The Pyloric Sphincteric Cylinder in Health and Disease

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Chapter 5 (page 20)

Chapter 5

The Mucous Membrane or Mucosa

The mucous membrane of the stomach and first part of the duodenum consists of 3 layers; an outermost muscularis mucosae, a middle lamina propria and an innermost glandular epithelial lining, bordering the lumen.


Muscularis Mucosae

The muscularis mucosae, a thin layer of smooth muscle, forms the border between the mucosa and submucosa. It consists of outer longitudinal and inner circular fibres; from the inner layer strands of smooth muscle cells extend through the lamina propria toward the luminal surface between the gastric glands. It is surmized that contraction of these strands may compress the glands, thereby facilitating their emptying (Bloom and Fawcett l975).

Lamina Propria

The lamina propria occupies the narrow region between the muscularis mucosae and the surface epithelial cells with their glands. It extends into the area between the necks of the glands and forms a basement membrane on which the epithelial cells rest. There is little lamina propria in the fornix and body, where the gastric glands are numerous and closely packed; it is more prominent in the cardiac and pyloric zones (Ito l967).

The lamina propria consists of a delicate network of collagenous and reticular fibres and a few fibroblasts or reticular cells. The meshes of the network contain plasma cells, mast cells, eosinophilic leucocytes and lymphocytes. Local accumulations of lymphocytes may occur in the cardiac and pyloric regions. Strands of smooth muscle traverse this layer, which also contains fine capillaries, lymphatic vessels and nerve fibres.

Epithelial Lining

The entire luminal surface of the mucosa is covered by a layer of simple columnar cells, called the surface mucus cells. Numerous tubular invaginations of the surface, the gastric pits or foveolae, are lined by the same type of cell (Ito l967). In the pyloric region the pits are deeper than in the remainder of the stomach, extending at least halfway to the muscularis mucosae. They are V-shaped, tapering off into the glands that open into them (Bevelander and Ramaley l979).

The mucus-secreting columnar cells lining the luminal surface and the pits are joined near their free surfaces to each other by tight junctions (Ham l974). It is surmized that this arrangement forms one of the mechanisms by which the underlying layers are protected against luminal acid. The supranuclear portions of the cells just below their free surfaces contain dense, homogeneous, spherical or ovoid granules consisting of a type of mucigen (Bloom and Fawcett l975). Upon release into the lumen, the granules give rise to the layer of mucus that covers the luminal surface of the mucosa. In the cells of the gastric pits, the granules become progressively less abundant at deeper levels, and in the bottom of the pits they from only a thin layer immediately beneath the cell surface. Cells of this kind continue into the necks of the gastric glands. Under physiological conditions, the surface mucus cells are continuously desquamated into the lumen and are completely replaced every 3 days (Ham l974; Bloom and Fawcett l975). Newly formed cells appear in the deeper parts of the foveolae and in the necks of the glands; these are slowly displaced upward and continually replace those lost on the surface.

Three Mucosal Zones

The entire thickness of the mucous membrane in all parts of the stomach is occupied by a multitude of glands which open into the bottom of the gastric pits. The blind ends of the glands extending into the mucosa are slightly expanded and coiled, sometimes dividing into 2 or 3 branches. On the basis of the cell population in the glands, the gastric mucosal coat is differentiated into 3 zones:

Cardiac Zone

This zone consists of a narrow, ring-shaped area around the gastro-oesophageal junction, containing the cardiac glands. These glands have wide lumina and shallow pits and are composed overwhelmingly of mucus-secreting cells. They may contain a few neuroendocrine cells, also called APUD cells on account of their capacity to synthesize monoamines through a process of Amine Precursor Uptake and Decarboxylation (vide infra). In the transitional area, where this zone is continuous with the second or oxyntic zone, a few parietal cells may be present. The glands of the cardiac zone secrete mucus and very little, if any, enzymes.

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