so today i'm talking about the esophagus stomach small intestine and large intestine if you go to the school where i work you probably don't need to know the histology of the large intestine but i'm including it anyway because i know sometimes there are professors that do cover it and also it just felt wrong not to talk about it when i'm already talking about the first three so i'm gonna go into the specific of all the layers and talk about the specialized features of each but while we have these all up next to each other i want to point out some key features that you can use to tell them apart easily so when you look at the esophagus it is the only one that has stratified squamous and it doesn't really matter how low the magnification is you can tell it's stratified squamous because there will be a darker border at the bottom of the stratified squamous layer so as soon as you see that dark border you know it must be the esophagus the stomach small intestine large intestine they all have ridges however you can look at the relative size of the ridges to decide which one you're looking at the stomach the ridges are really only the very top the small intestine they're more like big fingers so stomach think knuckles small intestine think fingers the large intestine also has some small ridges at the top but the ridges are even smaller than what you see in the stomach the mucosal layer overall you're going to see that it's proportionally smaller than what you see in the stomach and also typically the large intestine looks more organized and regular than the stomach so you're going to see the same pattern throughout the digestive tract from the esophagus to the anus all of the organs have four histological layers they all have a mucosa submucosa muscularis externa and an outer layer which will either be adventitia or serosa so the mucosa has three sublayers it has epithelium which is either going to be stratified squamous that's what you find from the mouth through the esophagus or simple columnar which is what you see from the stomach all the way to the rectum and then at the rectum it um changes back over to stratified squamous just think stratified squamous you need that to protect you from friction in any part of your digestive tract where there will be rubbing so your esophagus needs stratified squamous to protect it from food your anal canal needs stratified squamous to protect your anal canal from feces and everything in between is just going to be simple columnar now the epithelium always has to be anchored to something and that something is the lamina propria so if you've studied other systems you'll realize epithelium is always always always anchored to a lamina propria and underneath the laminar propria something that is unique to the digestive tract is a layer of smooth muscle called the muscularis mucosa so that's why we have to call the muscularis externa externa to differentiate it from the other muscle layer which is the muscularis mucosa the submucosa i cannot say much about it except it's going to be some type of connective tissue and depending on who you ask you're going to get a different answer as to the type of connective tissue so i'm just going to kind of ignore that and i just suggest you look at whatever materials have been provided to you if you want to know the type of connective tissue now the muscularis externa throughout the digestive tract has at least two layers the outermost layer the farthest away from the mucosa is the longitudinal layer the inner layer is the circular layer the stomach however has an additional layer which becomes the innermost layer which is the oblique layer and everything is going to be connected to surrounding structures either via adventitia or serosa so organs that are in your peritoneum they're in that sac that holds your digestive organs have serosa because the peritoneum is serosa and this serosa remember it is a double layered membrane that allows for movement because it has that fluid in between the layers which prevents friction you're going to see adventitia on organs that are typically less mobile they don't need to be that mobile so they're either going to be retroperitoneal meaning behind the peritoneum and anchored to your abdominal wall or not within the abdominal cavity at all like the esophagus so starting with the esophagus it's easy enough to see that there are one two three distinct layers starting from the top starting from the open space the lumen so those three layers all together will make up your mucosa and remember we said that the first layer will be [Music] the stratified squamous epithelium the second layer is going to be the lamina propria and the third the muscularis mucosa so you can orient yourself in the esophagus very easily just by counting down then you should know beneath the mucosa once you get beneath those three layers you will find the sub mucosa and below the submucosa is your muscularis externa with your inner circular layer and outer longitudinal layer and then on the very very outside because this is the esophagus we don't find serosa we have adventitia and i also want to mention uh one more thing about the muscularis externa so in this example it is entirely smooth muscle but you will see different types of muscle depending on what part of the esophagus you're looking at if you're looking at the very superior portion it is skeletal muscle in the middle it's a mix and the inferior portion is entirely smooth muscle so once you reach the inferior portion of the esophagus from then on out the muscularis externa in the digestive tract will be smooth muscle so to identify this i think first look for that stratified squamous and that dark purple border at the bottom although it doesn't necessarily need to be purple just see that it is darker than what's above it and you also will always see a pretty large obvious muscularis mucosa and then just some other things to keep in mind it does have adventitia not serosa and the type of muscle is going to vary depending on what part you're looking at now before we look at the actual slide of the stomach i just wanted you to see a drawing of what you're looking at so you're not gonna see um [Music] three nice layers for this for the mucosa like you did in the esophagus this is because in the stomach the epithelium is not one flat layer it goes down it's not just a flat layer therefore the lamina propria is not a flat layer so the laminar propria is in between these gastric pits and gastric glands the muscularis mucosa however will be a nice flat layer so if you look for that that will mark off the bottom of your mucosa also i just want to explain why you have a pit versus a gland the gastric pit is the opening of the gland and they gave it a different name because it is just the regular surface epithelium so the pit is regular surface epithelium when you get deeper inside now you have um tissue that is actually going to release products like hormones and pepsinogen so it's actually a gland so this is the mucosa like i said you don't see epithelium and lamina propria they're all kind of mixed in together but you do see the border at the bottom which is the muscularis mucosa and below that will be the submucosa and muscularis externa so these inward parts here i would call those gastric pits those are the openings and deeper in the layer any openings in there i would refer to as gastric glands in the muscularis externa you do have an oblique layer but oftentimes i don't really see the oblique layer on slides so you can't really count on it being there i would just know that it is the innermost layer you can clearly see though the circular layer and the outer longitudinal so when you're looking at the stomach um the top is either going to look like i said kind of more like knuckles than fingers which is what you would see in the small intestine or depending on how they cut the slide it might look kind of like cheerios little circles because those little circles will be the openings at the top of the epithelium those will be the gastric pits also just make sure you use that layer of muscularis mucosa to orient yourself once you find that then you know submucosa muscularis externa and i'll mention the epithelium is simple columnar but it is not always clearly identifiable as being simple columnar um same with the goblet cells they're there but they're not generally obvious you will see obvious goblet cells in a minute when we look at the small intestine and just remember like i said pits towards the top glands towards the bottom and because the stomach is a highly mobile organ located within the peritoneum it does not have adventitia it has serosa so in the small intestine [Music] you're going to see big fingers called villi so the villi are aligned with the epithelium and the inside of the villi is lamina propria and at the base of the villi you have parts that go deeper downwards those are the intestinal crypts also called intestinal glands and then i also want to mention that the edges of the simple columnar cells are going to have microvilli so those are just little nubs that increase surface area of the cells to make the cells better at absorbing which is the main function of your small intestine is to absorb your nutrients so if we look at this closely this is a part of a villi very close up you can clearly see the layers of simple columnar on the outside and the lamina propria is what the epithelium is anchored to on the inside of the villi you can also see very nicely the microvilli so sometimes you can actually see the fingers um [Music] sometimes it just looks like it does here like a dark border and you can also see the goblet cells so remember a goblet is a cup so any of these cells that have a kind of opening on the top would be goblet cells and those secrete mucus so here we're looking at it from much farther back so you can't clearly see those features but all of those features would be located up here in the mucosa uh you can still see all those finger-like villi and you can see that nice layer of muscularis mucosa makes the bottom edge of the mucosa layer just like it does in the stomach so below that is submucosa and below that muscularis externa and this is longitudinal and circular layers so the small intestine um in this particular example i know that this is an ilium um so it has serosa so the rule is the duodenum is kind of anchored tightly with the pancreas you don't want it to be moving around so it is retroperitoneal hanging out with the pancreas it has adventitia the other two have serosa the jejunum and ilium have serosa and i'll tell you in a minute how i know this is not the duodenum all right so remember you want to look for your finger like extensions um you if you're close enough you'll see the brush border and the simple columnar um any of these places where it goes deeper down um i would say it's safe to say those are intestinal crypts and some of these cheerios that we're kind of seeing in the bottom there i think those would also be intestinal crypts of villi that we can't see um and like i said before duodenum has adventitia because it's retroperitoneal the jejunum and ilium would have serosa now you can tell this is not the duodenum because if it was there would be a bunch of glands located in this submucosa which i will show you when we get to the practice portion of this um and to differentiate ilium from jejunum uh you're supposed to you know look for pyres patches but i cannot reliably find pyre's patches so i can't really explain that to you but just throwing it out there if you want to investigate that topic further all right so now we're at the large intestine mucosa with the muscularis mucosa making the bottom edge submucosa muscularis externa with both the layers now just like in the small intestine um whether you have adventitia or serosa is going to depend what part of the large intestine you're looking at um the ascending and descending colon are retroperitoneal and therefore have adventitia the transverse colon has serosa and to be honest i do not recall about the sigmoid colon um i believe that would have adventitia but i need to fact-check myself on that all right so what you're going to see here is you don't have villi but if you were close enough you might be able to see some microvilli um the goblet cells become more numerous so if you see just an amazing amount of goblet cells you're probably looking at a large intestine although they are not visible from this far away in this particular slide i would also note the mucosa looks more organized than what you see in the stomach and it's also proportionally smaller than what you see in the stomach so you know this mucosal layer is about the same size as the submucosa it's much smaller than the muscularis externa in the stomach or anywhere in the stomach um the mucosa would be larger than the submucosa all right so now we are ready for our practice so i want you to see can you identify the organ and also try to identify as many sub layers as you can i'm going to give you 10 seconds and then i'll just start pointing stuff out so pause if you would like more time to look at it so this is the esophagus which i hope you could very easily recognize that this is many layers of pancake cells with a dark border so it must be stratified squamous and if this is the epithelium that's the laminar propria and that's the muscularis mucosa so this is a stomach this has more ridges than what we saw in the first example i showed you but we have knuckles on the top here on ridges and like i said the mucosa is pretty large compared to the submucosa so this would be your lamin appropriate epithelium all mixed together some gastric pets glands down here muscularis mucosa at the bottom submucosa and muscularis externa so this is a really close-up view of the stomach this is what i had mentioned before where sometimes the stomach will appear to have cheerios at the top so we're only looking at the mucosa here we didn't even get to the muscularis mucosa so these would be the gastric pits you know in here would be gastric glands so as soon as you see these big fingers you should know it's a small intestine um we cannot tell what part of the small intestine because we can't see the submucosa remember these cells are simple columnar with goblet cells being specialized types of simple columnar microvilli on the border you just see kind of a fuzziness lamina propria in the villi and down here would be part of the muscularis mucosa so it doesn't matter how far out and terrible this slide is you still see that darker border at the bottom which tells you this is stratified squamous and therefore an esophagus um it is a little harder to tell laminar propria and muscularis mucosa apart here so in that case if you look at this that's clearly identifiable as the muscularis externa which makes this the submucosa which makes this the muscularis mucosa and the lamin appropriate is just too fuzzy to really tell clearly so this is a small intestine we can tell because it has fingers and also so here that's our muscularis mucosa which means down here that's our submucosa and you can see there are lots of cheerios there's lots of circles in there those are those brunner's glands i talked about before so because we're seeing broner's glands we know this must be the duodenum because you would not see that in the jejunum or ileum so if this is the duodenum what would the outer layer be serosa or adventitia it would be adventitia because remember duodenum is hanging out with the pancreas behind the peritoneum not in the peritoneum and this is a large intestine you can tell because it doesn't have fingers um the mucosa layer is relatively small compared to the submucosa and all of those white bubbly kind of things those are actually the goblet cells so there's just a ton of them [Music] here which is another sign you're looking at a large intestine all right that's all of the examples i have for you today i hope this was helpful have a great day and have fun learning