what's called ganglia. So the no-dose. Ganglia, the trigeminal ganglia, and the geniculate ganglia.
Why is that important? Because these are ganglia for cranial nerves. If you really wanted to know which ones, you guys go on our cranial nerve playlist, you'll see that we talk about these in that video, okay? But, that just to give you an idea of what those ganglia actually are and become. The last one that we'll talk about, which we'll talk about with more emphasis, is all the remaining.
So imagine everything else that's not where the placode and this brown neural plate is, all of that is called surface ectoderm. Okay, so all the remaining parts of the ectoderm not covered with placodes and neural plate is called the surface ectoderm. So now what I want to do is, is I want to talk about what these placodes make and what that surface ectoderm makes. Alright, so the next thing that we need to talk about now is what these placodes become.
So the first one here we're going to talk about is the olfactory placode, right? So this one is, these are obviously pretty straightforward. You guys could probably already tell by the drawings what we're talking about.
The olfactory placode takes place and becomes the tissue at the roof of the nasal cavity, very important. What is that tissue here called? It's called the olfactory epithelial tissue.
And actually I should say epithelium because what happens is this is actually made up of a bunch of different things. Obviously you have kind of like your receptor cells, but you also have like supporting cells. The big thing here is that some of these receptor cells are actually nerves, the olfactory nerves that are actually kind of in that area of the olfactory epithelium.
So what is the whole significance of the olfactory code? This is going to be for smell. It's going to be able to help us to pick up smell.
The next one, the actual next one that we're going to talk about what it becomes, obviously pretty obvious here is your lens placode. What do you guys think it becomes? The lens.
So the lens, what does that do? Well, really the lens is important for what's called accommodation. What the heck does that mean?
It helps us with our far vision and it helps us with our near vision. The whole process that we talk about in our special senses playlist. But basically, it's kind of the ciliary muscle contracting and making the lens either globular or flat. And that changes our kind of like the way the light is refracted off of it. So again, that's another important thing.
The next one is the otic placode. Okay, the otic placode. Otic is ear, right?
So what do you think this is going to become? The inner ear. When we talk about the inner ear, what do we mean? We're talking about a couple structures here. One is we're talking about the cochlea, which is for...
hearing, sound waves. The other one is for your vestibule. And your vestibule is for basically your static equilibrium.
And your last one is your semi-circular canals. And this is for your dynamic equilibrium. The last one is called our surface ectoderm.
Now the surface ectoderm is the one that actually is pretty cool and actually it's relatively simple really when you when it comes down to it surface ectoderm becomes epithelial tissue let's write that down now in some ways that can be like okay well there's epithelial tissue all over the body and you're exactly right what areas are we talking about we're talking about the epidermis people your epidermis all over your skin that's made up of epithelial tissue Your nails. Did you know your nails are actually epithelial tissue? Your hair.
That's epithelial tissue. And what else? Your sweat glands. That is also epithelial tissue.
But you know what else is important? You have epithelial tissue that lines the beginning of your cavities. In the upper part of the body and the lower part of the body. So if we talk about the epithelial tissue near orifices... It also lines the nasal cavity, except for what part of the nasal cavity?
The roof of the nasal cavity, because that comes from the olfactory placode. It also lines the oral cavity. And what else? The ear canal. We didn't talk about that.
What about the external ear canal? And last but not least, the inferior anus, right below the pectinate line. One last thing that I have to talk about because they love to ask this on exams You know the tissue right if I were to draw here kind of the side view here You have kind of the nasal cavity here right and then back here is your nasal pharynx. Well that Epithelial tissue from the surface ectoderm forms this what happens is some of these tissue starts forming like this little pouch that starts budding backwards You know that pouch is called It's called a Rathke's pouch. Rathke's pouch eventually buds off and then fuses with the structure coming off of the hypothalamus.
Do you remember what that structure was? The posterior pituitary. Let's switch the color though. Let's make the posterior pituitary, just so we know here, we'll make it orange, coming off of the hypothalamus. What does that actual Rathke's pouch become?
The anterior pituitary. Isn't that cool? So the Rathes pouts also helps to make the anterior pituitary.
Beautiful. All right, Ningeners. So let's end this off with a nice little mnemonic to help you guys to remember all the derivations of the ectoderm that we talked about here. E, within the ectoderm, guess what it can mean? Epithelial tissue.
What epithelial tissue? Both the epithelial tissue outside the body and the epithelial tissue that lines those beginning orifices, right? C for central nervous system. T, this one's a little bit of a stretch, but the lens placode. What does the lens placode become?
That becomes the lens. And then O is a double, the otic and olfactory placode. And this becomes the olfactory epithelium and the inner ear respectively, right?
D, those dang crest cells, they're so complicated. But we got our own mnemonic for those, right? E. E is for?
I forgot. No, it's eyes, right? But particularly what part of the eyes? We're talking about the retina. And again, the only reason I put this one in here is because it's a part of that central nervous system, but it's not common to think about it like that.
So I just added that one in there. The next one, we spent some time talking about this. Rathke's pouch. Remember that buds off of the epithelial tissue from that nasopharynx area and becomes the? anterior pituitary, right?
Last one here is melatonin, just because again, it's an interesting one. You don't commonly think about this, but this is going to be thinking about the pineal gland. And again, that's still a part of that central nervous system, but just something to help you guys to remember the ectodermal derivatives. I hope that helped.
All right, Nidgeners, so in this video, we talk about the ectoderm. I hope it made sense. I hope you guys did like it. All right, Nidgeners, we love you. Thank you.
And as always, until next time.