Histology
Lab-8-
Ass. Lec.
Wafaa H. M. Alhashimy
Dentistry College
Second Stage
Digestive system: small and large intestines
Small intestine
The small intestine is along convoluted tube , extends from the junction with the
stomach to join with large intestine. It is about 7 meters long , the main function of
the small intestine is the digestion of gastric contents and the absorption of nutrients
into blood capillaries and lymphatic lacteals .
Absorption
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Iron is absorbed in the duodenum.
Vitamin B12 and bile salts are absorbed in the terminal ileum.
Water and lipids are absorbed by passive diffusion.
Sodium bicarbonate is absorbed by active transport.
Fructose is absorbed by facilitated diffusion, glucose and amino acid cotransport.
Mucosa is lined by a simple columnar epithelium with Brush border (microvillus)
for increase the surface area over which digestion and facilitates absorption take place.
And contain absorptive cells (enterocytes) are responsible for absorbing nutrients, with
scattered goblet cells are secrete mucus to promote movement and effective diffusion
of gut contents.
The mucosa characterized by evagination into plicae and villi, and the crypts are short
tubular invaginations
The crypts are short tubular invaginations, which provide a protected site for the cells:
-Enteroendocrine cells secrete hormones to regulate secretion into the GI tract.
-Paneth cells, located at the bottoms of the crypts, secrete lysosomal enzymes and
other factors into the crypt lumen.
-Stem cells line the walls of the crypts and continually replenish the intestinal
epithelium, completely replacing all the absorptive and goblet cells approximately
once every 4days.
Villi (singular: villus) are small, finger-like projections that lined by epithelial.
Function the villi together increase intestinal absorptive surface and contains enzymes
on the surface for digestion.
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The lamina propria of each villus is consist:
* Capillaries : collect amino acids and simple sugars.
*A single lacteal (Lymph capillary): collect absorbed chylomicrons (lipoproteins
composed of triglycerides, cholesterol and amphipathic proteins).
*Thin strands of smooth muscle :(presumably allowing some motility for individual
villi, to encourage thorough fluid mixing at the absorptive surface).
Muscularis mucosa is a thin layer (only a few muscle fibers in thickness) beneath the
deep ends of the crypts.
Submucosa is relatively unspecialized, except in the duodenum where it is packed
with the mucous-secreting Brunner's glands.
Muscularis externa : has the standard layers of inner circular and outer longitudinal
smooth muscle, with ganglia of Auerbach's plexus scattered in between. Over most of
the small intestine, the outer layer is a serosa attached to mesentery. The exception is
the duodenum.
The small intestine is dividing into three regions:
* Duodenum: is the shortest, the widest, and has no mesentery, being only partially
covered by peritoneum , and readily distinguished from other regions of the small
intestine by the presence of submucosal Brunner's glands. Villi are rather flatter in the
duodenum than in the jejunum, and plicae are less frequent.
Brunner's glands provide abundant alkaline mucus to neutralize the acid contents
entering from the stomach.
It unites with the jejunum it turns abruptly forward, forming the duodendojejunal
flexure
* Jejunum is the longest and most "typical" region of the small intestine no contains
Brunner's glands and Peyer's patches like the duodenum and ileum respectively. Villi
are rather longer and more finger-like in the jejunum than in the duodenum, and plicae
are usually apparent.
* Ileum has proportionally more goblet cells than more proximal sections of the small
intestine. The ileum also displays an increase in the amount of mucosal Peyer's
patches, are lymphoid tissue may bulge out toward the lumen, displacing villi, and
inward across the muscularis mucosae into the submucosa. Overlying by epithelium is
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cuboidal (rather than columnar as on villi) and includes M cells which transport
antigen across the epithelium to immune cells.
The lymphoid tissue of Peyer's patches is similar to that of tonsils and appendix.
These structures, together with other more diffuse lymphoid tissue, constitute the GutAssociated Lymphoid Tissues, or GALT.
Intrinsic factor (IF) or gastric intrinsic factor (GIF) is a glycoprotein produced by
the parietal cells of the stomach. It is necessary for the absorption of vitamin B12 later
on in the small intestine.
Large Intestine
The mucosa of the large intestine does not have folds comparable to the plicae
circularis, except in the rectum. Also, the intestinal villi are absent beyond the
ileocecal valve.
1. colon
The mucosa of the colon is lined by a simple columnar epithelium with a thin brush
border and numerous goblet cells. ( no plicae or villi ). The crypts of Lieberkühn are
unbranched and lined largely with goblet cells. At the base of the crypts, stem cells
and Enteroendocrine cells are present; however, Paneth cells are not usually present.
The lamina propria are riche by Peyer's patches. The muscularis mucosae is a bit
more prominent compared to the small intestine, and consists of distinct inner circular
and outer longitudinal layers.
The submucosa is mixture of irregular connective and adipose tissue and nerves of the
submucosal plexus.
The muscularis externa is different from that of the small intestine in that the outer
longitudinal layer of smooth muscle varies in thichness and forms three thick
longitudinal bands, called taeniae coli .
2.Appendix : characteristics.
a. The mucosa resembles that of the colon
b. Its is rich in Peyer's patches accumulate in the submucosa, disrupt the
muscularis mucosae and extend into the mucosa, almost approaching the
luminal surface
c. The muscularis externa: is a complete layer (NOT bundled into taenia coli)
Rectum : which are similar to that of colon except for following differences:
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Its is rich in Peyer's patches.
Crypts of Liberkuhn are longest, almost goblet cells.
The muscularis externa: is a complete layer (NOT bundled into taenia coli)
Its serosa is partially replaced by fibrosa.
Mucosa of small intestine
Appendix :
Simple columner
epithelium
crypt
Lamina propria
Peyer’s patch
Submucosa
Muscularis externa
(
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