Now I want to zoom in on the stomach mucosa just a little bit more and talk about the stomach glands, or what we're going to call the gastric glands. That's because these gastric glands are going to produce the liquid that does the chemical digestion in the stomach. So we'll start off just by saying here that within the mucosae, there are specialized structures that secrete that liquid that we are going to call gastric juice. Gastric juice is going to be this acidic, enzyme-rich fluid. The acidity combined with those enzymes is what's going to do the chemical digestion.
As we look at the structure here, there are two major structures we want to identify: one we've looked at briefly before and that's those gastric pits. These are the tunnel-like structures connecting the gastric gland to the lumen, and as you travel down that gastric pit, travel down that tunnel, you will reach the gastric gland. In our image here, you can see this would be the lumen or that inside of the stomach here, and we see some gastric pits, and then we see one in cross-section, that tunnel-like structure, and as you go down it, we reach the bottom half of this image, which is the gastric gland. We have all these cells that are color-coded there that we're going to talk about as we go. The gastric pit is the top half of this image here, and the gastric gland is the bottom half that we're going to talk about.
Gastric glands, let's identify these cells. First up, we have the mucus neck cells. Here we have all these sort of purple cells near the top, and their name tells you most of what you need to know about them. They are in the neck, so the top part of the gland, and what do they make? They make mucus, and it's an acidic mucus. Now remember, the lining of the stomach has all these mucus cells that make an alkaline mucus, which protects the stomach from the acidic, gastric juice. The big difference here is, as you get into this gland, well, these mucus neck cells now make an acidic mucus. This acidic mucus, just to be clear, is not what makes your gastric juice acidic.
What makes your gastric juice acidic then? Well, that's going to be the parietal cells. The parietal cells, you can see here, spread around in blue, and they have this kind of unique, like, forked structure to them that you can see there in the image just a little bit. So they make the gastric juice acidic by producing hydrochloric acid, or HCl, and also importantly a protein called intrinsic factor. First, let's talk about the hydrochloric acid. Hydrochloric acid is going to participate in chemical digestion, chiefly by denaturing proteins. Now remember, proteins are three-dimensional structures made up of long chains of amino acids. To denature them means to break up that three-dimensional structure, but not to actually break the bonds between the amino acids yet. That's going to happen from enzymes later on. It's also going to kill bacteria. You take a lot of bacteria in when you eat, and you don't want all of that stuff colonizing your body. So one of the first lines of defense is this hydrochloric acid. It's going to do some other chemical digestion as well. For example, it helps to break down some cell walls so that enzymes can digest the insides of cells in plants and so on. But we want to focus in on those proteins and killing bacteria.
We also have intrinsic factor. Intrinsic factor is this protein that's going to be important for vitamin B12 absorption. Now you don't actually absorb your vitamin B12 until later on in the small intestine. But we need to call this out here because if you don't get the intrinsic factor in the stomach, you won't absorb that B12 later on.
Our next type of cells is going to be the chief cells. If the parietal cells made the hydrochloric acid that participated in chemical digestion, it's the chief cells that make the enzymes. We see these down near the bottom of these gastric glands, these orange cells there. They make enzymes pepsinogen and lipase. Technically, pepsinogen isn't an enzyme; it's an enzyme precursor. Under acidic conditions, like in the stomach, pepsinogen, this enzyme precursor, is converted into the enzyme pepsin. So we have two enzymes there that are now active, produced by these chief cells: pepsin, which digests proteins, and lipase, which digests fats. Lipase breaks down lipids. Protein digestion and lipid digestion are going to continue with enzymes in the small intestine. In fact, most lipid digestion happens in the small intestine, but it does get started here.
Our final type of cell that we want to talk about is going to be enteroendocrine cells. And enteroendocrine cells, you can see in green here, just a few of them compared to everything else. Entero means gut and endocrine refers to endocrine function, sending out hormones. So these cells release hormones and paracrine molecules. Hormones are chemical signals that travel in the blood. Paracrine molecules are chemical signals that signal cells nearby. They release both of those, and actually, some of the molecules, it just depends on what they attach to, whether they act as a hormone or a paracrine molecule. Some examples here are things like histamines, gastrin, serotonin, somatostatin. I just want to call out gastrin as one of those things that does both. Gastrin, if it binds to one of these parietal cells, is going to instruct them to start making stomach acid. However, if it ends up in the blood, there are also going to be target cells for that gastrin in, for example, the gallbladder and the pancreas, which is going to instruct them to get ready to release the fluids that they either store or produce.
Now as I look at all of this, honestly, I can remember mucus neck cells and enteroendocrine cells pretty well, but I always get parietal cells and chief cells confused. So to keep this straight in my head, we have this little memory tool. Parietal cells lower pH. Chief cells chop the proteins and also the lipids. So the parietal cells, remember, they make the hydrochloric acid that affects pH. Chief cells, they make those enzymes, which cut things up like proteins and lipids.
With that, we have examples and practice problems to follow. Give them a try.