Chapter 8: Gastrointestinal System
There is marked interspecies variations in the gastric compartment, exemplified by the difference between monogastrics (e.g. carnivores) and the multi-compartmentalized stomach of ruminants (e.g. cattle). The reader is referred to veterinary gross anatomy texts for the specifics pertaining to these structures, but the microscopic specifics will be noted here. Regardless, the primary function of the stomach is mechanical and chemical breakdown of digesta in preparation for absorption in the intestine.
Dogs and Cats
Domesticated carnivores (dogs and cats) have a single-compartment (simple) stomach that is entirely lined by glandular mucosa. The stomach is divided into several segments. From oral to aboral they are: cardia, fundus and pylorus.
The glandular stomach is so named due to the high secretory function of the mucosal epithelium, which produces mucus, hydrochloric acid, and pepsin. The mucosal surface is formed by numerous vertically oriented tubular glands. Superficially, the mucosa is composed of gastric pits, which are small mucosal depressions that form communications between the tubular glands of the mucosa and the gastric lumen. The gastric pits are primarily lined by mucus-secreting epithelial cells, columnar epithelial cells with pale to non-staining cytoplasm (mucus) and basal nuclei. These cells produce the mucus layer that coats the epithelial surface, protecting it from mechanical and chemical injury.
Below the gastric pits and comprising the majority of the mucosal epithelium within the fundus are two specialized epithelial cell populations. Parietal cells (oxyntic cells) are polygonal epithelial cells with abundant pale eosinophilic cytoplasm and round, central nuclei located within the central regions of the fundic mucosa. Parietal cells secrete the hydrochloric acid that acts as the primary chemical digestive effector of the stomach. Chief cells are low-columnar epithelial cells with granular basophilic cytoplasm and round, eccentric nuclei. Chief cells secrete pepsinogen, which is rapidly converted to the active form, pepsin, by hydrochloric acid. Pepsin is a proteolytic digestive enzyme (protease), thereby contributing to digestion. The distribution of the glandular epithelial constituents varies by region of the glandular stomach. The fundus contains large numbers of parietal cells, whereas the pylorus is rich in mucus cells but contains few parietal cells.
A third secretory epithelial cell type, gastrin cells, are located primarily within the pylorus. Gastrin cells secrete gastrin, a hormone induced by the presence of digesta within the stomach (e.g. eating) that stimulates secretion of the digestive enzyme pepsin (i.e. pepsinogen) and hydrochloric acid by chief and parietal cells, respectively. Gastrin cells are not readily discernible by routine histologic methods.
Similar to the other regions of the tubular digestive tract, a lamina propria underlies the gastric mucosa. External to the lamina propria are the gastric submucosa, tunica muscularis and serosa. The mucosa of the stomach may contain organized lymphoid aggregates, which are included in the gastrointestinal-associated lymphoid tissue (GALT; discussed later).