Digestive System I: The Gut
I) General Histological Organization
a. Common traits of the stomach, esophagus, and intestines
i. Layered Walls (three “true” layers*)
1. Innermost mucosa*
2. Muscularis mucosa
a. Considered part of the mucosa
b. Divides mucosa and submucosa
c. Very thin, not visible in all cross sections
3. Underlying Submucosa*
a. Meissner’s Plexus
i. secretory activity of glands in gut wall
ii. Contraction of the villi
iii. Exerts parasympathetic control
4. External Muscularis*
a. Inner circular smooth muscle layer
b. Outer longitudinal smooth muscle layer
c. Auerbach’s plexus
i. lies in between and outer/inner layer
ii. exerts parasympathetic control
5. Outermost Adventitia
a. Thin outer layer of connective tissue
b. Known as a “Serosa” when it is cover with
mesothelium (simple squamous epithelium)
b. Regional Distinctions (see chart on pg. 25)
i. Mainly distinguished by differences in mucosal epithelium and
variations in the layers
c. Epithelium
i. Mucosal epithelium, in general, is Simple columnar
ii. Each region of the gut has special cells that reflect the function of the
region (i.e. secretion, transport, absorption, etc)
1. Simple columnar cells
a. Abundant microvilli are present in absorptive regions
2. Mucous Cells
a. Mostly goblet cells
b. Large apical mucous droplets are visible
c. Extremely prominent in the colon
d. In stomach, mucous producing cells have dispersed
droplets in their cytoplasm
3. Parietal Cells
a. Located in Stomach
b. Eosinophilic
c. Abundant mitochondria
d. Intercellular canaliculi
i. Extensive invaginated membranes
ii. Involved in HCl secretion
iii. Secret Gastric Intrinsic Factor (GIF)
1. Required for ileal absorption of B12
2. Co-factor for RBC production
4. Enteroendocrine cells
a. Multiple types throughout the gut
b. Form a diffuse neuroendocrine system (DNES)
i. Secretion
ii. Gut motility
c. Classes described by staining properties or function i.e.
i. Argentaffin
1. silver staining
ii. Amine Precursor Uptake and Decarboxylation
(APUD)
1. special role in catecholamine production
5. M-Cells
a. Specialized intestinal cells
b. Uptake and presentation of antigens to lymphocytes
c. Required for activation of GALT
6. Progenitor Cells
a. Undifferentiated cells
b. Located in certain epithelial glands
c. Source of cells for renewal of epithelial surface
d. Replace epithelium every 5-7 days
e. Look for mitotic activity at the base of intestinal glands
iii. Specific distinctive features
1. Duodenum
a. Brunner’s Glands (submucosal mucous glands)
2. Ileum
a. Peyer’s Patches (submucosal lymphoid nodules)
3. Colon
a. Taenia Coli (unusual bundled arrangement of outer
longitudinal muscles)
d. Nerves
i. Autonomic regulation controls secretion, blood flow, and smooth
muscle contraction
ii. Sympathetic and Parasympathetic Nerves form two principal plexi
1. Auerbach’s Plexus
a. Located between inner/outer layers of the muscularis
b. Prominent Post ganglionic parasympathetic nerve cells
2. Meissner’s Plexus
a. Located submucosally with fibers that penetrate mucosa
b. Parasympathetic ganglion cells are less conspicuous
3. All postganglionic parasympathetic cell bodies are derived
from the vagus nerve
4. Sympathetic fibers are present in both plexi, but the cell bodies
are in the sympathetic chain and collateral ganglia
e. Blood Supply and Lymphatics
i. Sub-mucosal tissue is supplied by vast capillary networks that extend
into mucosal layer
ii. Lymphatics drain in the opposite direction, from lumen towards
muscularis
1. Primary lymphatic channels are visible at lacteals (core of the
villi in the small intestine)
II) Esophagus
a. Mucosa is lined by stratified squamous non-keratinized epithelium
b. Smooth muscle fiber bundles of muscularis mucosa mix with submucosal CT
c. Upper esophagus
i. Muscularis externa is striated skeletal muscle
d. Lower esophagus
i. Contains sub-mucosal and mucosal mucous glands
e. esophageal-cardiac junction
i. Contains sub-mucosal and mucosal mucous glands
ii. Has a sharp transition from mesothelium to highly folded surface of
the cardiac stomach (simple glands with cuboidal mucoid cells)
iii. Muscularis externa thickens to form a sphincter (prevents reflux)
III) Stomach
a. Cardiac Region
i. Shallow simple mucous glands
b. Fundic Region
i. Main body of the stomach
ii. Thickest mucosa with deep epithelial glands
iii. Mucous cells in gastric pits at the base of a gland
iv. Parietal cells—see I-C-3 above
v. Chief Cells
1. Located at the base of the glands
2. Cytosol has protease-rich, basophilic zymogen granules
c. Pyloric Stomach
i. Shallower glands
ii. Clusters of mucous producing alveoli at the base of the glands
1. Highly basic mucous neutralizes stomach acid before passing
the pyloric sphincter and entering the duodenum
IV) Small Intestine
a. General
i. Extensive villi for increased absorption
ii. Crypts of Lieberkuhn
1. Glandular invaginations between villi
iii. Lamina propria of each villus
1. Composed of loose CT
2. Contains various lymphocytes, plasma cells, leukocytes
iv. Jejunum (which has the longest villi) is the site of most absorption
b. Epithelium
i. Columnar epithelium
ii. apical microvilli (striated border)
iii. goblet cells
iv. nuclei displaced to basal region
v. Enteroendocrine cells
1. Release neurosecretory products
2. Adluminal nuclei
3. Cytoplasm and secretory granules face the capillaries of the
lamina propria (consistent with endocrine function)
vi. M-Cells
1. Overlie lymphoid nodules of the GALT
2. Transport antigens to activate mucosal lymphocytes
c. Absorption of metabolites
i. Absorptive cells
1. Microvillus border (striated)
2. Terminal Digestion via glycocalyx (proteins/carbohydrates)
a. Glycocalyx are membrane-associated glycoproteins
b. “Catch” digested amino acids and sugars by adhesion
c. Metabolites undergo “terminal digestion” at the surface
of the cells via glycocalyx-associated enzymes
d. Digested molecules are processed internally through the
lysosomal system
e. Final products pass through capillaries of the lamina
propria and then drain to the liver (via hepatic portal)
3. Intracellular Digestion (lipids)
a. Reduced to free fatty acids
b. Pass readily through the epithelial membrane
c. Converted (intracellularly) to triglycerides
d. Conjugated into chylomicra (lipoprotein complexes)
e. Chylomicra DO NOT enter capillaries, but rather enter
lacteals at the core of each villus
f. Lacteals drain to regional lymphatics and thoracic duct,
then enter venous circulation
d. Immunologic Defense of the Gut
i. Normally maintains a chronic immuno-active state due to dietary
antigens, bacteria, viruses, and parasites
ii. GALT
1. Active lymphoid infiltration (i.e. Peyer’s Patches, appendix,
colon, etc)
2. Produces IgA
a. Antibody dimmer, linked by a J-Peptide
b. Produced by lymphocytes
c. Coupled to a “secretory piece” to prevent degradation
during intestinal circulation
d. Secreted into the intestine by two methods
i. Across intestinal lumen
ii. From the liver
1. Passed via canaliculi and the bilary tract
2. essentially recycled because the liver
recovers IgA from circulating blood in
the ileum
e. Duodenum
i. Only region with submucosal glands (Brunner’s Glands)
1. Perforate muscularis mucosa & empty into crypts of Liberkuhn
ii. Prominent sub-mucosal foldings called Valves of Kierkring
iii. Crypts of Lieberkuhn at the base of villi
f. Jejunum
i. Region with the longest villi
1. Ideal for terminal digestion and absorption of nutrients
ii. NO Brunner’s glands, and FEW submucosal lymphoid nodules
g. Ileum
i. Easily identified by prominent Peyer’s Patches
ii. Look for Eosinophilic “Paneth Cells” at the base of each crypt
V) Colon (or Large Intestine)
a. Epithelium
i. Almost exclusively goblet cells
ii. Smooth surface with crypts, but NO villi (looks like you kept the
crypts and cut off the villi)
b. Outer longitudinal layer of muscularis externa is discontinuous
i. Forms three bundles called taenia coli
ii. Distinguishing feature in cross sections
c. Do not confuse with
i. Rectoanal Junction—hairs, apocrine sweat glands, more infolded
mucosa
ii. Esophageal-Cardiac Junction—virtually indistinguishable when very
zoomed in