Classification and Location
Based on their location and structural features, gastric glands are divided into three types:
- Proper gastric glands (Fundic glands). Located in the fundus and body of the stomach. These are the most numerous group (approx. 35 million). Their terminal portions are virtually unbranched, and their cellular composition is the most diverse.
- Cardiac glands. Located in the cardia. There are only 1–2 million of them, and their terminal portions are heavily branched.
- Pyloric glands. Located in the pyloric region (approx. 3.5 million). They feature branched terminal portions with wide lumens. An important anatomical feature of this region is that gastric pits are 2–3 times deeper here than in other parts of the stomach.
Cellular Composition of Proper Gastric Glands
Structurally, each gland consists of a duct (isthmus and neck) and a main portion (body and base). The cell population of proper glands includes several types of exocrinocytes and endocrinocytes:
- Parietal (oxyntic) cells. Responsible for secreting hydrogen and chloride ions (components of hydrochloric acid) as well as Castle's intrinsic factor (essential for vitamin B12 absorption). These are the largest cells of the gland, lying individually along the periphery. They feature marked oxyphilia (staining bright orange with Congo red) and an enormous number of mitochondria that provide energy for proton pumps.
- Chief cells. Synthesize the inactive enzyme pepsinogen as well as chymosin. These cells are smaller, arranged in groups, and have a basophilic cytoplasm due to a strongly developed protein-synthesis apparatus (rough endoplasmic reticulum and Golgi apparatus).
- Mucous neck cells (mucocytes). Produce a mucus-like secretion. They have dense nuclei displaced toward the basal pole and a light, reticular apical portion.
- Neck cells. Located in the ducts. In addition to mucous neck cells, this zone contains poorly differentiated (stem) cells (the cambium zone). From here, cells migrate upward (rapidly renewing the surface epithelium every ~4 days) and downward (slowly differentiating into glandular cells over months).
- Endocrine cells. In proper glands, these are represented by EC, ECL, and A cells.
Mechanism of Hydrochloric Acid Secretion
The production of hydrochloric acid (HCl) by parietal cells is a complex biochemical process. Inside the cell, the enzyme carbonic anhydrase converts carbon dioxide (derived from the blood) and water into carbonic acid, which dissociates into bicarbonate and a proton (H+).
At the apical membrane, protons are pumped into the gastric lumen by the H+,K+-ATPase (proton pump) in exchange for potassium ions (antiporter). Chloride ions (Cl-) enter the cell from the blood and are subsequently secreted into the glandular lumen alongside potassium (symporter). Due to the active export of acidic components, the cytoplasm of parietal cells becomes depleted of them, resulting in marked oxyphilia under light microscopy. An important ultrastructural feature of these cells is a ramified system of intracellular canaliculi with microvilli.
Mucocytes and Pyloric Glands
Gastric mucous cells (mucocytes) are divided into three types: surface mucous cells (lining the epithelium), neck mucous cells (in the ducts), and mucous neck/accessory cells (in the terminal portions).
In cardiac and pyloric glands, mucous cells (light-staining with flattened nuclei) make up the bulk of exocrinocytes. Pyloric glands also feature a specific endocrine apparatus: G cells (producing gastrin), as well as D, D1, and P cells.