Histology of the respiratory system + Anki flashcards

[00:00] In this session I will deal with the histology of the respiratory system, mainly the divisions of the bronchial tree. As you can see here that the bronchial tree is divided into a conducting portion and a respiratory portion. This is the conducting

[00:20] system. It includes the air passages that conduct air to and from the lungs and some of it is located outside the lung. The other part is present inside the lung. So you can see here that the trachea part of the main bronchus is located outside the lung and then the remaining part is located inside the lung here.

[00:40] No exchange of gases takes place. That's why the wall is thick. And then we have the respiratory portion, and it conducts the air, as well as it allow exchange of gases to take place between the air inside the bronchial tree and the blood vessels in the wall.

[01:00] So the conducting portion consists of the trachea, also it consists of the bronchi, the bronchus, we have a main bronchus, right and left main bronchus, and then as it goes into the lung, because the lungs have lobes, so it is divided into lower bronchi and then each lobe is divided into segments.

[01:20] and so they are divided into a segmental bronchi and then we have bronchioles, then we have the respiratory portion, we have the alveolar duct, alveolar sac, and the alveolar. We are going to identify all these structures in a moment. Let's start with the trachea and in the wall of the trachea.

[01:40] we have four layers. We have the mucosa and then the submucosa, the cartilage and then the adventetia and you can see here that the mucosa consists of an epithelium lying on a lamina property. Beneath that is the submucosa and the submucosa is mainly here occupied by the

[02:00] of the gland, serromucus glands. The cartilage is formed of hyaline cartilage. You can clearly see here the chondrocytes inside the lacunae and then we have the outer layer which is connective tissue containing some nerves and vessels and it's called the adventitiation.

[02:20] Here is another section showing the layers of the trachea wall. You can see here that the mucosa consists of epithelium lying on laminar property and then there is a submucosa containing glands. You can see here the ducts open on the surface of the mucosa and then there is the cartilage and then the edp.

[02:40] look at here at this section from the posterior part of the trachea you can see that these fibers in fact they are smooth muscle fibers the trachealis muscle. This is another section here at a higher magnification showing the layers of the trachea wall. You can see them mucosa here.

[03:00] the surface epithelial cells these are celiaced, clearly they are celiaced. Here again you can still see the celiac and look at the nuclei. It looks as if it is stratified, the nuclei are at different levels but in fact they are not stratified. It is the same layer but some of the cells are

[03:20] Some of them are short and that's why the nuclei they appear at different levels and give the impression that it is stratified. Of course, the epithelium lies on a laminar property and then we have the submucosa, look at the glands and then we have the cartilage and then we have the adventetia here. You can see some fibers of the tricharrhosmosin.

[03:40] muscle as well. This is a higher magnification showing the respiratory epithelium because this type of epithelium, the pseudostratified coruminar epithelium, is only present in the respiratory passages. That's why it's called the respiratory epithelium. And you can see that there are different types of cells, but we cannot differentiate them all.

[04:00] But at least we can differentiate that there are ciliated cells. These are tall corona cells. Look at the cilia here. And in between them, there are these goblet cells, which have mucus secretion in the upper part of the cytoplasm, while the nucleus is in the lower part of the cytoplasm. They are not ciliated.

[04:20] These are goblet cells and then these are the short cells, these are the nuclei of the short cells and all the cells they lie on the same basement membrane. So the nuclei are at different levels but all the cells they lie on the same basement membrane. These short cells they can be

[04:40] basal cells, which are the stem cells, or they can be what we call the small granule cells. We cannot distinguish them from the stem cells, from the basal cells. And there is another type of cell here, which is a columnar epithelial cell. These are called brush cells and these are in contact with nerve endings.

[05:00] They will initiate a reflex cough, for example. But we cannot differentiate them from other columnar cells. Here again, the ciliated cells, they are tall and the cilia project and the apical surface. These cilia are so fragile and they are vulnerable to damage by inhaled toxic blood.

[05:20] chemicals like cigarette smoke or car exhaust and city dwellers, also by bacterial and viral infections. Because of the coordinated beating of cilia, this will create a sweeping action that will drive the surface mucus to the outside.

[05:40] the surface here is covered with mucus. Some of this mucus is derived from goblet cells and the other part of the mucus is derived from the ducts of the acinet of glands that are present in the submucosa and this mucus will trap the particulate matter and then the mucus will be washed away.

[06:00] by the sweeping action of the cilia. So damage of the cilia will result in reduced efficiency of getting rid of the mucus, resulting in congestion. And prolonged and repeated damage of these ciliated cells will result in their replacement with a protective epithelium, squamous epithelium, and this is what we call.

[06:20] squamous metoplasia. Here again the goblet cells I have just mentioned that they accumulate the mucus in the apical part of the cytoplasm and that's why they have the shape of a goblet and because this mucus is mostly washed during the tissue preparation then the cytoplasm appear as a clean-out.

[06:40] And obviously they lack the cilia. Here in this inset you can see the long ciliated cells, surface mucus with debris on its surface. These are the mucigenes granules of the goblet cells. And look at these short cells, the basal stem cells mostly. They are basal stem cells. These are the three main.

[07:00] types of cells. Of course as I said that there are other cells like the brush cells which are sensory and the small granule cells. The small granule cells they look like the basal cells but you cannot differentiate them until you use certain specific stains like immunohistochemical preparation like this one.

[07:20] showing clusters of these cells. These are of the type of neuroendocrine cells. In other words, they secrete substances that act as neurotransmitters or local hormones and control the local environment, such as control the tone of the smooth muscle fibers that are present in the wall of the bronchial tree.

[07:40] as well as the smooth muscle fibers that are present in the blood vessels and they are the origin of an aggressive type of bronchial carcinoma. In the submucosa as shown in this section, let me orient you, this is the mucosa here and then it lies on laminar papria and this is the submucosa. You can see that there are

[08:00] are multiple acinae. Some of them have, the cells have clear cytoplasm. These are mucus acinae and some of them you can see that the cytoplasm is stained darker than in the mucus cells and these are the serous acinae. In some places you can see that the mucus acinus is capped.

[08:20] with serous cells and these are called serous demilunes. So these are called serous demilunes because they appear like a half-moon demilune. In the SINs the lumen is small but these SINI will give rise to ducts. As you can see here the duct is lined by simple cuboidal bacterial cell.

[08:40] wider lumen and these ducts will head toward the surface and secrete the mucus. This is to show you the cartilaginous layer of the trachia. Look at the profile of the trachia. It is flattened posteriorly where it lies on the esophagus. This is the location of the esophagus posterior to the trachia and it is flat.

[09:00] because there is no cartridge posteriorly. So the cartridge is in the form of a C-shaped ring, an incomplete ring. It is C-shaped and it is deficient posteriorly and it is posteriorly here that there are the smooth muscle fibers, the trachealis muscle. The cartridge will provide flexibility.

[09:20] and prevent the collapse of the tracheal lumen. Make it always patent for the passage of air. This is to show you a higher magnification in the posterior part of the trachea, showing the trachealis muscle, smooth muscle fibers that control the caliber of the trachea. So, so far about the trachea, now let's move to an end.

[09:40] part of the bronchial tree and this is the bronchus. The bronchus has the same general histological structure as you can see here that there is epithelium which is again pseudo-stragfoid columnar-ceviated epithelium lying on a lamina propria but then there is a muscular layer and there is the

[10:00] submucosa and the adventetia. Here the most important thing is that the cartilage rings, the C-shaped rings that were present in the trachea are absent here. In this preparation you can see that there are cartilage plates that are distributed all around the wall of the trachea.

[10:20] Here is another one here, another one here, and another one here, multiple cartilage plates. And so the smooth muscle fibers are not only restricted, like in the trachea, they are restricted for the trachea muscle posteriorly, but you can see that the smooth muscle constitutes

[10:40] complete circumferential layer all around the bronchus. So the main feature here is discontinuous layer of cartilage plates and circumferential layer of smooth muscle fibers. And obviously this section is of the part of the bronchus that is present inside the lung. You can see.

[11:00] see the alveoli of the lung here and these are blood vessels actually. You can see that they have a different histology than the bronchial tree. The lining epithelium is simple squamous epithelium and this is the tunica intima, tunica media and the tunica adventetia. This is another section showing

[11:20] you the layers of the bronchial wall. Again it is inside the lung. Look at the epithelium, so the stratified columnar, seriated, you can still see evidence of the seria here, lying on a laminar proprio and then we have the muscaratus. Look at the muscaratus layer and then more superficially, so this is the

[11:40] the submucosa and the glands, the several mucus glands. You can see them here as well. And then we have the cartilage. The cartilage, they are cartilage plates. They are present here, here, here. So they are present all around. Here again, they are present all around the bronchial.

[12:00] wall and to the outside look at the alveoli indicating that we are in the lung. This is a higher magnification of the bronchial wall. You can see these epithelium lying on the laminar proprio and this is the muscularis layer, this is the submucosa with the serromucus glands and

[12:20] look at this duct here heading to the surface and these are the cartridge plates again. So far about the bronchus and the bronchial wall now let's deal with the bronchial. So the bronchials are here as you can see they are present in the lung. Again they have the general features of the bronchial wall but they are.

[12:40] very small size, less than 1mm in diameter and the epithelium, as you can see here, it gradually transforms from pseudostratified into a simple columnar epithelium with ceria. We don't see abundance of goblet cells here and they will also disappear at the end

[13:00] of the at the terminal bronchioles. So as the mucus glans and the submucosa, so there are no submucosal glands, they disappear. And the most important thing here is that there is no cartilage in the wall. So the cartilage plates and the glands are not present in bronchioles. This is the hallmark of the bronchioles.

[13:20] Instead, we have a circumferential layer of smooth muscle fibers. These smooth muscle fibers relax during inspiration and contract at the end of expiration. These are the smooth muscle fibers that undergoes spasm in patients with asthma and result in

[13:40] expiratory wheezes. The bronchioles will give rise to the respiratory portion of the bronchioles like this one is a respiratory bronchioles. Still it has mucosa where the cells are cuboidal epithelial cells, simple cuboidal epithelium because they are becoming shorter and shorter.

[14:00] And there is some evidence of smooth muscle fibers here in the wall. But at the same time, you can see that the bronchioles are connected to alveoli. So that's why some gaseous exchange takes place here. They are directly connected to alveoli or alveolar sacs.

[14:20] We have an alveolar duct, which is an elongated duct. On either side of the duct, open the alveoli. So it looks like a corridor with multiple doors on either side. It's called the alveolar duct. And the alveolar duct will either give rise directly to alveoli or to what we call an alveoli.

[14:40] the alveolar sac, which are spaces like a lobby hall surrounded by multiple alveoli. The alveoli have a very thin septum so that they will allow gaseous exchange between the air that is present in the space of the alveolus and small capillaries

[15:00] that are present in the wall. Look at here, these are blood vessels in the wall. Of course, smaller than this are also present. So the lining epithelium here is of two types of cells, type I and type II alveolar cells. The type I alveolar cells are simple squamous epithelial cells that provide the thinnest possible wall for ex-

[15:20] exchange, they cannot multiply so they are replaced from the other type of alveolar cells, type 2 alveolar cells which are actually cuboidal cells. They are as numerous as these type 1 alveolar cells but because they occupy less surface area as they are cuboidal so they appear as a

[15:40] they are less, but they are as numerous as type 1 alveolar cells. Well, it is it will be difficult to differentiate between them in this section, but the cells whose nuclei are more flattened, they would be type 1 nemocytes like this one for example and the cells whose nuclei are right.

[16:00] rounded would be type 2 alveolar cell because they are cuboidal cells. These type 2 alveolar cells, they can act as progenitor cells for type 1 alveolar cells because they can undergo cell division and replace the damaged type 1 alveolar cells. But not only that, they produce a substance which is called the surfactant.

[16:20] that is produced on the surface here of the LVLI and this surfactant will reduce the surface tension and prevent the collapse of the LVLI. This surfactant is produced during fetal life by the 35th week of development onwards and that's why in a premature

[16:40] infants the amount of surfactant might not be that enough to prevent the collapse of alveoli and so they might undergo what we call neonatal respiratory distress syndrome. In the wall of the alveolus there is another type of cell and these are macrophages called alveolar macrophages they might be present

[17:00] and either in the wall and some of them might move to the space of the aliviores. You can see that they are filled with particles, dust particles, carbon particles. These are phagocytic cells, part of the immune system of the body and they not only phagocytize bacteria and viruses in case of infection but also

[17:20] they trivacetize particulate matter such as carbon particles and dust and so the name is that they are called dust cells. Many of them remain in the wall of the riviolis throughout the individual's life and they are responsible for the mottled appearance of the lung during dissection especially in people who are smokers.

[17:40] or city dwellers. Some of them they also phagocytize red blood cells that might enter the alveoli in case of heart failure and they might also in this case called heart failure cells. Finally let's deal with the pleura. Here this section is from the surface of the lung where you can see the alveoli here and you can see the surface of the lung.

[18:00] It is covered by the pleura. This part of the pleura is the visceral pleura and as you can see here the cells are simple squamous epithelial cells, they are mesothelial cells and they lie on connective tissue and then just deep to that are the alveoli. This concludes our study of the histology of the alveoli.

[18:20] respiratory system. Thank you.

[18:40] Thanks for watching.