Like the walls of the digestive tract, also the walls of the stomach are formed by more overlapping tunics. The gastric mucosa is the innermost layer of the organ; as such, it appears directly on the lumen of the stomach, in close contact with the products of digestion. Proceeding towards the outside, under the mucosa, the remaining tunics meet: the submucosa, the muscular and the serosa (peritoneal serous coating).
At the gastric level, the most developed tunics are the muscular and mucous membranes. The first is formed by three different layers of fibers, of which the most external are longitudinally oriented, the intermediate ones circularly and the inner ones obliquely. The contraction of the gastric muscles is essential to facilitate the mixing of the bolus and the emptying of the stomach.
The gastric mucosa covers the internal cavity of the organ and can in turn be divided into three layers: epithelium, lamina propria and muscolaris mucosae. The simple cylindrical surface epithelium is the same throughout the stomach mucosa and consists of columnar cells that secrete mucus and bicarbonate, while the glandular component - which has its roots in the connective lamina propria - differs in various sectors .
Mucus and bicarbonate secretion is essential to protect the gastric mucosa from the insult of acidic digestive juices secreted by its glands. When this protective layer is eroded by hydrochloric acid, a more or less extensive area of the mucous membrane suffers the digestion of gastric juice; the injured area is called gastric ulcer.
In the living, the gastric mucosa takes on pinkish shades at the end of the pylorus and reddish or brown-reddish on the rest of the surface; in childhood the shades are brighter and the vascular redness more pronounced.
About one millimeter thick, with a soft and velvety surface, the gastric mucosa observed under the microscope appears crossed by numerous furrows, which divide it into small polygonal-shaped areas. In the centers of these islands, slightly depressed and called gastric pits, the ducts of the glands located in depth emerge.
The gastric glands are divided, by position and structure, into three different types:
cardial glands (located in the proximal region of the stomach),
glands of the fundus and body (the most abundant), called oxintic or fundic
and pyloric glands.
The cardial region presents the homonymous cardial glands of tubulo-acinar type, with prevalent mucous secretion.
The body-bottom region presents simple tubular glands, called oxintic glands. This glandular component, which is responsible for the production of important digestive factors, is made up of different types of cells:
the parietal cells (which constitute the upper portion of the gland and secrete hydrochloric acid and intrinsic factor);
the principal zymogene cells (located in the lower portion of the gland, secrete pepsinogen, an important proenzyme for protein digestion that turns into pepsin on contact with hydrochloric acid);
endocrine cells, which mainly secrete histamine, serotonin and somatostatin;
and the mucosecreting cells of the collar (they constitute the precursors of the cells of the mucous surface).
Pepsinogen, hydrochloric acid and mucus are the main constituents of gastric juice.
In the gastric mucosa of the antrum and pylorus glands with prevalent mucous secretion, called pyloric glands, are found. The glandular component of this region is in turn divided into mucosernent cells, gastrin-secreting G cells (hormone sensitive to amino acid and peptide stimulus, promotes the synthesis of hydrochloric acid), serotonin-secreting enterochromaffin cells (stimulates smooth muscle contraction) and cells different types of endocrine hormones such as somatostatin (D cells), glucagon (A cells) and histamine (another stimulant for gastric secretion). The endocrine cells, however, are not exclusively located in the region of the antrum and the pylorus, but rather in the whole gastric mucosa.
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