Layers of the GI Tract

From innermost (near the lumen) to outermost:

1) Mucosa

2) Submucosa

3) Muscularis

4) Serosa

Layers of the GI Tract:

Mucosa

Mucosa also called Mucous Membrane 

Composed of (from innermost to outermost):

1) Epithelial lining - provides selectively permeable barrier, produce hormones & mucous

2) Lamina propria - loose connective tissue rich in blood & lymph vessels, & smooth muscle cells; rich in macrophages & lymphoid nodules & cells, provides protection from pathogens (IgA)

3) Muscularis Mucosae - usually composed of an inner circular layer & outer longitudinal layer of smooth muscle; promote movement of the mucosa independent of the other layers

Layers of the GI Tract:

Submucosa

Composed of:

1) Dense connective tissue w/ many blood & lymph vessels

2) Submucosal nerve plexus (Meissner's plexus)

3) May contain glands & lymphoid tissue

Layers of the GI Tract:

Muscularis

Composed of (from innermost to outermost):

1) Internal circular layer of smooth muscle

2) Myenteric nerve plexus (Auerbach's plexus) 

3) Connective tissue containing blood & lymph vessels

4) External longitudinal layer of smooth muscle

Layers of the GI Tract:

Serosa

Composed of:

1) Thin layer of loose connective tissue containing blood & lymph vessels & adipose tissue

2) Simple squamous covering epithelium (mesothelium)

In the abdominal cavity, the serosa is continuous with the mesenteries & peritoneum. In places where the digestive tract is bound to other organs or structures, the serosa is replaced by  a thick adventitia w/o a mesothelium.

Gastric Regions & Cell Types

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Superficially, all four regions are alike; they have a surface epithelium composed of tall columnar foveolar cells that secrete mucus. 

All four regions contain millions of pits (or foveolae) in the mucosa, which lead into various kinds of mucosal glands.  

1) Cardia - has cardia glands, containing cells that secrete mucus & lysozyme; contains a few parietal cells (HCl)

2) Fundus - has gastric glands, containing parietal & chief cells (HCl & pepsin), as wells as mucus-secreting cells

3) Body - has gastric glands, containing parietal & chief cells (HCl & pepsin), as well as mucus-secreting cells

4) Antrum - has antral glands, containing G cells & D cells (gastrin & somatostatin), as well as mucus-secreting cells

All four regions exhibit rugae (deep folds of mucosa & submucosa), but these rugae are more marked in the proximal regions and flatten out toward the distal regions.

Cardia Glands

Located:

     - In the cardia of the stomach

Contain:

     - Mucous & lysozyme secreting cells

     - A few parietal cels (HCl)

Similar to cardiac glands in the terminal portion of the esophagus

Gastric Glands

(aka oxyntic or fundic glands)

Located:

     - In the fundus & body of the stomach

Contain:

     - Mucous cells

     - Mucous neck cells

     - Stem cells

     - Parietal cells (HCl)

     - Chief cells (Pepsin)

     - Enteroendocrine cells

Three regions:

Antral Glands

(aka pyloric glands)

Located:

     - In the antrum of the stomach

Contain:

     - Mucous & lysozyme secreting cells 

     - Enteroendocrine cells:

          - G cells (gastrin)

          - D cells (somatostatin)

Gastric Chief Cells

(Also known as peptic cells or zymogenic cells) 

Located:

     - In the fundus & body of the stomach

         - In gastric (aka fundic or oxyntic) glands

             - Mainly in the base of the gland

Secrete:

     - Pepsinogen

     - Lipase

     - (Possibly chymosin, aka rennin)

Function:

  Pepsinogen - Converted to pepsin in the low pH of stomach (optimally at pH 2), deactivated in higher pH of duodenum; Pepsin is a proteolytic enzyme that breaks proteins down into polypeptides (actually there are several different pepsins which all do this)

  Lipase - Cleaves a fatty acid from the glycerol in a triglyceride or phospholipid (again there are several different lipases which have more specific actions)

  Chymosin - Present in the digestive system of young animals, chymosin is a proteolytic enzyme that curdles milk, allowing it to be digested. It may or may not be present in the digestive system of human infants.

Regulation:

Not well understood, but stimulated by gastrin (from G cells) and acetylcholine (from neurons).

May be inhibited by somatostatin.

Gastric Parietal Cells

(Also known as oxyntic cells) 

Located:

     - In the fundus & body of the stomach

         - In gastric (aka fundic or oxyntic) glands

             - mostly in the upper half (isthmus & neck) of the gland

Secrete:

     - HCl

     - Gastric instrinsic factor

Features:

     - Proton pump- pumps H+ out of cells in exchange for K+. (Simultaneously, Cl- move out of the cell through channels)

Function:

  HCl - kills many microbes ingested with food

        - Begins digestion by simple hydrolysis

        - Converts pepsinogen to pepsin

        - Allows absorption of dietary non-heme iron

  Gastric intrinsic factor - Binds vitamin B12, allowing it to be absorbed in the ileum

Regulation:

  Stimulation of acid secretion

- Histamine - Released by ECL (upon stimulation by gastrin & Ach) cells, binds H2 receptors

- Gastrin - Released from G cells

- Acetylcholine - Released by neurons in response to stimulus (such as gastric distention or smell of food); Ach mediates both short reflexes (involving enteric nervous system only) & long reflexes (involving vagal nerve & CNS)

- Alcoholic beverages & coffee - Both stimulate acid secretion, but probably not due to the alcohol or caffeine itself (possibly via certain amino acids)

  Inhibition of acid secretion

- Somatostatin - Released by D cells

- Prostaglandins

Gastric G Cells

Located:

     - In the antrum of the stomach

Secrete:

     - Gastrin

Function - Gastrin travels through the bloodstream & stimulates:

    - Parietal cells (acid)

    - ECL cells (histamine, which furthur stimulates parietal cells)

    - Chief cells (pepsin)

Regulation:

Stimulation of gastrin release

    - Gastrin releasing peptide (GRP) - Released by enteric neurons

    - Short peptides & amino acids derived from initial digestion of ingested proteins

Inhibition of gastrin release

    - Somatostatin - From D cells

Gastric D Cells

Located:

     - In the antrum of the stomach

Secrete:    

     - Somatostatin

Function - Inhibits:

     - G cells (gastrin)

     - ECL cells (histamine)

     - Parietal cells (acid)

     - Chief cells (pepsin)

Regulation:

     - Somatostatin is released when gastric pH falls below 3; The cells may be directly stimulated by a low pH, or they may be stimulated via a nervous pathway that senses low pH and releases CGRP (calcitonin gene related peptide).

Gastric ECL Cells

ECL = Enterochromaffin-like

Located:

    - In gastric glands in the body of the stomach

Secrete:

     - Histamine

Function:

Histamine diffuses to nearby parietal cells to stimulate acid secretion (binds H2 receptors)

Regulation:

Released under the combined influence of gastrin (from G cells) and acetylcholine (from nervous stimulation).

Gastric X Cells

An enteroendocrine cell

Located:

Secrete:

     - Endothelin 

Function:

Endothelin plays a role in the regulation of acid secretion

Gastric Mucus Neck Cells

Located:

     - In the fundus & body of the stomach

         - In gastric glands

              - Mainly in the neck of the gland

Function: Precursors for all the other differentiated cell types in the gastric gland.

Gastric Mucus Cells

Located:

     - Mucus-secreting cells are located in all the glands of the stomach; In addition, the surface epithelial cells are mucus-secreting 

Secrete:

     - Mucin, phospholipids, bicarbonate

Function:

     - Protects the lining of the stomach from the acidic environment of the lumen; The bicarbonate sets up a pH gradient

Regulation:

     - Secretion is regulated by several factors, including Ach & gastrin pathways and prostaglandins