Epithelium: usually glandular, with tubular glands extending into the lamina propria (mucosal glands), submucosa (submucosal glands), or outside of enteron (liver, pancreas, etc)
lamina propria: loose connective tissue; composed of compound glands and lymph nodules (mech of defense)
muscularis mucosa: thin layer of smooth muscle (stratified or simple) in longitudinal array; not present in oral cavity
Submucosa
(loose connective tissue (more fibrous than cellular) which attaches mucosa to muscularis externa)
coined the "outerbelt" for blood vessels, lymphatics, and nerve (Meissner) plexuses
arranged into an inner (tight) circular and outer (loose) longitudinal layer of smooth muscle
contains blood vessels, lymphatic vessels, and nerve (Auerbach) plexuses (situated between the 2 muscle layers)
contracts in order to constrict and shorten Þ peristalsis
Adventitia or Serosa
(encapsulating layer of loose connective tissue with oriented collagen/elastic fibers)
binds gut to surrounding structures
if coated with mesothelium (simple, squamous epith) Þ serosa (peritoneum); if absent Þ adventitia (retroperitoneum)
contains fat cells, and coined the "turnpike" for blood vessels, lymphatics and nerves
Autonomic Control of the Digestive Tract
The sympathetic fibers are postganglionic, with cells in the celiac and mesenteric ganglia
The parasympathetic contribution is from the vagus nerve, with cells in the wall of the digestive tube
exception: in the distal colon, the parasympathetic contribution is from the sacral spinal nerve
Auerbach
and Meissner are the 2 main plexuses which are not only modulated by preganglionic autonomic innervation, but are also capable of autonomous activity moderated by mechanical stretch and chemical stimuli
Esophogus
Mucosa
(star-shaped with longitudinal folds when empty)
lined with non-keratinized stratified squamous epithelium continuous with that of the oropharynx (continuously renewed)
unlike the GI’s basic plan, the epithelium is not glandular and the lamina propria is sparsely cellular
mucus-secreting glands in the lamina propria and submucosa near both ends of this straight 1ft long tube
at upper end Þ esophogeal glands (in submucosa)Þ compound tubulo-alveolar
at bottom end Þ cardiac glands (in lamina propria) Þ branched tubular (variable)
muscularis mucosa is roughly continuous with the elastic layers of the pharynx
Submucosa
Muscularis externa
upper ¼ Þ inner circular and outer longitudinal layers of skeletalmuscle Þ hypopharyngeal sphincter
as it progresses down, transitions to smooth muscle
Adventitia
(no mesothelium coat)
Peristalsis
swallowing reflex Þ peristalsis of bolus Þ meets cardiac sphincter in ~10sec Þ relaxes to allow bolus into stomach
swallowing reflex Þ liquid shoots down and accumulates at sphincter Þ peristaltic wave arrives Þ inhibits sphincter Þ relaxes to allow liquid into the stomach
sphincter does not close in infants leading to frequent occurrences of reflux
reverse peristalsis occurs in animals but not in man (under normal circumstances)
Cardiac Sphincter
surface epithelium changes abruptly from a stratified squamous to a simple columnar epithelium
deeper layers change gradually
not an anatomical sphincter, but a physiological one (no evident change in nerve or muscle to suggest its function)
Cardiospasm
– achalasia when chronic
cardiac sphincter fails to open promptly Þ difficulty in swallowing Þ esophogus dilates with accumulated food
due to loss or absence of ganglion cells in the myenteric plexus (Auerbach)
Stomach
Mucosa
(deep, glandular, scant lamina propria)
contains many folds (rugae) that divide mucosa into gastric areas a few millimeters in diameter
these areas contain gastric pits (foveolae) which extend down into the mucosa
gastric glands
extend the entire thickness of the sparse lamina propria and open into the bottom of the gastric pits
muscularis mucosae Þ 2 indistinct layers (circular and longitudinal) Þ sends fibers thru lamina propria
Submucosa
(loose connective tissue without glands)
Muscularis externa
contains an inner oblique layer in addition to the other 2
the pyloric mucosa (covers the lumen of the gastric antrum) releases gastrin via G cells into the blood and gastric juice in response to the presence of amino acids or mechanical distension
gastrin stimulates the secretion of acid by parietal cells (H+ Cl-)
when pH < 2.5, gastrin secretion is inhibited (negative feedback loop)
muscle of the antrum is responsible for most of the mixing occurring in the stomach Þ forms chyme
pyloric sphincter is formed by a thickening of the circular muscle of stomach and controls rate of gastric emptying
rate of emptying depends on gastric pressure and digestive hormones (ie secretin from duodenal endocrine cells ß contraction of gastric musculature which ß gastric pressure thus slowing gastric motility)
with pyloric sphincter closed, contraction of antrum propels food into corpus (large reservoir); antrum relaxes and muscles of corpus contract pushing food to antral region Þ sequence repeated Þ mixing and breakdown of food
coat the mucosal surface of the stomach including the gastric pits
have a long clear zone of cytoplasm with the nucleus pushed towards the base of the cell
secretes mucus with an entrapped alkaline fluid (receives bicarbonate from byproduct of parietal cell secretion) which serves as a protective coat for the stomach lining
Three cell types found in gastric glands
(1) chief or peptic cells
most numerous; large pyramidal cells found in lower portion of the glands
nucleus and lots of RNA at base of cell while the apical cytoplasm is clear and vacuolated with zymogen granules
the granules contain pepsinogen (inactive precursor of the proteolytic digestive enzyme pepsin), rennin, and other protein-splitting enzymes
an acid medium or pepsin itself triggers the splitting off of pepsin from pepsinogen
largely under parasympathetic control via vagus nerve and effects are accentuated by hormones such as gastrin
(2) parietal cells
scattered singly among the other cells
cells are large, oval, reddish when stained due to numerous mito, and contain large spherical nucleus
produces HCl and intrinsic factor (binds vitamin B12 and facilitates intestinal absorption; without it Þ pernicious anemia)
largely under parasympathetic control via vagus nerve and effects are accentuated by hormones such as gastrin
(3) mucous neck cell (chief neck cell) – acidic polysaccharide with protein core
located in small groups near the opening of the glands into the gastric pits
cuboidal cells with oval nucleus situated basally
non-differentiated mucus-secreting cell that can migrate upward to the surface (replace surface mucosal cells) or downward into the bottom of the glands (differentiate into parietal, chief, or more mucous neck cells) every 3-6 days
Enteroendocrine cells
(enterochromaffin)
scattered singly among the other cells in the gastric glands and in lining of small intestine as well (mostly in pylorus)
are also called argentaffin cells due to their detection via special silver stains
contain granules of biogenic amines (serotonin, histamine) and polypeptide hormones (gastrin)
Enterogastric reflex
arrival of food (low pH) in duodenum Þ nervous reflex (vagus n) Þ slows stomach contraction until food is processed
Ulcers
(loss of tissue from the surface)
found more often in males and smokers; occurs in duodenum or stomach due to failure to protect against stomach acid or enzymes, or due to increased levels of acid (stress increases gastric secretion via vagus n)
may involve mucosa only or penetrate the entire wall of the stomach/intestine
Small Intestine
simple columnar epithelium with goblet cells
Sections of small intestine
(1) Duodenum (retroperitoneal portion of GI tract, covered by adventitia)
in proximal portion, numerous mucus glands in submucosa
Þ Brunner’s glands (alkaline mucus to protect from acid)
(2) Jejunum (covered by serosa)
(3) Ileum(covered by serosa)
differentiated lymph nodes
Þ Peyer’s patches (only found here)
(4) Ileocolic sphincter (projection of small gut into colon, created by thickening of the circular muscle)
reflexly contracts when the proximal colon fills up
Þ controls rate of release of ileal contents
Structures which increase SA
(1) Plicae circulares (valves of Kerckring)
number of circular folds with a submucosal core and include all layers of mucosa
found in all of small intestine but most numerous in duodenum and jejunun
(2)Villi
further increases SA (8x) and covers entire surface; aids in ingestion of absorbed materials
consists of a core of lamina propria with an epithelial lining; greater in number in duodenum than the more distal parts of the small intestine
everything in lumen must pass thru lamina propria which contains fibroblasts, mast cells, etc
central lacteal
: single small lymph vessel, absorbs and transmits fat to the lymphatic system
small blood vessels
: located beneath basement memb of surface epith; absorbs nutrients
(3)Microvilli (see below)
2 types of surface cells
(1) absorptive cells: columnar with ovoid nuclei situated basally; apical end has many microvilli covered by mucopolysacch
microvilli are involved in the absorption of foodstuffs from the lumen (increase SA 20x more than villi)
(2) goblet cells: mucus-secreting
Þ protective layer of mucus covering the absorptive cells
the # of goblet cells increase from duodenum towards anus
Crypts of Lieberkuhn
(the glands of the small intestine)
tubular glands in the mucosa and extend into the muscularis mucosa; open between the villi
cells of Paneth
: are found at the bottom of the crypts; filled with large, round acidophilic granules
contain lysozymes, as well as precursors of intestinal digestive enzymes (pancreas is major source for these)
undifferentiated cells
: above the Paneth cells
Þ migrate to the tip of villi and are shed Þ epith renewed every 3-6 days
intestinal endocrine cells
: (neural crest origin; releases polypeptide GI hormones)
releases secretin into the bloodstream in response to acid in duodenum
Þ acts on centroacinar cells of exocrine pancreas Þ bicarbonate (alkaline) secretion;inhibits gastric acid secretion and stimulates pepsinogen secretion
releases secretin in response to products of fat digestion in duodenum
Þ decreases contractions of gastric muscles and increases contractility of pyloric sphincter Þ decreased delivery of fatty chyme from stomach
releases cholecystokinin (CCK) in response to fat and amino acids in duodenum
Þ acts on gall bladder and pancreas Þ contraction of gall bladder and enzymatic secretion from acinar cells of pancreas
releases biogenicamines that affect neighboring cells (paracrine secretion)
Large Intestine
columnar epithelial cells on smooth surface with goblet cells
Colon
(ascending, transverse, descending and sigmoid portions; serosal)
mucosa lined with columnar absorbing cells and numerous goblet cells (contained in Crypts of Lieberkuhn along with undifferentiated cells
Þ replaces epithelium weekly) Þ main function to absorb water and secrete an alkaline mucus
no paneth, no villi, enterochromaffin cells are rare, and no circular folds in the colon
Plica semilunaris
: core of muscle from the inner layer of muscularis externa
In colonic wall, the outer longitudinal muscle layer is arranged in 3 longitudinal bands (taeniae coli)
Appendix
: finger-like process extending from blind pouch of Colon (Cecum)
contains enormous mass of lymphatic tissue in its lamina propria (may extend into submucosa)
lamina propria infiltrated with copious quantities of small lymphocytes, eosinophils and plasma cells
lumen is small, villi and taenia coli are absent, crypts are irregular and variable, goblet cells are abundant
acute appendicitis
– due to bacterial infection (less frequent in infancy and old age)
Þ polymorphonuclear leukocyte infiltration of the muscularis
Rectum
(retroperitoneal Þ coated with adventitia)
simple columnar epith with goblet cells, becomes more cuboidal toward the junction of the anal canal
extends from the sigmoid colon to the beginning of the anal canal; crypts of Lieberkuhn increase in #
presence of 3 or more large foldings
Þ rectal valves
contain a core of muscle from the inner circular layer of muscularis externa
originate from alternate sides of the wall (encircle only 1/3 to ½ of the lumen)
in the muscularis externa, longitudinal layer lacks taeniae coli but completely surrounds the circular layer
Anal canal
(retroperitoneal Þ coated with adventitia)
distinct change in mucus memb occurs at junction between rectum and anal canal (simple columnar