Hi Ninja Nerds, in this video today we're going to be talking about the development of the respiratory system. Before we get started, please make sure you guys continue to support us by hitting that like button, commenting down in the comment section, and please subscribe. Also down in the description box, we'll have links to our Facebook, Instagram, Patreon.
Go check that out. All right, Nidgeners, let's get into it. All right, Nidgeners, so when we talk about the development of the respiratory system, let's start again understanding how we get the respiratory system. We actually developed the respiratory system from two germ layers. One is going to be the endoderm, and the other one's going to be the mesoderm.
So first thing we have to talk about is going from our bilimiter disc to our trilimiter disc, do a quick folding process in both the lateral. folding process and cranial caudal folding process and then we'll talk about some of the structures that are developing from these layers that being the larynx the trachea and the lungs. Alright so first things first start up here at the top we have our bilimiter disc right which is around week two right so this is going to be made up of our epiblast layer on the top epiblast layer and the epiblast layer remember above that is going to be the amniotic cavity right and then below the epiblast layer is going to be the hypoblast layer. And then below the hypoblast is going to be like the little yolk sac layer, right? What happens is, you guys remember, you develop a nice primitive streak, right, in the epiblast layer.
Some of those epiblast cells, move through the primitive streak and convert the hypoblast layer into the new endoderm layer, this bottom green layer. So here's your endoderm. Then more epiblast cells move through that primitive streak.
As it moves through the primitive streak it forms a new layer between this new endoderm and this epiblast layer. What is this called? That's called your mesoderm. And then we also know that this little black dot here is your node accord.
That's from the epiblast cells moving through the primitive node. And then you go through the process of what's called nurulation, and you make this maroon color structure called your neural tube. Then finally, the epiblast cells... cells start differentiating themselves and turning into ectoderm. And we should all know what this process is called, where we take a bilimiter disc and turn it into a trilimiter disc.
We have done this at least a million times. This is called gastrulation. And if you guys remember what else did we have kind of happening at this process here as well besides making two layers to three layers we also developed the notochord so there's also this notogenesis that's also occurring. All right sweet now we have have our three layers. Now it just looks like a nice little pancake, right?
We need to look at this kind of in a three-dimensional way. So what we're going to do now is I want you guys to imagine that we're going to fold this pancake in two directions. One is we're going to fold this, this is a cross-section.
I'm going to fold this in a lateral type of way and bring this together or you can only, you can't really see it in this view, but if I were to pull out from this point, here would be the top of the embryo or the cranial portion and going back into the bore would be the caudal portion. It's going to fold this way which is called cranial caudal folding. So we're going to go through the two folding processes here.
One is going to be the lateral folding. Alright, and then the other one is going to be the cranial caudal folding process and what this does is this kind of just gives us a little bit of an idea of more of the anatomy and kind of how these layers will arrive into the structures that look familiar to us in the adult. Alright beautiful. So this top one is the after lateral folding and this bottom one is after cranial caudal folding. Now what you guys need to remember here is this structure here.
Remember I told you that there's two things that are making up your respiratory system, two germ layers, your endoderm and your mesoderm. All right, so this first thing here is this green layer. What is that called?
Your endoderm. Now this endoderm, you guys need to remember that the endoderm is basically going to give way to what's called your gut tube, the epithelial lining and the glands of your GI tract, right? So it's going to make that thing called your gut tube. And there's different portions of the gut tube. We'll talk about that.
It's easier to see in the crane. your caudal section. The other thing that we need to know is this mesoderm.
Now you guys remember what are the mesoderm layers? This one closest to the neural tube is called your paraxial mesoderm. You guys are going to be pros at this, right?
Paraxial mesoderm. Then after that a little bit more lateral is your intermediate mesoderm. And then the ones that we care about, you have the two layers here, right? This one that's closest to the ectoderm. What do we call this one?
is your somatic layer of the lateral plate mesoderm. We're just going to abbreviate that lateral plate mesoderm. And then this one here closest to the endoderm or the gut tube is going to be your splanchnic layer of, we're going to abbreviate it lateral.
plate mesoderm. The two layers I really want you guys to remember here is the endoderm that's gonna make the epithelial lining of the respiratory tract in this case which we're going to talk about larynx trachea and what else lungs and the splanchnic layer of the lateral plate mesoderm is going to help to make some of the cartilage the muscle and the connective tissue around the larynx and the trachea and the lungs we'll get a little bit more detail on the larynx because that's a little bit different for but for the most part endoderm is going to make the epithelial lining and the splanchnic layer is going to make the cartilage muscle and connective tissue of the respiratory tract now let's come down here and look at this cranial caudal type of fold all right beautiful so we We've seen this in the lateral type of process, the lateral folding to make this nice cross-sectional diagram. So we kind of have an idea of what layers are going to give us most of the respiratory system. We'll get a little bit more specifics, but for right now just understand those two things are going to make up most of the respiratory system. Now this sagittal view after cranial caudal folding also gives you a really nice representation of kind of where the respiratory system is coming from.
So if you guys remember, this green tube here is your gut tube. We only see it at one section. For example, maybe I just took it at that section. We're only seeing a portion of it.
But I want you to remember that there is three portions of it. You have the foregut here, the midgut here, and the hindgut here. We don't really care about the midgut and the hindgut. We really only care about this foregut.
And then at the top of this endodermal tube, we're going to see that there's like these little pouches that are part of your pharyngeal apparatus. And these are... also the structures that we care about.
So we care about two layers of this endoderm, the foregut, and this area, what we're going to just denote it as the primitive pharynx, but that's going to be the pharyngeal apparatus. And we'll talk about that a little bit later. All right.
So this is a beautiful view to kind of give you an idea. So what I want us to know here is what are the three structures we are going to make from this endoderm and from that splanctic layer of the lateral plate mesoderm. You're going to make the larynx.
You're going to make the trachea. and you're going to make the lungs. Okay? These are the three structures. Now, just for the basics, we'll talk about a little bit more.
The larynx is going to have two layers. They're all going to have two layers, if you will. An endoderm layer and a mesoderm layer.
Which mesoderm layer, though? The splanchnic layer of the lateral plate mesoderm. So they'll all have this. What I want you to understand is what they give rise to. Okay?
That's kind of the big thing here. So, first things first. When we talk about the larynx, this is really the one that I kind of want to focus on because we're going to talk about this next.
The endoderm is actually going to be particularly coming from a structure called the pharyngeal pouch. It's going to be coming from a structure called the pharyngeal pouches. We'll talk about the fourth and sixth pharyngeal pouches. These are the ones that are really going to give rise to the epithelial lining of the larynx.
The mesoderm is going to come from the part of the pharyngeal arches. the fourth and sixth mesoderm component of pharyngeal arches. The rest of all of this stuff is endoderm from what? This is endoderm from the foregut. And we'll talk about how it actually buds off the foregut.
Then the mesoderm is still that lateral plate mesoderm the lateral plate mesoderm Lateral plate mesoderm which component the splanchnic layer the lateral plate mesoderm So that's why I wanted to just specify a little bit more here on the endoderm and mesoderm portion for the larynx the tree trachea and the lungs. Alright, so now what we gotta do is we have to take and make the larynx, then make the trachea, then make the lungs. Let's go through the larynx first.
Alright, so now that we've kind of built the foundation for developing our respiratory system, let's go by each component first. So we're gonna develop the larynx, then we'll do the trachea and the lungs. So first things first, we gotta take a look here at a nice little sagittal section of the embryo. This kind of gives us a really good kind of start for the development of the larynx.
What we're going to do is we're going to take here, remember we had foregut up here, we had your midgut, and you got that good old hindgut, right? And right here, you have these kind of like your pharyngeal apparatus, if you will, right? What I want us to do is to take a nice little section kind of like this here, Take a look at the layers, right, starting with the endoderm, mesoderm, and ectoderm of this good old pharyngeal apparatus. Okay, so this is going to be kind of our pharyngeal apparatus. All right, sweet deal.
So first thing I want us to go through is obviously start here at the top. This is your buccal pharyngeal membrane, right? So the buccal pharyngeal membrane is what will become the mouth. Now the inner portion here, the green portion, is the endoderm. But we give this kind of portion a special name.
So this inner portion here, this green portion, this green portion of the endoderm, right this portion here, we actually call this the pharyngeal pouches. So this is going to give way to what's called your pharyngeal pouches. Okay? And this is going to make the epithelium and the glands, in this case of the larynx. The next thing is in between that you have this next layer, this red layer.
And this red layer here is your mesoderm. Alright? So this right here is your mesoderm.
And the mesoderm is a part of this pharyngeal arches, right? So this would be your first pharyngeal arch, second, third, fourth. Sometimes your fifth either develops and then it actually digresses over time or it doesn't develop at all. So we don't even consider the fifth one.
And then your sixth pharyngeal arch. The core of these pharyngeal arch is made up of mesoderm. And this is what gives way to like muscle, right?
This is going to give way to your connective tissue, give way to your cartilage, all that good stuff, right? And then the outer part here of this pharyngeal apparatus, you have this blue portion, which is your ectoderm. And we actually kind of give these kind of like little things here, these little divots, we call it the pharyngeal clefts.
So the ectoderm, we give a nice special name, and we call that your pharyngeal clefts. But what I really want us to focus on here is, again, what did we say were the two components that make up the respiratory system? The endoderm, particularly the pharyngeal pouches, and the mesoderm, which is going to make the muscle, the connective tissue, and the cartilage. But here's the next part that we've got to build on.
What point? portion of this pharyngeal apparatus is actually making the larynx. That's what's important.
And it's really the fourth and sixth pharyngeal arch. Okay. So now what we're going to do is we're going to take this fourth and sixth pharyngeal arch and zoom in on it.
Approximately around week five. Okay. So somewhere around week five, within that fourth and sixth pharyngeal pouch, you develop a little cavity.
Okay. You develop a little cavity here and we call that cavity the laryngeal orifice. So you develop a cavity around week five called the laryngeal orifice. Now here's what's really cool.
Within that orifice, some of this endodermal tissue within these actual pharyngeal pouches start invading into this laryngeal orifice and after it kind of invades into the laryngeal orifice, guess what it makes? Guess what that endodermal lining makes? It makes your vocal cords.
So the vocal cords will develop from endodermal cells invading into the laryngeal orifice. Pretty cool, right? Alright, what else happens? Around week six, that mesoderm, right, around week six, that mesoderm starts forming cartilage and muscles that are surrounding this portion of the larynx and it starts to make these swellings, if you will.
So you start to develop these swellings around the orifice, okay? So around the orifice. What are these swellings that you start to see?
Alright, one of the swellings, let's do it here in this blue color, develops on the top of the laryngeal orifice. And this swelling here is called the epiglottic swelling. So the first swelling that you'll have here develops above the laryngeal orifice, this is called the epiglottic swelling.
Then below, you're going to develop another swelling that comes like this. And what is this swelling here called? This is called your arrythrocyte. arytenoid swelling. Okay, so this is called your arytenoid swelling.
Eventually, over time and over time, that mesoderm is continuing to make more cartilage. The endoderm is continuing to invade into laryngeal orifice and again, help those pharyngeal pouches to make a nice little epithelial lining of the larynx. At around week 12, we finally make somewhat of our mature type of larynx. So now we're going to come down here to about week 12. But along the way, here's what I want you to know. So in week 12, we'll make kind of our somewhat mature larynx.
Okay? Along the way that we develop some things. Here's what I want you to remember.
Remember, the endoderm, okay, the endoderm portion is going to give way to what? The epithelial lining of the larynx. What is the epithelium of the larynx?
You know there's a particular type of epithelium that develops within the larynx? This is... called we're gonna abbreviate it because there's no way in heck I'm gonna spell this the pseudo stratified ciliated columnar epithelial tissue all of that is derived from the actual endoderm the other thing is it's gonna help with making your good old vocal cords. And the other thing I want you to remember is that laryngeal orifice eventually becomes what's called the laryngeal inlet. So the laryngeal orifice becomes a laryngeal inlet.
Next thing I want you to remember is the mesoderm. We talked about how the mesoderm becomes muscle, it becomes cartilage, it becomes connective tissue. It's going to give way to a bunch of different cartilage, all right, all that laryngeal cartilage.
And I know you guys probably probably know that, right? What are all those cartilage? I'm not going to name them all.
I'm not going to write them all, but I'm going to name them. Let's see if you guys remember. You have your thyroid, your cricoid, then you have your arytenoid cartilage, you have your cuneiform cartilage, and your corniculate cartilage.
So all of that is going to be derived from the mesoderm. But it's also going to make some of those laryngeal muscles. The laryngeal muscles, right?
So all of those cricorytenoids, all your cricothyroid, thyroid muscles, all that good stuff. One more thing, what's the nerve that comes down and supplies the larynx? Because it's one of the things that are derived from the French Arches, your vagus nerve, right? So you also have to remember that your vagus nerve is also going to be derived from the fourth and sixth pharyngeal arches and that's going to be actually supplying what?
These laryngeal muscles. That's pretty cool. As well as giving sensory information to the epithelial lining of the larynx. Pretty darn cool. cool.
Alright, so now at around week 12 we develop somewhat of what looks like our nice mature larynx. You start to see here you have the epithelial lining of the larynx here, nice epithelial lining. Remember what happens to the laryngeal orifice? The laryngeal orifice becomes what? Becomes the laryngeal inlet.
And also what else starts to form here? Okay, so you have the epithelium, you have your vocal cords, laryngeal orifice becomes the laryngeal inlet, you even get some of your epiglottic folds here, and then you get all the cartilage around. What was the cartilage made from? The mesoderm. Then you get your muscles, the cricoretinoids, the cricothyroids, right?
Then you have your vagus nerve supplying those muscles, and what else can the vagus nerve do? Depending upon which portion it is, it can also penetrate through here and give sensory information to the actual laryngeal epithelium. Boom!
We have our mature larynx. Now that we've made our larynx, let's go on to making the trachea in the lungs. All right, so we've actually made our larynx. Now, let's go ahead and start making that trachea, those lungs, all that good stuff.
All right, you guys remember we got that nice good old sagittal section here. So again, what do we have here? We have that foregut.
We got that midgut, we got that highgut, right? And we already went over this top portion here if I were to draw like a little bit of, it's a little like sackies here, right? We already went through that pharyngeal apparatus up there, which made our larynx.
All right, so now what we're going to do here is we're going to take a portion of the foregut. So I'm going to go through a portion of the mesoderm, and I'm going to go through a portion of the mesoderm here and just focus on this portion of the foregut and look at it, okay? So this is going to be...
looking at it from the side and this is going to be looking at it kind of from the front end. Okay so you're looking at this from the front and you're looking at this gut tube from the side. What is happening to the foregut okay that's helping us to develop our trachea and lungs?
Around week four so sometime around that week four period off of the foregut so here's your foregut tube right so I'm going to abbreviate this foregut tube here's your foregut coming off of that some of the endodermal cells start kind of coming out and budding off of the foregut. And as they start budding off of the foregut, they make this tiny little bud, pretty straightforward, right? They make a tiny little bud called the lung bud.
So they make a bud called the lung bud around week four. Now, that's what you can see from the side view. From the frontal view or anterior view, something else is kind of important to define here. You see how coming off of this, we're going to draw... draw this in blue some of the endodermal cells come out here into this bud and create kind of this Nice little lumbar here.
Well they create a little groove if you will that goes into this anterior lumbar. What is this groove here called? This is called, it's a heck of a thing, it's called the tracheo esophageal groove or ridge and this is basically going to be where the endodermal cells are kind of coming out and forming that nice little long but ok now here's the next thing I want you guys to understand we're just looking at that gut But again, what is wrapping around that gut tube? What is this red layer here? Can't forget that.
What is this red layer here called? That's your mesoderm. What part of the mesoderm? That's your lateral plate mesoderm.
What part of the lateral plate mesoderm? Your splint. splanchnic layer of the lateral plate mesoderm.
What does that become? Cartilage, connective tissue, and muscle. What does the endoderm become?
Epithelial lining of the trachea lungs. Beautiful. We have an idea here.
As we continue to grow from week four all the way down to here about week five, we're going to continue to kind of see some small changes happen here. So we had that lung bud. Coming off of the lung bud, it starts to continue to replicate, replicate, replicate, and buds off into these two little sacs here.
Then what happens is if you look here, the lung bud... creates one little bifurcation right there. And we call this now, it went from a lung bud to what's called a bronchial bud, to what's called a bronchial bud.
And we can see that very nicely from this side view. But if you look here in the frontal view, remember how we had those tracheosophageal grooves or ridges? Look what happened to them. They completely come together and meet in the middle. And whenever they do that, they form a nice little septum between the anterior lung butt and the posterior foregut, which would become, in this case, the esophagus.
So now, not only do we have a groove kind of coming together and closing, that creates that nice little septum. So now we have what's called the tracheoesophageal septum. And that is beautiful because what it does is, is it starts helping us to separate the foregut, right, from the diverticulum coming off of it, which is going to be that respiratory diverticulum which will become the trachea and we know where's the trachea with respect to the esophagus anterior beautiful as we continue continue down what happens is that tracheoesophageal septum will completely close and separate posteriorly what's gonna be posteriorly here so whenever we have this continuously happening here that lung bud will continue to grow and what it'll do is will create a nice separate between what?
It'll create a separation and anteriorly you're going to have your lung bud, which will give way to your trachea and then eventually your lungs. And then posteriorly, you're going to have what? That is going to give you your foregut. In this case, this is actually going to become the esophagus.
Again, to kind of In this, kind of giving you guys an idea here, so we continue to repeat it, the endoderm, which came from that actual gut tube, will invade into this actual lung bud and bronchial bud and make what structures here? The endoderm will give way to the epithelium and the glands. And then what will that lateral plate mesoderm become?
That lateral plate mesoderm will give way to The muscle. So the bronchial smooth muscle that you'll start to have there, the trachealis muscle, the connective tissue, and the cartilage. Beautiful. We now have an understanding here how we started to make what?
The beginning of our trachea and lungs. Now let's continue. So we're at about where have we finished here from week four at this point where we completely separate the anterior lung bud from the posterior foregut tube, which will become the esophagus.
We're around week five. So. So now what we have to do is go from week five all the way until birth, actually even eight years of age, and start really developing all the intricacies of the respiratory system.
Boom. Got it. All right.
Such a cool part. All right. So we now have our bronchial buds and our lung buds, which came off of the foregut. So we have an idea where the origin of the trachea and the lungs are coming from. Now let's keep building that trachea and lungs.
So if we start up here, we're at where we ended, which was. again starting at week four where we develop the lung bud to where we develop the bronchial buds and the separation of the lung developing lung away from the foregut. So we're around week five. So we're going to start here at week five with our Which portion here?
The top portion coming off of the foregut was our lung bud and then here we're going to have the bronchial buds. Alright beautiful. So now what we're gonna do is we're gonna go through these stages.
This is a lot, this is sometimes asked a lot in questions on your exam. So let's start with the first stage. The first stage we're going to go with, continuing to build from these bronchial buds, is going to be what's called the pseudoglandular stage.
So this is called your pseudoglandular stage. The pseudoglandular stage starts at around week 5 and continues to occur until about week 16. Okay, so about week 5 to week 16 what we're gonna do is we're after we continue to keep growing and growing growing we get A structure somewhat like this So if we start here we have our trachea that came off of that foregut right from the lumb bud now We're gonna do is Here was the beginning of the bronchial buds. They're going to continue to keep growing and growing, growing outwards and downwards.
So, let's start here kind of color coding them and then naming them. The bronchial buds as they grow, the first thing that they're going to make is these primary bronchi. So the first thing you're going to develop is a right and left primary bronchi.
Beautiful. Then these primary bronchi are going to bifurcate. Alright, so we have our right and left primary bronchi. These are going to split. Now it's important to remember that within the adult lung, generally you have about three lobes in the right lung and two lobes in the left lung.
So that means that this primary bronchi on the right side is going to have to split into how many? Low bar. It's going to have to split into one, two, three low bar bronchi on the right. So on the right side, you're going to have right. On that one, you're going to have.
three lobar or also called secondary bronchi. On the left side, it's only gonna split into two because we only have two lobes on the left side. So you're gonna have on the left side, you're gonna have two lobar or secondary bronchi.
Beautiful. Then what happens is those secondary bronchi break up even more. And they break up into a ton of these actual small little bronchia. bronchi.
So we're going to have a ton of these here. And what I want you to know is that these next ones here I'll call your tertiary. And for the tertiary bronchi that you actually make here, you're going to have about 20 in the right lung and about 18 in the left lung.
So on the right side, you're going to have a total of 20 tertiary bronchi. And then on the left side you're gonna have about 18 tertiary bronchi. Beautiful.
Then these tertiary bronchi feed into even smaller bronchi, but now we actually like to call these small bronchi. bronchioles. So then you have to see these little squiggly guys here. These little squiggly guys that are going to be fed by the tertiary bronchi here, these are going to be called your terminal bronchioles.
So all of these little guys here are called your terminal bronchioles. So then you're going to have your nice good old terminal bronchioles. All right. So what I want you guys to take away from the pseudoglandular stage is we start week five, two 16. You start with that lung bud bronchiole, but, and by the end, what have you made right and left primary bronchi you've made three low bar on the right side two low bar on the second side on the left side 20 tertiary bronchi on the right side 18 on the left side and then you've even fed into these small little guys called terminal bronchioles That happens to about week 16. Then we go into the next stage. The next stage here is called the canalicular stage.
So we call this stage here the canalicular stage. And this continues at week 16. So the canalicular. stage goes from about week 16 to about week 26. And what happens here is let's continue off with our actual terminal bronchioles. So now we're just taking this section here and looking at it.
So this right here is going to be the terminal bronchioles. And we're going to take this section to be our terminal bronchioles. From the terminal bronchioles we're going to continue to keep making more and more structures.
What are these next structures that we're going to make? Coming off of this you're going to have these little guys here. So then the terminal bronchioles will break into what's called respiratory bronchioles.
So then they're going to make these next structures here called respiratory bronchioles. And these are very very little special things because what they do is they break off into smaller little things called alveolar ducts. So then they're going to break off into about three to six of these things called alveolar ducts.
So then the respiratory bronchioles will give way to what's called alveolar ducts. The alveolar ducts though are very interesting because what they do is they feed into what's called a little primitive alveolar kind of sack if you will. So they're going to feed into these next structure here which is called your primitive alveoli. So it's called your primitive alveoli. Now here's what I want you guys to remember.
The primitive alveoli are very interesting. And the reason why is if you were to actually kind of cut open the primitive alveoli and look at them, they're actually just made up of cuboidal cells. So that's not really good for gas exchange process because they're too thick.
That's a lot of cytoplasm for the respiratory gases like oxygen and CO2 to have to move across. So at this stage these primitive alveoli it's very important to remember that they are very immature. They're not very good at their job yet.
So we're gonna have to continue to keep growing and developing off of that. Okay? So now the next thing that happens here is after we have these primitive alveoli some pulmonary capillaries start kind of growing here.
So you see how you have these like little structures that start growing around these primitive alveoli? That's the next structure. So the next structure here is you start forming what structure here? Your pulmonary capillaries. Alright, beautiful.
So now at this point we have an idea of what's starting to kind of develop into our respiratory system. Now what we're going to do is, is we're going to take these actual primitive alveoli and continue to keep going with them. So now if we take and continue...
keep going through this process here we got to take a cut one of these actual primitive alveoli and zoom in on them there's a reason why when you look at this primitive alveoli cutting into and zooming in on it at this stage it's very very immature. And the reason why it's very immature is if you look at this, it's only cuboidal cells. So this is a primitive or immature alveoli that we form by the end of that kind of canalicular stage.
Now, when you look at this, what you see is a lot of cuboidal cells. And why is that important to understand that? Cuboidal cells are not good at gas exchange, okay? So this is going to have very little function with gas exchange. So there's a decreased ability for gas exchange in this process.
So what we need to start doing in the next stage is kind of changing some of these cuboidal cells to make them smaller. So make some of them smaller so that they'll be easier for that gas exchange process to occur. So that leads us into the next stage. So over the next following weeks from about week 26 to about birth, We go to the third stage.
What is this third stage? This third stage here is called the saccular stage. So we call it the saccular stage or the terminal sac stage, doesn't really matter. It's generally about week 26 to around the time of birth.
And what happens here is very important. Obviously you make more respiratory bronchioles because you're going to continue to increase the number of alveoli, right? So one thing that's happening here is we're increasing the number of alveoli.
So if you're increasing the number of alveoli, you're also respectively going to have to fill more air into the alveolar ducts and more alveoli. So there's also going to be an increase in the number of respiratory bronchioles. So increase in the number of alveoli and increase number of respiratory bronchioles. There's also going to be another thing here.
You see how we have this brown structure here forming around this actual kind of primitive but somewhat differentiated alveoli this brown tissue here is called your basement membrane so you're also going to start forming a basement membrane around that actual primitive alveoli we're going to increase the number of pulmonary capillaries so we're going to increase the number of pulmonary capillaries Beautiful. Now here's what I want you guys to really take away from the saccular stage. So from the canalicular stage we made more primitive alveoli, more respiratory bronchioles, but something else happens to these alveoli that I really want you to distinguish this stage from the other one. And this is it. Remember at the primitives, this canalicular stage, this is what our alveoli looked like.
So at the canalicular stage it was all cuboidal cells. As we went into the saccular stage, now look what happens to this alveoli. This alveoli is a little bit more mature.
Not completely, but it's starting to become a little bit more mature from the canalicular stage. stage to the saccular stage. How? Look at the cell difference here. It used to be all cuboidal, but now we went from all cuboidal, so it was all cuboidal, to two different types of cells.
One is these little flat squamous cells. And the flat cells are called your type 1 cells, your type 1 pneumocytes. These are the ones where, guess what happens between them? them. Gas exchange occurs.
So in other words, oxygen will move across these cells and then CO2 will move across these cells. And it's much easier for the gases to move across these flat cells than it would be across these cuboidal really big cells. So that's one kind of phenotypic change.
The other thing is that some of these cuboidal cells, they stay cuboidal, but they come a little bit more specialized. And these cuboidal cells... That kind of remain cuboidal, become a little bit more specialized, and they're called type 2 pneumocytes or type 2 alveolar cells. And you know what these guys do? They're very, very special.
And they make a kind of protein and lip... kind of complex here that coats the inner surface of the alveolar membrane. You know what this is called? Surfactant.
And that surfactant is super important whenever the baby's born because if there's a decreased number of surfactants guess what surfactant does? It prevents the alveoli from collapsing, keeps them open, reducing surface tension. If there's less surfactant, whenever the baby's born, guess what happens?
Those alveoli naturally wanna collapse. It's really hard for the baby to bring in air because it has to pop open already collapsed alveoli. So that's why this is important. So big thing to take away here is between canalicular stage and saccular stage is not only is the number of primitive alveoli increasing and making a basement membrane and making more capillaries, but the alveoli. alveoli are becoming more specialized and differentiated.
Beautiful. Now let's go to the last stage of lung development. All right, so the last stage here is the, probably one of the cooler stage I think is very interesting about this stage.
This is kind of the alveolar stage. So we're really kind of zooming in and looking at that alveoli and seeing what's happening. Because we've already seen one thing already, right?
One of the things that we've already kind of noticed here is that the cell change, the, That was a big one. What happened to the cells? We formed type one alveolar cells and type two alveolar cells.
That was a big thing that happened within that saccular stage. Well, you're gonna continue to make more primitive alveoli in the alveolar stage, but here's what else is happening, something that's very, very important. So not only are we increasing the number of the alveoli, right?
The primitive alveoli, converting more of those primitive alveoli into having type 1 and type 2 pneumocytes. But you see what else is different here? Remember how this was just one big cavity, one big alveoli?
we start to form a little septa or partition within the alveoli and that really helps with kind of increasing the surface area. So one of the things that's going to be changing here in the alveolar stage is you're increasing the septa or the partition within these alveoli and one of the things that helps to do is it really helps to increase the surface area and that's very important when it comes to gas exchange. It also just helps to mature.
So this again just contributes to the maturation process. Okay? Now, that's the big thing that happens within the alveolar stage. So the big thing to take away from the alveolar stage is obviously you're making more alveoli, differentiating them into type 1, type 2 from just straight cuboidal, and then forming septa to increase surface area.
The other thing is, again, it's pretty obvious here, we're kind of thickening that basement membrane again. We're making a good old basement membrane that we're going to have here. And the other thing is we're really kind of helping with those pulmonary capillaries as well. So again, we're kind of increasing the number of the pulmonary capillaries as well.
All right. The next thing that we need to go through here is actually not only just to talk about the alveolar stage, but we should... should denote on how long this stage actually occurs.
Which is very interesting, because you would think, oh, by the time you're born, your lungs are pretty much perfect. No. So the alveolar stage generally starts around week 36. And then, This kind of occurs around the same time as the saccular stage, but it extends very, very far past birth, eight years of age to about eight years of age. Now, here's the big thing I want you guys to understand. At birth, you have a decent number of alveoli.
You know how many alveoli you have around by birth time to give you kind of an understanding of this? Around birth, you start off. with approximately, maybe close to 100 million of those primitive alveoli.
But by the time you get to about eight years of age, guess what happened? You increased the number of alveoli to about 300 million. That is insane.
So that kind of just gives you an appreciation of what's happening even after birth. So again, alveoli number goes from 100 million at birth. birth to about 300 million alveoli by about eight years of age.
And again, this is because you're continuously going through this alveolar stage, forming septations, increasing the number of alveoli and making them specialized, making more basement membranes and more pulmonary capillaries. Beautiful. We're not done here though.
What I want to do now is I want to talk about what happens as you go through the birthing process, whenever they're basically in utero to the point of birth and what happens if you don't have that surfactant. All right. So now Now what I want us to go through is let's say that we're before birth.
We're basically before the birthing process. Before birth, the baby does have a little bit of fetal breathing movements. The purpose of this fetal breathing movements is to allow for the lungs and the muscles of the respiratory system to work with one another. But generally before birth, the baby is in amniotic fluid. So whenever the the baby breathes, guess what the baby actually breathes in?
The baby breathes in amniotic fluid. So that amniotic fluid from the amnion will actually come into the baby's alveoli and kind of cover all of this area whenever it's in utero before the birthing process. And again, what is the purpose of this? It's to kind of allow for some, to have the lungs and muscles.
work together, okay, to really kind of strengthen up that actual baby's lungs so whenever it is born it can help with bringing in air from that first cry. Now once the birthing process actually occurs, what happens? So at the time of birth, What happens is the amniotic fluid gets sucked up into the pulmonary capillaries. So all that amniotic fluid that was in the pulmonary capillaries gets sucked in.
All the actual amniotic fluid in the alveoli gets sucked into the pulmonary capillaries. Then after that happens, do you guys remember what those type 2 alveolar cells were secreting? It was secreting that surfactant.
And after the amniotic fluid is removed, all that's pretty much remaining here should be surfactant. All right. why is that important? Because again what does surfactant help us to do? Reduce surface tension and keep those alveoli stented.
If the baby is born premature, right, if the baby's born premature what can happen? Let's say it's born a little bit earlier before you know like maybe week 26 maybe week 27 something around that time. The type 2 alveolar cells haven't had time to really produce enough surfactant.
So if there's very little surfactant what's gonna happen to the surface tension? It's gonna increase. As surface tension increases, what happens to the collapsing pressure?
It increases and now the baby's alveoli will collapse. So whenever it's born, it has to take in its first breath, push the amniotic fluid into the pulmonary capillaries, leave a layer of surfactant. That surfactant layer is very thin and because of that, alveoli collapse whenever it tries to breathe, it has to take in so much more air than it even has the ability to take in to open up those already collapsed alveoli.
What is that condition called? called it's called infant respiratory distress syndrome so whenever there is significantly decreased surfactant related to what kind of a premature birth this can lead to infant respiratory distress syndrome sometimes referred to as the hyaline membrane disease but again what are the stages that are happening at birth to kind of recap here at birth baby takes in air so it takes air in as it takes the air in what happens the amniotic fluid gets absorbed in surfactant layer. remains.
And this helps to reduce surface tension to allow for the baby to breathe and take in air without a lot of effort. That is the process of the development of the respiratory system. Alright Ningenieurs, so in this video we talked about the development of the respiratory system.
I hope it helped and I hope you guys enjoyed it. Alright Ningenieurs, as always until next time. Thank you.