- Wall Structure of the alimentary tube

Mucosa, Submucosa, Muscularis, Adventitia (Serosa)

- contains epithelium, basement membrane, lamina propria, glands/areolar CT, lymphatic tissue, and muscularis mucosae (inner circular/outer longitudinal)

Mucosa

- contains fatty areolar CT, nerve plexus (Meissner's) & ganglion cells , Cajal cells, blood plexus, lymphatics, glands

Submucosa

- inner circular

- nerve plexus & ganglion cells (myenteric plexus), Cajal cells

- outer longitudinal

Muscularis

- loose CT (and mesothelium)

Adventitia (or Serosa)

- non-neural pacemaker cells that receive, transmit and integrate neurogenic signals and muscular activity in the Enterit Nervous System

- defects in these cells lead to motility disorders

Interstitial cells of Cajal

- Cajal cells express these receptors designated CD117 (c-kit)

Tyrosine kinase

- non-keratinized stratified squamous epithelium

- lymphoid tissue may be present

- present muscularis mucosae

Mucosa of Esophagus

- scattered mucous esophageal glands

- submucosal veins drain to portal and systemic

Submucosa of Esophagus

- upper 1/3 (or less) is skeletal, middle 1/3 is mixed, lower 1/3 is smooth muscle

Muscularis of Esophagus

- outer layer of alimentary tube in thorax/abdomen

Adventitia/Serosa

- non-permanent longitudinal folds of mucosa and submucosa that distend following ingestion of a large meal

Rugae

- function as ducts for gastric and pyloric glands

- crevices leading down from the surface

- transition to gland is isthmus/neck region

- straight ducts that connect the branched, coiled glands

Gastric pits (foveolae)

- contain rugae and gastric pits

- simple columnar cells which produce alkaline mucus

Mucosa of Stomach

- filled with glands that extend from gastric pits to muscularis mucosae

- cardiac and pyloric glands secrete mucous, oxyntic/fundic produce enzymes and HCl

Lamina Propria of Mucosa of Stomach

- the stem cells of the mucosa of the stomach that move up to create surface mucous cells or down to create parietal, chief, enteroendocrine or pyloric mucous cells

- between glands and gastric pits

Mucous Neck/stem cells

- in this region of the stomach the thickness of the glands is greater than the thickness of the pits/foveolae in a longitudinal section as opposed to them being close to equal (in the antrum)

Fundus

- scattered between chief cells

- more common in the upper portion of fundic glands

- stain pink and tend to be round with central nuclei

- most mitochondria (for pumping H+)

- lined with microvilli

- produce intrinsic factor (necessary for B12 absorption)

Parietal Cells

- this membrane receives extra microvilli following a meal from the tubulovesicular system in the cytoplasm

Canalicular/apical plasma membrane

- stimulates parietal cells to produce HCl

Ach (cholinergic input), histamine and gastrin

- zymogenic cells that produce protein for export (zymogen granules)

- RER in a basal location and apical zymogen granules (dark)

- produce pepsinogen and gastric lipase

Chief cells

- scattered individual endocrine cells of multiple types

- basal secretion granules

- Ex: enterochromaffin-like cell secreting histamine & stimulating HCl production --> which are stimulated by gastrin made by G cells

Enteroendocrine cells

- produce mucous and lysozyme

- contain parietal cells, ECL cells, and G cells

Pyloric glands

- this layer of GPAT is distinct in the stomach and separates the mucosa from the submucosa

Muscularis mucosa

- does not contain glands

- forms core of the rugae

Submucosa of the stomach

- has an added inner oblique layer of muscle (3 total)

- a thickened middle circular layer forms the pyloric sphincter

- serosa is present

Muscularis of Stomach

- non-keratinized stratified squamous changes to simple columnar (foveolar) with associated mucus-secreting glands

Cardiac (Gastroesophageal) Junction

- glands above the muscularis mucosa  --> glands below the muscularis mucosa (Brunner's) AND villi/goblet cells

Pyloric Junction

- made up of mucosa and submucosa in the small intestine

Plicae circulares

- in cross section, these structures show epithelium surrounding lamina propria

intestinal villi

- in cross section, show epithelium surrounded by lamina propria

- found between the villi

- epithelium is continuous from the villi into this structure

Crypts (glands)

- simple columnar epithelium that has absorptive enterocytes, M cells, Goblet cells, Paneth cells, Enteroendocrine cells, and Stem cells

Mucosa of the Small Intestine

- move from bottoms of the intestinal glands up onto villi until lost on the tips (5 days)

- columnar cells with numerous microvilli

- thick glycocalyx (enteric coat)

- has motility due to actin/myosin interaction

- Junctional complexes surround apical ends

Absorptive enterocytes

- these integral membrane proteins on absorptive enterocytes help digest oligo and disaccharides at the microvilli before absorbing monosaccharide

Disaccharidases

- this structure is found on the lateral and basal plasma membrane of absorptive enterocytes

- its action result in movement of salt and water (co-transport glucose) into nearby capillaries

Sodium-potassium ATPase pumps

- peptidases in the enteric coat break down proteins into amino acids which facilitates absorption (by enterocytes) to this region where they enter extracellular space and then capillaries

basal and lateral plasma membrane

- this molecule is reformed after glycerol, free fatty acids, and monoglycerides (from emulsified micelles) are absorbed by enterocytes and move to the apical smooth ER

triglycerides

- the product of triglycerides and proteins made in the apical smooth ER of enterocytes that is exocytosed at the basal/lateral plasma membrane

- blind-ended lymphatic vessels (lacteals) pick them up from there

Chylomicrons

- specialized enterocytes closely found on intramucosal lymphoid aggregates

- transport antigens from gut lumen to underlying lymphoid tissues

M cells

- scattered throughout the epithelium and produce mucus

- increase in abundance from duodenum to ileum to colon

- not found in stomach

Goblet cells

- clustered cells located at the bottom of the intestinal glands

- have large and abundant red-orange secretion granules

- produce lysozyme

Paneth cells

- have open (microvilli and contact lumen) and closed (contact basement membrane) types

- have secretion granules near basement membrane

- named by location or secretion

Enteroendocrine cells

- produce serotonin

enterochromaffin cells

- differentiate into absorptive, goblet, endocrine or Paneth cells that all slough at tips of villi over time (due to migration) except for Paneth cells

Stem cells

- these structures are found in the duodenum, secrete mucous, and are more abundant closer to the pyloric stomach

- secrete alkaline mucus

- other portions of small bowel do not have submucosal glands

Brunner's glands

- only portion of the small bowel that has an adventitia rather than a serosa due to its retroperitoneal location

- contain Brunner's glands, ampulla of Vater, and border the pancreas

Duodenum

- these cells work in conjunction with Peyer's patches in the ileum by endocytosing antigens and passing them to the underlying lymphocytes

M cells

- transition from villi to glands with no villi

Ileocecal junction

- no plicae circulares or villi, just glands

- no Paneth cells, more goblet cells

- more lymphatic tissue than small intestine but not aggregated like Peyer's patches

Mucosa of Large Intestine

- other than stomach, this organ has no submucosal glands

- produces mucus and absorbs water

- stores feces

- flatus, water & short-chain fatty acids are produced as byproducts of carb-consuming bacteria

Large Intestine

- creates taenia coli from the 3 bands of outer longitudinal layer of muscles

- weak points in this where vessels penetrate contribute to the formation of diverticula (from low-fiber diet + increased intracolonic pressures)

Muscularis propria of Large Intestine

- transition from simple columnar epithelium (abundant goblet cells) to stratified squamous epithelium

- non-keratinized --> thin skin

- hemorrhoidal veins are sometimes quite prominent under rectal and anal mucosa

Anorectal junction

- mini colon with abundant lymphoid tissue

- numerous M-cells cover lymphoid nodules

- sporadic crypt epithelium includes goblet cells, absorptive columnar, endocrine, and stem cells

Appendix