General Organisation of the Gut Wall

00:00 So it's important that you understand all these concepts. First of all, I wish to describe the histology of the esophagus. Here is a rather image filled piece of information, there's two pictures here. One on the left is a diagram that starts to explain all the major components that make up the wall of the gut. And I'm going to go through this very carefully because, as I mentioned earlier, it's an essential feature of understanding the histology all the way along the elementary tract. On the right-hand side, there is a section taken through the esophagus and then viewed using a scanning electron microscope. And I want to take you through this very slowly, because as I stressed a couple of times now, it's very important to identify all the different components of the wall of the gut and then understand their function. When looking at sections through a tube, such as the gut tube here, the tube through the esophagus, try and locate the lumen first. So, locate the lumen on the right-hand side scanning electron micrograph image of the esophagus. It's a star-shaped or rather folded shaped lumen here because the esophagus is collapsed.

01:34 It's only going to open out when a bolus of food is passing down through it, as part of the swallowing process. The surface of the lumen is where you find the mucosa.

01:49 The mucosa is that component of the gut wall that is adjacent to the lumen, the cells open on to the lumen. So the mucosa consists of the epithelium of the gut wall, the gut tube, and also the supporting connective tissue, the lamina propria.

02:16 And often we include the very small thin muscle layer around the mucosa called the muscularis mucosa. So just going through that once again, the lumen is in the centre, and then you have the mucosa which consists of the epithelium that lines that lumen, supported by a lamina propria. And then we have the muscularis mucosa.

02:44 Sometimes, I will refer to both those components, the mucosa and muscularis mucosa as just the mucosa. Then there is a space of loose connective tissue we call the submucosa. That's a very important layer because it contains nerves and blood vessels that are going to branch and provide the mucosa, the very busy epithelial surface, with all the blood nutrients that need to carry out the functions all the way along the gut.

03:26 Then you have on the outside, the muscularis externa. And all the way along the gut tube, that muscularis externa consists of at least two layers, mostly it's an inner circular layer and an outer longitudinal layer. So let's work back from that muscularis externa and just describe some of the main roles of each of these layers, generally, and then we'll see how they become specialized when we look here at the esophagus, and then again, at the stomach. And in later lectures, I will describe the roles they serve when looking at other parts of the elementary tract. Well, the muscularis externa, consisting of those two layers, is responsible for contracting and moving food along the gut tube. And by using both contraction of circular muscle and then longitudinal muscle, the gut wall can have a peristaltic wave of contraction that forces the food along. That's the major role of the muscularis externa. On the outside of that muscularis externa is going to be connective tissue called adventitia, which blends or holds that part of the gut wall against surrounding tissues. Or sometimes, as indicated here, it's called the serosa, because the surface is lined or free and lined by part of the peritoneum.

05:07 Now, that muscularis externa is working independently of what's going on in the lumen. The epithelial cells, particularly in the stomach and further down the gut, are busy carrying out functions, secreting, absorbing all the necessary jobs they have to do, supported by the lamina propria.

05:32 The muscularis mucosa around that mucosa helps local mixing. It contracts and relaxes, and therefore, changes the dimensions of the environment immediately at the luminal surface. And allows this local mixing, and therefore, the mixing of secretory products to aid digestion and also the mixing of products to enhance or increase the surface area for absorption.

06:04 That muscularis mucosa works independently of the muscularis externa, and they're controlled by different components of the enteric nervous system. So think of one tube inside another tube, and the inner tube is working independently from the outer tube. But overall, the outer tube is responsible for moving the food along as it's broken down and as the nutrients are absorbed. So, the submucosa acts as a sort of a cushion. It allows a lot of movement within the lumen, of course by the muscularis mucosa. It allows the lumen to expand, as you see here, if a bolus of food passes down the lumen, and therefore, that submucosa is going to allow that expansion. And it's also going to give the freedom of movement from the inner tube and then the outer muscularis tube. So again, let me emphasize those important components of the wall of the gut. You should know them because they're going to change, particularly the mucosa, as we move down along the gut tube.