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The stomach (in ancient Greek στόμαχος, stòmachos, hence Latin stomachus; in Latin also ventriculus) is an organ of the digestive system, located between the esophagus and the small intestine, whose function is to store food and initiate its digestion. The prefix “gastro-“, which identifies medical terms related to the stomach, comes from the Greek γαστήρ, gastèr, “belly, stomach”.
The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. It is here that the digestive processes begin, made possible by both the presence of digestive enzymes and the periodic contractions of the stomach itself. From here, the food passes into the intestine, where the digestive processes can continue, allowing for the absorption of the nutrients in the ingested food.
The main role of the stomach is to digest into linear filaments protein molecules ingested with food (denaturation), through the action of hydrochloric acid and certain enzymes, in order to then allow their absorption at the level of the small intestine. Specifically, gastric juices and enzymes initiate the digestion of fats and proteins by breaking them down into their building blocks (fatty acids and amino acids, respectively). Digestion of carbohydrates in the stomach, on the other hand, is limited because of the highly acidic environment within it.
Digestive functions are facilitated by contractions made possible by the gastric musculature, which stir up the contents of the stomach. In this way, in a maximum time of five hours, the organ is able to digest solid food from the esophagus, reducing it to a semifluid substance (the chyme) that is sent toward the intestine thanks to the opening of the pyloric sphincter, which closes again soon after to prevent the chyme from returning to the stomach.
Digestion is made possible by the gastric glands, which secrete the three basic components of gastric juice. The first is pepsinogen, which after being converted to pepsin is involved in the breakdown of proteins into amino acids. The second is hydrochloric acid, which is necessary for pepsin to perform its function. Finally, the intrinsic factor. Produced by the gastric glands, it is also essential for the absorption of vitamin B12 in the intestines and of iron.
Molecules such as water and alcohol can also be absorbed directly in the stomach.
The function of the stomach is to store ingested food until it is digested, to agitate it internally by its motility, and finally to release it gradually in the form of chyme into the duodenum and then into the rest of the digestive system. The food that enters the stomach through the action of gastric motility and gravity is placed above the food that was previously ingested, in what is physiologically called the “oral” portion of the stomach, that is, the lower and upper two-thirds of the body. By means of sensory endings that terminate in the brain stem and efferent fibers that return to the stomach (“vagus-vagal” reflex), the stomach senses its degree of fullness, reduces its muscle tone, and facilitates its distension when food is ingested. The stomach can hold up to 1.5 liters of chyme.
Food, after being attacked by hydrochloric acid and gastric enzymes, is transformed into a semi-liquid, opaque substance of varying consistency (depending on the amount of water in relation to the consistency of the ingested food) called chyme. Gastric juice, which is an integral part of chyme, is secreted by the gastric glands, which are located in all the walls of the stomach except for a tiny part of the small curvature.
Weak stirring waves (peristaltic waves) are generated in the stomach every 15-20 seconds and propagate from the body to the antrum, increasing in intensity. Stirring waves are generated from slow waves resulting from the intrinsic ability of gastric smooth muscle to maintain a basal electrical rhythm consisting of fluctuations in the membrane potential of smooth muscle fibrocells on the order of 5-15 mV. Some stirring waves are particularly intense and propagate in all directions, generating a circular peristaltic contraction that pushes food from the body of the stomach toward the antrum and pylorus. Peristaltic contractions are generated by action potentials as opposed to slow waves. In the latter case, however, the contraction does not move significant amounts of chyme from the antrum to the duodenum through the pylorus because the pylorus is particularly narrow and the stirring waves cause it to contract rather than expand. Thus, the vast majority of the chyme tends to be pushed back toward the antrum or body instead of into the duodenum.
Although only a very small fraction of the chyme previously present in the stomach passes into the duodenum after each circular peristaltic contraction, the process is useful for gastric remixing. Antral circular peristaltic contractions, on the other hand, are primarily responsible for gastric emptying. These are waves that originate in the antrum of the stomach and propagate towards the pylorus, tending over time to originate higher and higher in the stomach as they reach the body. This particular motility allows the progressive pushing of food in the body toward the antrum. Although again only a few milliliters of chyme pass through the pylorus, it should be remembered that these waves are repeated over time and gradually succeed in emptying the stomach. In addition, the chyme “repelled” by the barrier formed by the pylorus participates in the gastric movement. Another type of rhythmic peristaltic contractions characteristic of the gastric body are hunger contractions. As their name and common experience suggest, they are muscle contractions that occur when the stomach is deprived of food to digest for many hours or days. The starvation contractions can overlap, creating a single tetanic contraction that can last for several minutes and cause the subject pain (called “hunger pangs”).
The pylorus is the sphincter of the stomach, a structure in which the layer of circular muscles is twice as thick as the rest of the stomach. The pylorus is almost always slightly contracted but never completely closed, and liquids can pass through it easily, unlike food that has not yet been well digested and converted into chyme of the correct semi-liquid consistency. Gastric emptying is mainly controlled by signals from the duodenum and stomach and is regulated to match the rate of emptying to the absorptive capacity of the small intestine. Increased gastric content, for example due to ingestion of food, facilitates emptying because the distension of the gastric walls due to the ingestion of food activates the myonteric plexus, which increases the frequency of antral circular peristaltic contractions and pyloric distention. A second factor that aids gastric emptying is the secretion of gastrin, which is increased when the gastric walls are distended by food and when proteins are digested. Gastrin modestly increases gastric motility, particularly circular peristaltic contractions of the antrum; it also promotes glandular secretion of the stomach walls. Duodenal factors that trigger gastric emptying are mainly responsible for this.
Three types of reflexes originate from the walls of the duodenum and act by inhibiting gastric emptying. A first modality is the activation of the enteric nervous system of the duodenum, which gives rise to inhibitory enterogastric reflexes. This activation can occur depending on various factors such as the degree of duodenal wall distension, excessively acidic pH, the amount or osmolarity of chyme entering the duodenum, the significant presence of products of protein and lipid catabolism, and irritation of the gastric mucosa. Inhibition is then possible by extrinsic fibers that reach the spinal cord and then travel to the paravertebral orthosympathetic ganglia and then to the gastric wall via sympathetic inhibitory fibers; finally, a third mode, of lesser importance, by vagal fibers that travel to the brainstem and there inhibit excitatory stimuli from the vagus nerve itself. All three types act by inhibiting circular peristaltic contractions and causing the pylorus to contract.
Protein digestion is carried out by lytic enzymes, chymosin, also called rennin or labferment because it is specific for milk casein, the presence of mild gastric lipase, and pepsin (proteins are broken down into smaller chains called polypeptides). In addition, there is the absorption of water, some ions and fat-soluble compounds such as alcohol, acetylsalicylic acid, caffeine and, last but not least, the sterilization of ingested food by hydrochloric acid. Pepsin only works in a low pH environment. This is always ensured by the presence of hydrochloric acid.
The combination of all these elements is called gastric juice, which is activated even when we only think about eating (in fact, “watering” comes, i.e. salivation is stimulated). The walls of the stomach would be damaged by the inherent acidity of the gastric juice if there were no protective factors, but the stomach secretes a substance, mucin, which prevents this problem. Finally, the thick musculature ensures the stirring of the food, which is transformed into chyme during its stay in the stomach, which can vary from one to three hours.
Another function is absorption, which begins in the cells of the stomach: a minimal amount of water, some short-chain fatty acids, and some drugs such as aspirin are absorbed, and finally alcohol.
Anatomy and structure
The human stomach is a muscular, unequal, paramedian abdominal organ, shaped like an elongated sac, flattened antero-posteriorly, occupying topographically the regions of the left hypochondrium and epigastrium; however, it should be emphasized that it exhibits considerable variability in shape and position both between living and cadaveric, and depending on the physical constitution, its filling and the position assumed: In fact, it shows a greater vertical axis in the longilineal, while in the brevilineal it tends to assume a greater horizontal axis. On observation, it presents an anterior and posterior surface, a concave right border or small curvature and a convex left border or large curvature.
The small curvature forms the postero-superior border of the stomach; it extends to the right and inferiorly, then it rises more gently at the level of the angular incisura and continues to descend, ending at the level of the pylorus. The hepatogastric ligament, which connects the liver and the stomach, is attached to it anteriorly, and the small omentum is formed by the hepatoduodenal ligament.
The great curvature is four to five times longer than the small curvature (about 40 cm), begins at the cardiac incisura and then rises, forming the dome-shaped edge of the gastric fundus, reaches the esophagus, then, starting from the apex of the fundus (the point of maximum convexity, just below the nipple), it descends inferiorly and medially until it reaches the intermediate groove separating the pyloric antrum from the pyloric canal. The great curvature is lined anteriorly by the peritoneum, while laterally, farther to the left, the two anterior and posterior peritoneal laminae join to form the gastrojejunal ligament, which connects the gastric wall to the splenic hilum. Also on the wall of the great curvature is the gastrocolic ligament, which extends from the great curvature to the transverse colon, right colic flexure, and duodenum to form the anterior root of the great omentum.
The anterior or upper surface of the stomach is lined by the peritoneum and contracts with the diaphragm, with the spleen, determining its gastric surface, with a part of the left and square lobes of the liver and with the transverse colon. The posterior or inferior surface of the stomach contracts relations with the left adrenal gland, with the body and tail of the pancreas, with the aorta and the lienal and hepatic arteries, and with the portal vein. It is completely lined with peritoneum, except near the cardia, where it is in contact with the diaphragm.
Four main parts (fundus, body, pyloric antrum, pyloric canal) and two orifices (cardia, pylorus) are recognized in the stomach.
- Cardia (lat. pars cardiaca): represents the opening that connects the stomach and the esophagus; externally, this junction is not covered by the peritoneum and has no thickening of the muscular tonaca. The cardia allows the passage of saliva-soaked food (food bolus) in only one direction, from the top to the bottom, and prevents its reflux into the esophagus by a number of mechanisms, such as maintaining a certain muscular tone and oblique fibers of the internal muscular tonaca of the stomach, which form a virtual valve closing the lumen. The demarcation between the esophagus and the stomach is represented by the Z-line, which consists of a portion of the gastric mucosa that deepens for a few centimeters inside the esophageal lumen and then ends with a zigzag profile and a squamous or columnar structure. The cardia’s mucosa rises in characteristic reliefs called “mucosal rosettes” that help prevent gastroesophageal reflux.
- Fundus (lat. fundus ventriculi/stomachs): is a glandular portion resting posteriorly on the diaphragm, distinguishable by drawing an imaginary horizontal line from the cardiac incisura. It corresponds radiologically to the gastric bulla, which is the part of the stomach filled with air and therefore radiolucent, since it is not reached by radiological contrast. Its projection on the chest wall is called the semilunar space of Traube, bounded inferiorly by the lower edge of the 9th costal cartilage and the xiphoid process of the sternum, superiorly by the 5th-6th rib, laterally to the left by the costal arch and to the right by the anterior edge of the liver. The mucous membrane of the fundus of the stomach has temporary folds which disappear completely when the stomach is distended.
- Corpus (lat. corpus ventriculi/stomach): is the largest part, glandular in shape with a vertical axis, slightly inclined to the right and narrowed at the bottom. It is located between the base of the gastric fundus and the angular incisura. The mucous membrane of the body of the stomach has permanent gastric folds, which are particularly widespread in the postero-medial, medial and antero-medial regions, that is, in the area proximal to the small curvature. The inner walls of the greater curvature have folds with a more convoluted pattern that become more defined and elevated as one moves from the bottom of the stomach to the border of the pyloric antrum. This slows the passage of fluids and the food bolus. It is hypothesized that fluids can pass more quickly along the small curvature than along the large curvature, which is why the folds of the former together form what is called the “short gastric pathway”.
- Pyloric antro (lat. pars pylorica): is a cylindrically shaped portion that runs laterally and superiorly to the body. It is located between the angular incisura and the pyloric groove. Its internal mucosa is mostly smooth, but in a contracted state, at the border of the pyloric canal, there are noticeable folds; these are longitudinal folds, more similar to those of the short gastric path than to those of the great curvature.
- Pyloric canal: is a hemispherical portion between the intermediate sulcus and the pylorus; it runs inferiorly and laterally to the pyloric antrum.
- Pylorus (lat. pylorus): is a muscular sphincter connecting the stomach to the duodenum, the position of which can be determined by the narrowing of the pyloric canal. It consists of thickened circular smooth muscle fibrocells intertwined with some oblique muscle fibers.
Gases generated by bolus digestion tend to rise and concentrate in the fundus of the stomach, which is the most cranial area of the organ. In humans, the stomach has a capacity of 0.5 L when empty, and an average capacity of about 1-1.5 L when completely full. After a normal meal, it generally expands to hold about 1 L of bolus, but it can also expand to hold up to 4 L and more, compressing other organs in the abdominal cavity and often the chest as well.
All major gastric arteries arise from the celiac trunk of the abdominal aorta. The aorta branches into 3 major arteries, the left common iliac artery, the right common hepatic artery, and the left gastric artery. Because of its wide and branched arterial circulation, gastric ischemia is rare. However, numerous variations of the gastric arteries are possible.
- The left gastric artery, starting from the celiac trunk, ascends to the cardia, where it branches into an esophageal branch that ascends to the esophagus, then curves and follows the course of the small curvature of the stomach, remaining within the peritoneum. Along the small curvature it sends branches to both the upper and posterior sides of the stomach, thus contributing to the vascularization of the cardiac area, sometimes a small part of the fundus of the stomach and the upper part of the fundus of the stomach in the anterior and posterior sides.
- The right gastric artery is a branch of the hepatic artery propria; it runs superior to the gastroduodenal artery, then descends to the level of the pylorus and follows the small curvature, anastomosing with the left gastric artery. Like the left gastric artery, it sends branches to the upper part of the pyloric canal, the pyloric antrum and the body of the stomach in their anterior and posterior faces.
- The short gastric arteries, which vary in number from 5 to 7, are small arteries that branch from the lienal artery at the hilum of the spleen, they ascend until they lead anteriorly and posteriorly to the fundus of the stomach, anastomosing with branches of the left gastric artery and the left gastroepiploic artery.
- The left gastroepiploic artery is the largest branch of the lienal artery and originates from it in the lower part of the posterior aspect of the spleen, remaining within the gastrolienal ligament. From here it follows the course of the great curvature of the stomach, sending branches to the anterior and posterior surfaces, which anastomose with the short gastric arteries and the left gastric artery. It then perfuses the lower part of the gastric fundus. Some of its branches, however, penetrate the great omentum and irrigate its upper part.
- The right gastroepiploic artery is a large branch of the gastroduodenal artery. It follows the great curvature of the stomach, sending branches anteriorly and posteriorly to the lower area of the pyloric canal, the pyloric antrum and part of the body of the stomach, and then anastomoses with the left gastroepiploic artery.
- The gastroduodenal artery is the main branch of the common hepatic artery. It passes inferiorly and posteriorly to the pylorus and duodenum, and then branches into the right gastroepiploic artery and the antero-superior pancreatic-duodenal artery.
- The posterior gastric larteria is not always present; when it is, it is a branch of the lienal artery that decorates the posterior part of the body of the stomach and ascends until it branches into the upper part of the stomach and the fundus of the stomach.
The gastric veins drain primarily into the portal vein, the inferior vena cava, and the superior mesenteric vein. As with the arteries, variations in venous distribution are common and numerous.
- The left gastric vein drains blood from the abdominal part of the esophagus through the esophageal tributary vein, from the right side of the fundus of the stomach, from the right upper part of the body of the stomach. It forms a kind of undulating semicircle from the small curvature, receives the esophageal tributaries at the level of the cardia, then passes anteriorly to the celiac trunk, the inferior vena cava, descends and finally flows into the portal vein.
- The right gastric vein drains blood from the left upper part of the gastric body, the pyloric antrum and the pyloric canal. It follows the course of the small curvature, then rises vertically and heads posteriorly to the portal vein. At the level of the pylorus it also receives blood from the prepyloric vein.
- The short gastric veins, numbering 4-5, drain blood from the fundus of the stomach and part of the body of the lower stomach and then flow into the lienal vein.
- The left gastroepiploic vein drains blood from the lower part of the body of the stomach and parts of the greater omentum. It runs along the greater curvature and joins the right gastroepiploic vein, with which it anastomoses.
- The right gastroepiploic vein drains blood from the right lower body of the stomach, antrum and pyloric canal. It runs along the great curvature, receives blood from the left gastroepiploic vein, and then flows into the superior mesenteric vein. It may receive blood from the superior pancreaticoduodenal vein before flowing into the superior mesenteric vein.
- The posterior gastric vein, if present, decorates the posterior gastric artery and drains blood from the posterior middle part of the gastric body and part of the fundus of the stomach, draining into the lienal vein.
The gastric lymphatic vessels are part of the continuous lymphatic network in the upper part of the abdomen; in particular, they are in continuity with the esophageal and duodenal vessels, as well as with the pancreatic, hepatic and spleen vessels. The lymphatic vessels themselves can be described as divided into three zones. The first drains the upper part of the anterior and posterior surfaces of the fundus, body, pyloric antrum and pyloric canal. These vessels drain into the cardiac lymph nodes (at the heart, within the peritoneum) and the suprapyloric lymph nodes (above the pylorus), which in turn drain lymph into the celiac lymph nodes (anterior to the celiac trunk). The second drains the lower part of the anterior and posterior body, the antrum and the pyloric duct. These vessels flow into the subpyloric lymph nodes (located below the pylorus and its right side) and the right gastroepiploic lymph nodes. A third zone drains the lymph from the lower left part of the gastric body and the left part of the gastric fundus. Its vessels drain into the left gastroepiploic lymph node, which in turn drains lymph into the lienal lymph nodes located at the hilum of the spleen. Lymph draining the entire stomach eventually drains to the celiac lymph nodes and from there to the chyle cistern.
The sympathetic innervation of the stomach is derived from the celiac plexus, the hepatic plexus, and the large and small splanchnic nerves, corresponding to the anterior branches of the thoracic nerves T5 to T12. The nerves of the celiac plexus reach the stomach by following the course of the arteries, which they innervate and surround, and then distribute to the organ. The sympathetic branches of the celiac plexus tend to distribute to the posterior (inferior) surface of the stomach and to the antrum, while those of the hepatic plexus distribute to the anterior (superior) surface and to the fundus. The sympathetic system of the stomach is vasoconstrictive to its vessels, inhibitory to the gastric musculature, while making the pylorus contract, also transmitting sensitivity and pain.
The parasympathetic innervation of the stomach is derived from the anterior and posterior branches of the vagus nerve. The anterior vagus nerve, which originates from the left vagus nerve and the esophageal plexus, adheres closely to the outer muscular layer of the esophagus and divides into hepatic, gastric, and pyloric branches at the small curvature. Some of the anterior gastric branches detach from the anterior vagus to innervate the fundus and part of the body of the stomach (upper face), while the main branch, called the anterior gastric great nerve, runs in the small omentum and the great curvature, giving off branches to the body of the stomach and the antrum, whose course follows that of the right and left gastric arteries, and then branches to the pylorus (pyloric nerves). The posterior vagus nerve runs in the tonacic adventitia of the esophagus and is less closely attached to it. It gives off gastric branches that run behind the cardia and distribute to the lower surface of the stomach and antrum, and celiac branches that join the celiac plexus. Other plexuses are found in the external muscular tonaca (Auerbach’s plexus) and submucosa (Meissner’s plexus). The parasympathetic of the stomach is responsible for gastric motility, glandular secretion, and pyloric release.
The stomach, like all organs of the gastrointestinal tract, has four tonacas, from the innermost to the outermost: mucosal, submucosal, muscular, and serous.
The mucosa of the stomach is composed of a superficial epithelium in contact with the lumen of the organ, a lamina propria of connective tissue and muscularis mucosae. Its color varies from red in the fundus and body of the stomach to pink at the pylorus. The mucosa rises in folds of different shapes depending on the area of the stomach considered; some are temporary (fundus, pyloric antrum), in which case they are wrinkles of the submucosa that appear during contraction; others are permanent (body), in which case they are true folds of the mucosa.
The lining epithelium consists of a simple monolayered cylindrical epithelium with a secretory and lining function. The cells that make it up are called “mucoid” cells. They have an average lifespan of 3 days. They have a highly developed Golgi apparatus in a perinuclear position and a central nucleus. On the cis side of the Golgi apparatus there are numerous protosecretory vesicles containing the neutral proteoglycans characteristic of the mucosal secretion of these cells. These proteoglycans serve to protect the mucosal epithelium from acidic pH and the action of gastric juice enzymes. Epithelial cell secretions are directed toward the apical pole of the cell. The epithelium has numerous invaginations, about 0.2 mm in diameter, with an irregular lumen, called gastric pits, at the base of which the gastric glands of the stomach open, which then deepen into the lamina propria. The gastric glands, which discharge their secretions into the crypts of the gastric areolae, are divided into three different types: the cardinal glands, the main gastric glands and the pyloric glands. The gastric glands differ in structure according to the different regions of the stomach considered.
The cardia glands are located in a band about 3 cm from the cardia. These glands are simple tubular or compound tubular and produce a mucous secretion containing neutral glycoproteins. There may also be some large gastric glands intermingled with the cardial glands. Sometimes such glands are found in the esophagus, in which case they constitute ectopia.
The main gastric glands are located in the fundus and body of the stomach and are the most widespread in the organ. They are located in the gastric pylorus, for each pylorus there are 3 to 7 tubular glands. The part of each gland connected to the bottom of the gastric pit is called the isthmus, immediately below it is the collar, then the body and finally the base of the gland. This regional distinction is useful in understanding the distribution of the cells that combine to form these complex glands. There are at least five types of cells in each gland, some of which have subtypes.
- The superficial mucosal cells cover the walls of the gastric pit and are therefore distributed in the isthmus and in the apical part of the collar. They have a cylindrical and elongated shape, with a brush-like edge at the apical pole of the cell, which protrudes towards the lumen and is the same where the mucus is secreted. The nucleus, roundish or oval, is located at the base of the cell, centrally is a developed Golgi apparatus, and in the apical portion are concentrated large vesicles containing mucus about to be secreted. Numerous mitochondria, often elongated and bastoncellular in shape, rather developed smooth endoplasmic reticulum.
- The mucous cells of the collar are the main type of cells in this area of the gastric gland. They have either a cylindrical shape, more squat than that of the superficial mucosal cells, or a prismatic shape, with an equally developed brush-like rim. The nucleus is oval with the major axis perpendicular to that of the cell. The Golgi apparatus is centrally located and well developed, while the smooth endoplasmic reticulum tends to concentrate around the nucleus, enveloping it. Mitochondria are quite numerous, and mucin vesicles, smaller than those of superficial mucosal cells and containing chemically distinct acidic proteoglycans, tend to concentrate at the apical zone of the cell. These cells are completely replaced every 3 days due to the highly acidic environment of the stomach.
- Parietal (oxyntic) cells are mainly distributed in the body of the gastric gland, but more rarely in the collar or isthmus. They are very large, pyramidal cells with a characteristic structure; in fact, their apical surface invaginates into numerous canaliculi lined with microvilli containing proton (H+) and potassium (K+) pumps on the plasma membrane. These microvilli appear to form and split continuously according to the secretory activity of the cell. The canaliculi are connected to a tubulo-vesicular system that permeates the cytoplasm of the cell. The flow of protons and chloride ions leaving the parietal cells determines their main function, which is the secretion of hydrochloric acid, which contributes to maintaining the gastric pH at values between 1 and 3. Parietal cells have an eosinophilic cytoplasm very rich in mitochondria, predominantly smooth endoplasmic reticulum concentrated at the base of the cell, while the nucleus is centrally located and round in shape. The Golgi apparatus is present, but not as well developed as in mucus cells. In addition to secreting hydrochloric acid, they produce intrinsic factor, a protein essential for cobalamin (vitamin B12). They are completely replaced each week.
- The main (zymogenic) cells are located in the base and body of the gastric glands. They are prismatic or cuboidal cells with a less developed brush-like rim than mucosal cells and a strongly basophilic cytoplasm due to the abundance of ribosomes and RNA. Their nucleus is roundish or oval, euchromatic, and located in the basal zone of the cell, just above the wrinkled endoplasmic reticulum, which is instead concentrated just below the nucleus and on its flanks. Mitochondria are scarce, whereas the Golgi apparatus is well developed. The zymogen granules, round and electron dense, are concentrated at the apical pole of the cell, but are also easily found centrally above the nucleus. They contain the digestive enzymes pepsinogen and lipase.
- Neuroendocrine cells are widely distributed in the basal zone and body of the gastric glands, in all areas of the stomach, including the cardia and pylorus. They are highly variable in shape, sometimes vaguely pyramidal, with irregularly shaped but often rounded nuclei. The cytoplasm contains mitochondria, a highly developed Golgi apparatus, several cisternae of wrinkled endoplasmic reticulum. Small secretory vesicles (0.3 µm in diameter) with highly electron dense contents are concentrated in an area close to the nucleus. Based on vesicle content, gastric neuroendocrine cells are classified as G-cells (gastrin), δ-cells (somatostatin), and ECL-cells (histamine). They also secrete factors that control intestinal motility and glandular secretion.
- Stem cells are undifferentiated cells often found in the mitotic phase in the isthmus and especially in the body of the gastric gland. They are cylindrical in shape with an underdeveloped brush-like margin. Their central position in the gastric glands allows them to differentiate and migrate either to the isthmus or to the base of the gland, differentiating into the different possible cells based on stimuli received from the environment and interactions with neighboring cells.
Pyloric glands are branched tubular glands (usually consisting of 2-3 tubules) located at the base of the pyloric antrum. They consist of mucous cells, neuroendocrine cells such as G-cells (which secrete gastrin) and others that secrete serotonin, and some parietal and large cells.
The lamina propria consists of loose connective tissue, bundles of collagen and elastic fibers, with fibrocytes, macrophages, eosinophilic granulocytes, and plasma cells. In the lamina propria there are a large number of blood capillaries, aggregates of lymphoid tissue, sometimes in the form of true gastric lymphatic follicles, and nerve plexuses with sensory and motor endings. It is separated from the epithelium by a weakly PAS-positive basement membrane.
The muscolaris mucosae of the stomach is a layer of smooth muscle cells located beneath the lamina propria. Three layers can be distinguished: an inner circular one, which sends bundles of smooth muscle fibers between the glands, facilitating the emptying of the secretion into the pits and thus into the gastric lumen; a longitudinal one, which continues the inner circular one; and finally an outer circular one, which is discontinuous with respect to the other two.
The submucosal tonaca of the stomach is mainly composed of loose connective tissue with numerous elastic and collagenous fibers. It is abundant and allows the formation of transient mucosal folds. Macrophages, eosinophilic granulocytes, lymphocytes, plasma cells and fibroblasts are present. The elements of the immune system may sometimes aggregate to form true lymphoid follicles with a germinal center. Houses Meissner’s submucosal nerve plexus.
The stomach has a complex smooth muscle structure that completely encloses it in several layers. The inner layer consists of smooth muscle fibers arranged obliquely from a region called the collar of Helvetius, located at the cardiac incisura. The fibers descend from the cardial incisura on the anterior surface of the body of the stomach until they reach the pyloric antrum. However, these fibers are rather scattered and do not form a very dense layer. The middle muscular layer has fibers with a circular course, concentrically wrapping around the fundus of the stomach and then wrapping around the body of the stomach, the antrum and the pyloric canal (which then continues into the duodenum) in a postero-anterior direction. The myofascial layer of the stomach has longitudinal fibers.
The outer esophageal muscle fibers divide at the level of the cardia into two large bundles, one running along the fundus of the stomach (passing over the circular musculature) and then along the great curvature to the pyloric canal, the other running along the small curvature, then joining the former at the level of the pylorus and continuing as a single muscle layer into the duodenum. Thus, a window in the anterior surface of the stomach is left without the outer longitudinal layer and is covered only by the inner oblique layer and the overlying middle circular layer. The pylorus consists of a thickening of the middle circular musculature and the oblique fibers, which deepen between the circular fibers. The muscular layers of the stomach are covered by the visceral peritoneum, to which they are intimately connected. Muscle contraction is regulated by a network of amyelinated nerve fibers of the enteric nervous system, located between the muscle layers to form Auerbach’s myenteric plexus.
The serosal tonaca is formed by the visceral peritoneum, which is arranged to cover the stomach almost completely. It consists of mesothelium and a submesothelial layer of loose connective tissue with elastic and collagenous fibers. Serosa is absent at the insertion of the great omentum and the small omentum at the great curvature and the small curvature, because the peritoneal leaflets are separated by blood vessels and nerves. There is also a triangular area behind the cardia not covered by the peritoneum and in direct contact with the diaphragm.
The stomach is one of the organs most prone to neoplasia, about 23% of all neoplasms (in Europe), which seems to be favored by the use of salt and smoked foods.
In addition to neoplasms, the organ is also prone to other diseases such as gastritis and related ulcers.