GI Tract Wall Layers

GI Tract Wall Layers

• Order from lumen outwards: Mucosa, Submucosa, Muscularis externa, Serosa.

Mucosa

• Nutrient absorption is a primary function
• Principal functions include protection, absorption, and secretion
• Not involved in neural signaling
• Divided into three layers: epithelium, lamina propria, and muscularis mucosae
• Mucosal glands extend into the lamina propria
• Epithelial cell renewal occurs in the isthmus

Surface Area Increase

• Microvilli provide the most increase in absorptive surface area

Microvilli

• Apical projections of absorptive cells

Extramural Glands

• Lie outside the GI tract delivering secretions into it.

Muscularis Mucosae

• Boundary between mucosa and submucosa
• Consists of inner circular and outer longitudinal smooth muscle.
• Contraction of is responsible for absorptive and secretory movements.

Submucosa

• Primarily composed of dense irregular connective tissue composed mainly of
• The Meissner plexus is located

Submucosal Glands

• Can be found in the duodenum and esophagus
• Release contents into the lumen via ducts

Muscularis Externa

• Typically has two layers of smooth muscle (inner circular, outer longitudinal)
• Striated muscle is found in the upper third of the esophagus
• The stomach's muscularis externa uniquely contains oblique smooth muscle in addition to the typical two layers
• Peristalsis between muscle layers is regulated by the Myenteric (Auerbach) plexus

Sphincters

• Control the passage of contents in the GI tract
• The upper esophageal is also called the pharyngoesophageal

Esophagus

• Lined with nonkeratinized stratified squamous epithelium
• Proper glands are located in the submucosa
• Proper glands produce mucus for lubrication
• Cardiac glands are found in the lamina propria near the esophagogastric junction
• The distal one-third of the esophageal muscularis externa contains smooth muscle
• Anchored mostly by adventitia to surrounding structures, but the region in the abdominal cavity is surrounded by serosa
• Myenteric (Auerbach) plexus controls peristalsis in the muscularis externa
• Proper glands utilize stratified squamous ducts to release mucus
• Longitudinal folds are responsible for the appearance of a branched lumen when collapsed
• The muscularis mucosae is especially thick in the proximal aspect
• Mucus functions to lubricate and protect the mucosa

Enteric Nervous System

• Does not include the visceral motor cortex
• Includes the Auerbach and Meissner plexuses

Serosa

• Primarily made up of mesothelium and connective tissue

Stomach Regions

• Cardiac region is closest to the esophageal orifice
• Divided histologically into cardiac, fundic, and pyloric regions

Fundic Region

• Contains branched tubular glands

Surface Mucous Cells

• Secrete bicarbonate-rich mucus

Gastric Pits

• Also known as foveolae
• Lined with simple columnar epithelium

Rugae

• Longitudinal submucosal folds

Epithelial Cell Renewal

• Occurs in the isthmus

Lifespan of Surface Mucous Cells

• 3–5 days

Chief Cells

• Secrete pepsinogen
• Have basophilic basal regions and eosinophilic apical regions

Pepsinogen Activation

• Activated into pepsin by HCl

Parietal Cells

• Secrete HCl and intrinsic factor

Intrinsic Factor

• Required for B12 absorption
• Absorbed in the ileum

Parietal Cell Secretion of HCl

• Use canaliculi

Enteroendocrine Open Cells

• Possess microvilli

Gastrin

• Stimulates HCl production

Esophageal Glands Comparison

• Cardiac glands are most similar

Pyloric Glands

• Branched and coiled
• Secrete viscous mucus

PGE2 (Prostaglandin E2)

• Increases mucus and blood flow

Nitric Oxide (NO)

• Increases mucosal blood flow to promote tissue repair

Gastric Cytoprotection

• Protection without acid inhibition

Duodenum

• Shortest and widest part of the small intestine
• Contains Brunner glands which secrete alkaline mucus

Jejunum

• Villi are more finger-like
• The principal site of nutrient absorption

Ileum

• Peyer patches are mostly located

Small Intestine Epithelium

• Lined with simple columnar epithelium

Villi Core

• Contains lamina propria with blood vessels and a lacteal

Lacteal Function

• Transports lymph and fat

Microvilli Support

• Supported by actin microfilaments

Paneth Cells

• Secrete antimicrobial peptides

Intestinal Cell Number

• Goblet cells increase in number from duodenum to ileum

M Cells

• Function in transporting antigens

Intestinal Glands

• Also called crypts of Lieberkühn

Enteroendocrine Hormones

• Insulin is not produced by enteroendocrine cells

M Cell Microfolds

• Characterized by short folds instead of microvilli Here's a summary of the additional information, framed as study notes:

Large Intestine Specializations

• Plicae circulares and Villi: Absent in the large intestine.
• Goblet Cells: Increase in number down intestinal glands (crypts of Lieberkühn)
• Collagen Table: Collagen and proteoglycans located between basal lamina of epithelium and venous capillaries
• Tuft Cells: Are potentially exhausted Goblet cells
• GALT: Gut-associated lymphatic tissue is more extensive in the large intestine.
• Functions: It reabsorbs electrolytes and water; it eliminates waste.
• Outer longitudinal muscle: Arranged as Teniae coli

Segmentation

• A type of contraction in the colon that mixes contents without forward propulsion

Appendix

• It is a thin, finger-like projection of the cecum rich in lymphatic nodules.
• Location: Projects off the cecum.
• May play a roll immunologically
• Carcinoid cell tumors commonly originate from enteroendocrine cells in the Appendix

Histological Characteristics

• The surface is smooth, lacking villi.
• Peyer Patches: Rich supply

Rectum vs Anal Canal

• It has a simple columnar epithelium with lots of goblet cells. Absence: Hair Follicles , Teniae Coli

Anal Canal Zones

• Lining : Goes from simple columnar to stratified squamous

Anal Canal transitions

• Anal canal: There is also a transition in Epithelia and features absent
• Colorectal zone (upper)*: Simple columnar epithelium.
• Anal transitional zone (middle)*: Changes with interposed stratified columnar/cuboidal cells.
• Squamous zone (distal)*: Contains hair follicles and is lined by stratified squamous epithelium.

Glands and Sphincters

• Circumanal glands: Apocrine glands in anal skin.

• Location: Contains anal glands inside the anal mucosa

• Internal hemorrhoids: enlarged submucosal veins

• Internal Anal Sphincter: From the muscularis externa.

  • The Muscularis Muscosa : Absent at the ATZ

• External: contains distinct skeletal (striated) Muscle Here's a summary of the Liver, Gallbladder, and Pancreas, framed as study notes:

Liver

• Function: Produces bile.

• Capillaries: Sinusoidal

• Features*:

  Organized: Hepatocytes are arranged into lobules with a central vein.

  • Blood flow: From portal triad to central vein; bile flows the opposite direction.

-The space of Disse Location: Located between sinusoidal endothelial cells and hepatocytes

1- Exchange: Site where molecules are exchanged.

• Liver Cells* - The primary cells include

• Key Features: Ito, stellate, Kupffer ,Hepatocytes Hepatocytes: They form the bile canaliculi walls

• Connected: With tight, gap and desmosomes junctions

• Vitamin A: They store

• • Ito / Stellate cells**

The Liver Store Vitamin A

• The stellate liver cells*: Has this key role to store The Macrophages : Are known as Kupffer cells (they deal with the RBCs)

Functions -Removes: debris and older RBCs -Location: sinusoidal lumen

• Blood Supply

The key features blood is supplied by: The hepatic artery + portal vein

• Limiting plate*: Hepatocytes border the portal tract
• -Hering's canals -* collect from bile canaliculi.
• The acinus model blood zones: based on zones of O2 in blood and nutrient delivery 2-. Which of the following cells have a role in immune defense in the liver?: Kupffer Cells
• Hepadocytes - Functions Key structures and Key terms
• Space of Disse* key for exchange of molecules between blood and hepatocytes.
• Kupffer Cells*:* Macrophages of the Liver *
• The Bili Caniculi: Connects* to the bile duct

Gallbladder (GB)

• Functions: Concentrates and stores bile.
• Epithelium*: Lined with simple columnar epithelium.
• No submucosa*
• Stimulated by: CCK to contract
• Rokitansky-Aschoff sinuses*: Outpouchings in mucosa.

Pancreas

Function: Exocrine: Produces digestive enzymes. Endocrine: Produces hormones

• Structure *: Has both endocrine/exocrine parts
• Acinar Cells:* Secrete pancreatic digestive enzymes.
• Zymogens activated*: By enterokinase.
• Centroancinar & Duct Cells*: Modify bicarbonate content in secretions.
• Features*:
• islets of Langerhans*: Are the endocrine portion.
• Beta cells: Produce insulin -Functions*
• Hormones*: Alpha: Secretes glucagon Delta: Secretes somatostatin (Inhibits insulin and glucagon). Secretin - increases pancreatic bicarbonate secretion
• CCK hormone*: Regulates pancreatic enzyme secretion
• The location of cells in the pancreas:
• Centroacinar cells*: Form the initial portion of the intercalated duc
• Pancreatic lipase digests Fats
• *Arterial Structure: Splenic Artery
• **The Liver receives from?: *** -
• The Hepatic Artery:**** supplies blood
• Pancrease has*** 1-Amylase digests: Carbohydrates

2- Trypsin: Digests Proteins

###Key Features & Differences

• HCV: - The Acini Is very similar across