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MICROSTRUCTURE

The gastric wall consists of the major layers found elsewhere in the gut, i.e. mucosa, submucosa, muscularis externa and serosa, together with gastric vessels and nerves (Figs 65.15, 65.16). The microstructure reflects the functions of the stomach as an expandable muscular sac lined by secretory epithelium, although there are local structural and functional variations in this pattern.

  

Fig. 65.15  Principal regions of the interior of the stomach and the microstructure of tissues and cells within its wall. Undifferentiated, dividing cells are shown in white.

  

Fig. 65.16  Low power micrograph showing the stomach wall, thrown into longitudinal folds or rugae which are visible macroscopically. The surface epithelium is infolded microscopically to form gastric pits. Gastric glands extend through the thickness of the mucosal lamina propria and open into the bases of the gastric pits. A muscularis mucosae layer and submucosa follow the contours of the rugae. Part of the external muscularis layers is seen below left.
(By courtesy of Mr Peter Helliwell and Dr Joseph Mathew, Department of Histopathology, Royal Cornwall Hospitals Trust, UK.)

Mucosa

The mucosa is a thick layer with a soft, smooth surface that is mostly reddish brown in life but pink in the pyloric region. In the contracted stomach the mucosa is folded into numerous folds or rugae, most of which are longitudinal. They are most marked towards the pyloric end and along the greater curvature. The rugae represent large folds in the submucosal connective tissue (see below) rather than variations in the thickness of the mucosa covering them, and they are obliterated when the stomach is distended. As elsewhere in the gut, the mucosa is composed of a surface epithelium, lamina propria and muscularis mucosae.

Epithelium

When viewed microscopically at low magnification, the internal surface of the stomach wall appears honeycombed by small, irregular gastric pits approximately 0.2 mm in diameter (Figs 65.15, 65.16). The base of each gastric pit receives several long, tubular gastric glands that extend deep into the lamina propria as far as the muscularis mucosae. Simple columnar mucus-secreting epithelium covers the entire luminal surface including the gastric pits, and is composed of a continuous layer of surface mucous cells which release gastric mucus from their apical surfaces to form a thick protective, lubricant layer over the gastric lining. This epithelium commences abruptly at the cardiac orifice, where there is a sudden transition from oesophageal stratified squamous epithelium.

Gastric glands

Although all gastric glands are tubular, they vary in form and cellular composition in different parts of the stomach. They can be divided into three groups – cardiac, principal and pyloric. The most highly specialized are the principal glands.

Principal gastric glands

The principal glands are found in the body and fundus, three to seven opening into each gastric pit. Their junction with the base of the pit is the isthmus, immediately basal to this is the neck, and the remainder is the base. The walls of the gland contain at least five distinct cell types: chief, parietal, mucous neck, stem and neuroendocrine.

Chief (peptic) cells (Fig. 65.15) are the source of the digestive enzymes pepsin and lipase. They are usually basal in position and cuboidal in shape, and their nuclei are rounded and euchromatic. They contain secretory zymogen granules and their abundant cytoplasmic RNA renders them strongly basophilic. Parietal (oxyntic) cells are the source of gastric acid and of intrinsic factor, a glycoprotein necessary for the absorption of vitamin B12. They are large, oval and strongly eosinophilic, and have centrally placed nuclei. Parietal cells occur intermittently along the walls of the more apical half of the gland, but can reach as far as the isthmus; they bulge laterally into the surrounding connective tissue. They have a unique ultrastructure related to their ability to secrete hydrochloric acid. The luminal side of the cell is deeply invaginated to form a series of blind-ended channels (canaliculi) that bear numerous irregular microvilli covered by a plasma membrane rich in H+/K+ ATPase antiporter channels. The latter actively secrete hydrogen ions into the lumen; chloride ions follow along the electrochemical gradient. The mitochondria-rich cytoplasm facing these channels contains a tubulo-vesicular system of abundant fine membranous tubules directed towards the canalicular surface. The precise structure of the cell varies with its secretory phase: when stimulated, the number and surface area of the microvilli increases up to five-fold, probably as a result of the rapid fusion of the tubulo-vesicular system with the plasma membrane. This process is reversed at the end of stimulated secretion, when the excess membrane retreats back into the tubulo-alveolar system and microvilli are lost.

Mucous neck cells are numerous at the necks of the glands and are scattered along the walls of the more basal regions. They are typical mucus-secreting cells, displaying apical secretory vesicles containing mucins, and basally displaced nuclei: their products are distinct histochemically from those of the superficial mucous cells.

Stem cells are relatively undifferentiated mitotic cells from which the other types of gland cell are derived. They are relatively few in number, and are situated in the isthmus of the gland and the bases of the gastric pits. Stem cells are columnar and possess a few short apical microvilli. They periodically undergo mitosis; their progeny migrate either apically, to differentiate into new surface mucous cells, or basally, to form mucous neck, parietal, chief or neuroendocrine cells. All of these cells have a limited lifespan, especially the mucus-secreting types, and so they are constantly replaced. The typical replacement period for surface mucous cells is 3 days, and for mucous neck cells is 1 week. Other cell types appear to live much longer.

Neuroendocrine (enteroendocrine) cells occur in all types of gastric gland, but more frequently in the body and fundus of the stomach. They are situated mainly in the deeper parts of the glands, among the chief cells. They are basally situated, pleomorphic cells and display irregular nuclei surrounded by granular cytoplasm that contains clusters of large (0.3 μm) secretory granules. Neuroendocrine cells synthesize a number of biogenic amines and polypeptides that are important in the control of gut motility and glandular secretion. They are part of the system of dispersed neuroendocrine cells: in the stomach they include G cells, which secrete gastrin; D cells, which secrete somatostatin; and ECL (enterochromaffin-like) cells, which secrete histamine.

Cardiac glands

Cardiac glands are confined to a small area near the cardiac orifice: some are simple tubular glands, others are compound branched tubular. Mucus-secreting cells predominate; parietal and chief cells are present, but sparse.

Pyloric glands

Pyloric glands empty via groups of two or three short convoluted tubes into the bases of the deep gastric pits of the pyloric antrum: the pits occupy about two-thirds of the mucosal depth (Fig. 65.17). The glands are populated mainly by mucus-secreting cells, but they also contain neuroendocrine cells, especially G cells, which secrete gastrin when activated by appropriate mechanical stimulation (causing increased gastric motility and secretion of gastric juices). Although parietal and chief cells are scarce, parietal cells are always present in both fetal and postnatal pyloric glands, and may also appear in the duodenal mucosa, proximally near the pylorus, in adult tissue.

  

Fig. 65.17  Micrograph showing the pyloric region of the stomach. Pyloric glands are stained with the periodic acid-Schiff (PAS) technique to show mucin (magenta) in the gastric pits and glands. Pale staining cells are the larger parietal cells (P) and smaller enteroendocrine cells (E).
(By permission from Dr JB Kerr, Monash University, from Kerr JB 1999 Atlas of Functional Histology. London: Mosby.)

Lamina propria

The lamina propria forms a connective tissue framework between the glands. It contains small masses of lymphoid tissue, gastric lymphatic follicles, which resemble solitary intestinal follicles (especially in early life). It also contains a complex periglandular vascular plexus involved in the maintenance of the mucosal environment, e.g. the removal of bicarbonate produced in the tissues as a counterpart to acid secretion, and neural plexuses rich in both sensory and motor terminals.

Muscularis mucosae

The muscularis mucosae is a thin layer of smooth muscle fibres lying external to the layer of glands, arranged as continuous inner circular and outer longitudinal layers, and a discontinuous external circular layer. The inner layer sends strands of smooth muscle cells between the glands: their contraction probably assists in emptying into the gastric pits.

Submucosa

The submucosa is a variable layer of loose connective tissue. It contains thick bundles of collagen, numerous elastin fibres, blood vessels and nervous plexuses, including the ganglionated submucosal (Meissner’s) plexus.

Muscularis externa

The muscularis externa is a thick muscle coat immediately under the serosa, with which it is closely connected by subserous loose connective tissue. From innermost outwards, it contains oblique, circular and longitudinal layers of smooth muscle fibres. The layers are not always easily separated: the circular layer is poorly developed in the oesophageal region, but is thickened at the distal pyloric antrum to form the anular pyloric sphincter; the outer longitudinal layer is most pronounced in the upper two-thirds of the stomach; the inner oblique layer is most obvious in the lower half.

The actions of the muscularis externa of the stomach produce a churning movement that mixes food with the gastric secretions. When the muscles contract, they reduce the volume of the stomach and throw the mucosa into longitudinal folds or rugae (see above). The folds flatten as the stomach distends with food and the musculature relaxes and thins. Muscle activity is controlled by a network of unmyelinated autonomic nerve fibres and their ganglia which lie between the muscle layers in the myenteric (Auerbach’s) plexus.

Serosa or visceral peritoneum

The serosa is an extension of the visceral peritoneum. It covers the entire surface of the stomach other than along the attachments of the greater and lesser omenta to the greater and lesser curvatures respectively, where the peritoneal layers are separated by vessels and nerves, and over a small posteroinferior area near the cardiac orifice, where the stomach contacts the diaphragm at the reflections of the gastrophrenic and left gastropancreatic folds.

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