Hi everybody, Dr Mike here. We are now looking at our last layer, the most external layer of the gastrointestinal tract when we're looking at the histology and that is called the serosal layer or adventitia layer. So what we've gone through so far is the most internal layer being the mucosal layer, then the submucosa, then the muscularis externa and now this very final layer termed the serosa or adventitia.
Now why do I say serosa or adventitia? Well, a serosa or serosal layer is a double membrane layer made up of epithelia. One of these layers will be stuck to the organ and that's going to be called the visceral layer. And then there's going to be another layer that's going to be most closest to the body cavity.
And that's going to be called the parietal layer. In between those two epithelial layers is a fluid called a serous fluid that helps produce Basically, it's a lubricant and helps reduce friction between the organ and the surrounding structures. Now, serosal membranes are going to be surrounding organs that sit within the cavity of the gastrointestinal area, called the peritoneal cavity.
So, serosa wraps around organs that sit within the peritoneum, known as intraperitoneal. These organs include the liver, the stomach, the spleen. The very first part of the duodenum called the duodenal bulb or bulb of the duodenum.
The rest of the small intestines being the ilium and jejunum. The transverse colon, which I've drawn behind these structures, but in actual fact, the transverse colon sits in front of all these structures. And the sigmoidal colon.
So all of these areas that I've highlighted in blue is covered by serosa. Like I said, double membrane epithelia produces serous fluid and it wraps organs that sit within the peritoneal cavity. Now adventitia is not epithelia, it is connective tissue.
It's loose connective tissue and it wraps and binds organs that sit outside of the peritoneal cavity, specifically sit behind the peritoneal cavity called retroperitoneal. And what you can see is organs that sit retroperitoneal that are bound by adventitia include the pancreas, most of the duodenum, the cecum and the ascending colon, the descending colon, and they're all bound by adventitia connective tissue, and it binds them to the walls of the abdominal cavity, okay? So it basically is there to hold it into place.
So what's the difference between the serosal layer or the adventitia layer, both of which is the most external layer of the gastrointestinal tract? Serosa is epithelia, double membrane, serous membrane that produces serous fluid as a lubricant. It binds organs that sit within the peritoneal cavity. Adventitia is loose connective tissue. It grabs a hold of organs and anchors them to the walls of the body cavity and it's for organs that sit outside of the peritoneum known as retroperitoneal.