Light and electron microscopy of the European beaver (Castor fiber) stomach reveal unique morphological features with possible general biological significance.

Anatomical, histological, and ultrastructural studies of the European beaver stomach revealed several unique morphological features. The prominent attribute of its gross morphology was the cardiogastric gland (CGG), located near the oesophageal entrance. Light microscopy showed that the CGG was formed by invaginations of the mucosa into the submucosa, which contained densely packed proper gastric glands comprised primarily of parietal and chief cells. Mucous neck cells represented <0.1% of cells in the CGG gastric glands and 22-32% of cells in the proper gastric glands of the mucosa lining the stomach lumen. These data suggest that chief cells in the CGG develop from undifferentiated cells that migrate through the gastric gland neck rather than from mucous neck cells. Classical chief cell formation (i.e., arising from mucous neck cells) occurred in the mucosa lining the stomach lumen, however. The muscularis around the CGG consisted primarily of skeletal muscle tissue. The cardiac region was rudimentary while the fundus/corpus and pyloric regions were equally developed. Another unusual feature of the beaver stomach was the presence of specific mucus with a thickness up to 950 µm (in frozen, unfixed sections) that coated the mucosa. Our observations suggest that the formation of this mucus is complex and includes the secretory granule accumulation in the cytoplasm of pit cells, the granule aggregation inside cells, and the incorporation of degenerating cells into the mucus

Ultrastructure of mucous cells in the pyloric glands.

<p>Note the three types of granules: 1) irregular, filled with electron-lucent material and “speckled” with electron-dense spots (arrows); 2) round, bipartite (double arrows); and 3) granules with intermediate properties.</p

Ultrastructure of chief cells in the proper gastric gland of the corpus mucosa.

<p>(A) Bipolar organization of chief cells. Note the well-developed Golgi apparatus (<i>AG</i>). L: lumen of the gland. (B) Numerous secretory granules, varying in size and electron density, located in the supranuclear cytoplasm. (C) Well-developed rough endoplasmic reticulum in the basal region.</p

Gross morphological features of the European beaver stomach.

<p>(A) An external view of the stomach. Asterisk: the CGG on the lesser curvature; <i>E</i>: oesophagus; <i>D</i>: duodenum. (B) The mucosa after cutting the stomach along the greater curvature. Note differences in mucosal colour between the corpus - <i>C</i> and pylorus - <i>P</i>. Asterisk: the CGG. (C) The CGG viewed from the stomach lumen. Note the large crater-like (arrow heads) and small (arrows) orifices. <i>Z</i>: gastroesophageal junction (D) The stomach cut along the lesser curvature. Note the continuity between the muscular coat of the CGG and the muscularis of the esophagus. Asterisk: the CGG; <i>E</i>: oesophagus. (E) Section of the CGG showing its internal organization. Arrows: branched tubes that open on the luminal mucosal surface.</p

Ultrastructure of the neck of a proper gastric gland in the stomach corpus.

<p>Mucous neck cells - <i>M</i>, parietal cell - <i>P</i>, and differentiating chief cell - <i>C</i> surrounding the lumen of the gland - <i>L</i>. Note the bipartite secretory granules (arrows) and well-developed Golgi apparatus – <i>AG</i> present in the mucous neck cells as well as the numerous cisternae of rough endoplasmic reticulum (asterisk) and round, electron-dense secretory granules (double arrows) in the developing chief cell.</p

Ultrastructure of endocrine cells in the proper gastric gland of the corpus mucosa.

<p>(A) Endocrine cell with well-developed endoplasmic reticulum and polymorphic granules. (B) Endocrine cell with small, round, electron-dense granules and numerous filaments.</p

Cellular composition of the proper gastric glands.

<p>(A) Fundus mucosa. (B) Corpus mucosa at the greater curvature opposite to the GCC. (C) Mucosa surrounding the CGG orifices. (D) Corpus mucosa close to the pylorus. (E) The CGG. Cells were counted in ca. 500 000 µm² areas that contained longitudinal sections through the gastric glands, as described in “<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0094590#s2" target="_blank">Material and methods</a>”. Total number of cells forming the glands was taken as 100%. The presented data are means and standard deviations (n = 12).</p

Surface epithelium cell morphology and mucus structure.

<p>Note differences in the appearance of cells that line various parts of the pits as well as the presence of cell debris in the mucus.</p

Ultrastructure of the surface epithelium cells in the basal (A), middle (B, C), and upper (D, E) regions of the corpus gastric pits and mucus structure (F).

<p>(A) Pit cells containing electron dense granules in their apical parts. Note the bundles of tonofilaments (arrow). (B) Cells with numerous electron-dense granules that form oval clusters in the supranuclear cytoplasm. (C) Cells with numerous granules that vary in electron density and aggregate with each other. (D) Degenerating cells with flattened nuclei and cytoplasm filled by the mucous mass. (E) Cells with destroyed apical regions. Note the mucus is partially released from the cell. (F) Mucus ultrastructure. Cell debris and fine fibrils are visible.</p

Ultrastructure of the neck of a proper gastric gland in the CGG.

<p>(A) Longitudinal section through the neck. Parietal cells - <i>P</i> situated above each other and undifferentiated cells - <i>U</i> between them. (B) Mucous neck cell. Note its small size and bipartite secretory granules. <b>C</b>. Two parietal cells - <i>P</i> forming the intercellular canaliculus (arrows). (D, E) Well-developed tubulovesicular system in the parietal cell at low (D) and high (E) magnification.</p