The three subdivisions of the small intestine are the duodenum, the jejunum and the ileum. The general microscopic structure is similar in all three parts. In the 21 to 23 feet of small intestine, digestion of food materials continues after leaving the stomach, and its selective absorption occurs.
A. === Mucosa===
1. Epithelium. The absorptive process is a function of columnar epithelial cells that possess on their surface numerous microvilli forming a striated border. The microvilli increase the surface area of the lining cells at least twenty fold. Irregularly scattered among the absorptive cells are goblet cells that provide a protective secretion of mucus. The absorptive intestinal cells and goblet cells are distinctive for the small intestine, and they serve to distinguish the small intestine from the stomach that has only mucous columnar cells lining it.
2. Plicae circulares (= valves of Kerckring). Within the small intestine are circular folds of mucosa and submucosa that can be seen grossly. These visible folds, called plicae circulares or valves of Kerckring, are most prominent in the distal duodenum and proximal portion of the jejunum. They extend to the mid-ileum.
3. Villi. The mucosa also exhibits other projections or evaginations called intestinal villi that are covered mainly by columnar intestinal cells with striated borders and goblet cells. These finger-like processes, about 0.5 to 1.5 mm in height, increase the absorptive area of the small intestine in man at least five-fold. The center of each villus contains loose connective tissue of the lamina propria, a central lacteal (a lymphatic capillary), an arteriole and two venules, capillaries and slips of smooth muscle. In general the length and surface of the villi are maximal in the proximal duodenum, and they decrease gradually towards the terminal ileum.
4. Intestinal glands (crypts of Lieberkuhn). These simple tubular glands occupy the lamina propria proper. They open between the bases of the villi and extend for 0.1 to 0.7 mm through the lamina propria to the muscularis mucosae. The epithelium of the villi is continuous with the epithelium of the intestinal glands, but the striated border is progressively reduced towards the blind ends of the glands. In addition to absorptive cells and goblet cells within the crypts, cells of Paneth at the bases of the glands and scattered enteroendocrine cells (argentaffin cells) are also found.
5. Lamina propria. The intestinal glands are not as closely packed as are the gastric glands. A highly cellular lamina propria fills in the spaces between the glands or crypts and as previously mentioned, it also extends as a core of connective tissue into each villus. The main connective tissue fibers are argyrophilic reticular fibers. Elastic and delicate collagenous fibers are also present. Located within the lamina propria are large numbers of lymphocytes and plasma cells, other white cells, mast cells and scattered smooth muscle fibers. The lymphocytes may be organized into solitary nodules scattered all along the intestine. In particular, aggregates of nodules known as aggregated nodules or Peyer’s patches are found in the mucosa and submucosa of the ileum.
6. Muscularis mucosae. This thin region of smooth muscle is organized into an inner circular layer and an outer longitudinal layer, and sends strands of fibers into the lamina propria.
B. === Submucosa.===
1. The submucosa is a layer of loose connective tissue between the muscularis mucosae and the muscularis externa.
2. It contains the large blood vessels, lymphatics, nerve plexus and small parasympathetic ganglia (submucosal or Meissner’s plexuses).
3. Only in the duodenum are large duodenal glands (submucosal glands; Brunner’s glands) located in the submucosa.
C. === Muscularis externa.===
The muscularis externa of the small intestine has smooth muscle arranged into an inner circular layer and an outer longitudinal layer. In connective tissue between the two layers can be found the myenteric plexus containing parasympathetic ganglion cells and nerve fibers.
D. === Serosa.===
The serosa is typical, being a layer of mesothelial cells covering a thin loose connective tissue layer.
PYLORUS== Pylorus-DUODENAL JUNCTIONDuodenal Junction ==
=== Slide 150 Pylorus-duodenum, (H&E). ===
1. Find under low power the typical appearance of the pyloric stomach. Identify the simple columnar mucus-secreting epithelial cells covering the surface and the coiled pyloric glands towards the muscularis mucosae.
2. Scan the slide to identify the junction of the pyloric stomach with the duodenum. This is indicated by the appearance of mucous glands in the submucosa of the duodenum.
a. Note the somewhat abrupt change where typical stomach mucosa ends and small intestinal mucosa begins. Identify:
(1) *villi and the covering epithelium of absorptive cells and goblet cells.
Note that the absorptive cells of the duodenum are much taller than the mucus-secreting cells of the stomach lining. (Goblet cells are not easily identified here.)
(2) *the striated border of the absorptive cells.
(3) *the lamina propria forming the core of the villi and supporting the short
intestinal glands (crypts of Lieberkuhn) below the villi.
(4) *the muscularis mucosae of the duodenum, continuous with that of the stomach.
(5) *the duodenal glands (of Brunner) lying beneath the muscularis mucosae.
These highly branched tubular mucus-secreting glands reside in the submucosa, and their ducts penetrate the muscularis mucosae to empty into the bases of intestinal glands. The glands may extend for a short distance into the submucosa of the pyloric stomach.
(6) *the pyloric sphincter, formed by a great thickening of the circular muscle layer and inclusion of the inner oblique muscle layer of the stomach. The outer longitudinal layer does not take part in the sphincter. *the muscularis externa of the duodenum, an inner circular and an outer longitudinal layer, continuous with those of the stomach.
(7) the muscularis externa of the duodenum, an inner circular and an outer longitudinal layer, continuous with those of the stomach*serosa.
(8) serosa.<peir-vm>UAB-Histology-00150</peir-vm>
DUODENUM== Duodenum ==
On slide 140, Duodenum (PASH):