Week 1: GI Histology Flashcards
Identify
The mucosa consists of
- consists of an epithelial lining (stratified squamous or columnar) an underlying lamina propria of loose connective tissue rich in blood vessels, lymphatics, lymphocytes and often containing small glands
- and a thin layer of smooth muscle called the muscularis mucosae separating the mucosa from the submucosa and allowing local movements of the mucosa
The mucosa is also frequently called
a mucous membrane
Digestive accessory organs that empty into the GI tract
- salivary glands
- liver
- gall bladder
- pancreas
Layers of the GI tract
- mucosa
- submucosa
- muscularis
- serosa/adventitia
Mucosa structure
the mucosa consists of an epithelial lining (stratified or columnar) an underlying lamina propria of loose connective tissue rich in blood vessels, lymphatics, lymphocytes, and often containing small glands and a thin layer of smooth muscle called the muscularis mucosae separating the mucosa from the submucosa and allowing local movements of the mucosa
Submucosa structure
this layer is under the mucosa and is fibroelastic loose connective tissue
this layer contains larger blood vessels and lymphatics, often glands and an important network of nerves and ganglia belonging to the Autonomic nervous system (Submucosal or Meissner’s plexus)
Muscularis
- With a few exceptions, the muscularis consists of smooth muscle with inner circular and outer longitudinal orientations
- Between these layers theres is another network of nerves and ganglia known as the Myenteric or Auerbach’s plexus
- Contractions of the muscularis which mix and propel the luminal contents forward (peristalsis) are generated and coordinated by the myenteric plexus
Adventitia AKA
Serosa
Adventitia or serosa
Connective tissue surrounding the muscularis externa
When facind the peritoneal cavity, distal esophagus, stomach, jejunum, ileum and parts of the large intestine, this connective tissue is encased with a thin layer of simple squamous epithelium known as the mesothelium
When mesothelium forms the outermost layer of the gut, the combined connective tissue and mesothelium is referred to as the serosa
The moist serosa performs an important function in allowing adjacent portions of the GI to come into contact and be able to slide over each with minimal abrasion
In non-peritonealized or retroperitoneal surfaces, such as the thoracic esophagus, duodenum and ascending and descending colon there are some areas that are devoid of this mucosal lining and only have connective tissue present
Here the outer tunic is referred to as adventitia
Identify
What is the esophagus?
A muscular organ that extends between the pharynx and the stomach mostly within the thoracic cavity
The empty esophagus is collapsed and thrown into longitudinal folds
Esophagus structure
It has the same basic structure as other portions of the GI Tract with a few differences
The mucosa is lined by thick nonkeratinized stratified squamous epithelium to withstand abrasion, also overlies a very thin lamina propria containing lymphocytes which appear as dark purple staining clusters) and a variably thick muscularis mucosa
Submucosa has mucous secreting glands and are known as esophageal glands, in the distal 8 cm of the esophagus the submucosal veins anastamose with branches of the portal vein
Muscularis externa the proximal 1/3 is skeletal muscle, middle 1/3 is a mix of skeletal and smooth muscle, and the distal 1/3 is smooth muscle
Adventitia/serosa the outermost layer of the thoracic portion of the esophagus is a loose connective tissue that blends into the surrounding tissues and is therefore classified as an adventitia
after passing through the diaphragm mesothelium covers this connective tissue and this connective tissue plus the mesothelium layer is referred to as serosa
Stomach main functions
digestion of carbohydrates initiated by the salivary enzyme amylase
acidic fluid to the ingested food and mixing its contents into a viscous mass called chyme by the churning activity of the muscularis
The continued digestion of triglycerides initiated by the pancreatic enzyme lipase
the initital digestion of proteins with the enzyme pepsin
Stomach anatomical divisions
fundus
cardia
body
pyloric antrum
Mucosa of the stomach
simple columnar epithelium consisting entirely of muucous secreting cells
the surface epithlium forms invaginations into the underlying lamina propria giving rise to the gastric glands
the branching gastric glands within the fundus and body open into gastric pits which empty their contents into the lumen of the stomach
Identify stomach
Identify stomach section
Cell types within the gastric glands
regenerative/stem cells
mucous cells
parietal cells
chief cells
enteroendocrine cells
Describe stomach regenerative cells/stem cells
Undifferentiated cells that give rise to the other cell types in the glands and the gastric pits
Describe stomach mucous cells
Line the lumen of the gastric pits and secrete a thick adherent and highly viscous protective mucous layer that is rich in bicarbonate ions
Describe stomach parietal cells
predominate in the upper segment of the gastric gland and produce hydrochloric acid, important for the conversion of pepsinogen to pepsin and intrinsic factor essential for the absorption of vitamin B12
Describe stomach Chief cells
Secrete the inactive proenzyme pepsinogen
Pepsinogen is released into the lumen of the gland and converted in the acidic environment of the stomach to pepsin, a proteolytic enzyme capable of digesting most proteins
Describe stomach Enteroendocrine cells
Several types that secrete hormones that give effects on gut motility (serotonin) and HCl secretion (gastrin)
The difference in glandular cells throughout divisions of the stomach
Identify stomach mucosa
Identify stomach mucosa
Submucosa of the stomach
there is a characteristic loose connective tissue with coarse collagen bundles
the increased thickness of this layer is responsible for the rugae of the mucosa
nerve supply including Meissner’s plexus is present
Muscularis of the stomach
Three layers of smooth muscle are present including an inner oblique, middle circular and outer longitudinal
Auerbach’s/myenteric plexus is present between the outer 2 layers
Serosa of the stomach
The stomach is completely surrounded by the mesothelium and so has an outer serosal layer
Small intestine
The small intestine consists of a tube approximately 5-7 meters long divided into 3 regions
duodenum, jejunum and ileum
The small intestine continues digestion of chyme from the stomach using pancreatic enzymes and liver bile and absorbs the resulting nutrients
The msmall intestine contains all the usual layers (mucosa, submucosa, muscularis and serosa)
Identify sections of small intestine
Describe the luminal surface of the small intestine
the luminal surface of the small intestine is thrown into circular folds called plica that contain small finger like projections called villi (approximately 1.5 mm long)
The plica circulares and villi significantly increase the luminal surface area necessary for absorption of nutrients from the digested food
Describe the mucosa of the small intestine
the epithelium of the small intestine is a simple columnar epithelium joined by extensive tight junctions several cell types are present including
enterocytes (absorptive cells)
goblet cells (mucus-secreting cells)
Paneth cells (lysozyme-secreting cells)
Enteroendocrine cells (hormone-secreting cells)
Stem cells (replace enterocytes and goblet cells every 3-5 days)
Describe absorption in the small intestine
to further increase the surface absorptive area, each enterocytes is covered with numerous microvilli (brush border)
enterocytes absorb carbohydrates, proteins, lipids, vitamins, Ca2+ and Fe2+ from the lumen and transport these into the blood or lymph
Describe the handling of carbohydrates in the small intestine
carbohydrates are digested to monosaccharides and transported to the portal blood
Describe the handling of proteins in the small intestine
Proteins are digested to amino acids, dipeptides and tripeptides
some further digestion may occur in enterocytes and then amino acids are transported to the portal blood
Describe the handling of triglycerides in the small intestine
triglycerides are emulsified by bile salts and digested to fatty acids and monoacylglycerols
long chain fatty acids are packaged as chylomicrons that enter the lacteal in the center of the vilus
short and medium chain fatty acids enter enterocytes directly and are delivered to the portal blood
Describe the handling of vitamin B12 in the small intestine
Water-soluble Vitamin B12 is absorbed in the ileum and requires intrinsic factor
Describe the handling of Ca2+ in the small intestine
Ca2+ requires Vitamin D for absorption
Identify small intestine section
Goblet cells in the small intestine
- Goblet cell are interspersed among the enterocytes covering the villi
- These cells secrete mucus that provides lubrication throughout the small intestine
- The further down the GI tract the greater the number of goblet cells to meet the need for increased lubrication of the luminal contents
- Goblet cells appear clear in H&E stained sections
Crypts of Lieberkuhn
The epithelium at the base of the vili invaginates into the lamina propria to form the short intestinal glands or Crypts of Lieberkuhn
Identify
What are paneth cells?
Paneth cells are found at the base of the crypt and stain a characterisitc orange-red color with H&E stain
Their color is due to the presence of numerous cytoplasmic granules filled with lysosome (an antibacterial substance) tumor necrosis factor-α TNF-α (a proinflammatory factor) and defensins (anti-bacterial and anti-parasitic)
These cells synthesize and secrete substantial quantities of antimicrobial peptides and proteins as well as factors that help sustain and modulate the epithelial stem and progenitor cells that cohabitate in the crypts and rejuvenate the small intestinal epithelium
Dysfunction of Paneth cell biology contributes to the pathogenesis of chronic inflammatory bowel disease
What are these?
Enteroendocrine cells secretions
Cholecystokinin
Secretin
Gastric inhibitory peptide
glucagon-like peptide
What is Cholecystokinin
Secreted by enteroendocrine cells in response to small peptides, amino acids and fatty acids in the gut lumen
stimulates secretion from pancreatic acinar cells and release of bile from gall bladder
What is secretin?
Secreted by enteroendocrine cells in response to H+ and fatty acids in gut lumen
stimulates release of HCO3 from pancreas and biliary tract of the liver
What is gastric inhibitory peptide?
Secreted by enteroendocrine cells in response to glucose, amino acids and fatty acids in the gut lumen
Stimulates insulin secretion from pancreatic islets
What is glucagon-like peptide?
Released in response to glucose, amino acids and fatty acids in the gut lumen
Stimulates insulin secretion and inhibits glucagon secretion
Brunner’s Glands
A specialized set of glands found only in the submucosa of the duodenum adjacent to the stomach.
Secrete alkaline mucus via ducts that empty into the bases of the crypts to help neutralize the acidic contents of the stomach entering the small intestine
Peyer’s Patches
large numbers of lymphocytes critical for the immunological defense of the small intestine are found throughout the lamina propria
The highest concentration of lymphocytes will be present in the ileum as individual cells or as groups of lymphoid nodules referred to as Peyer’s PAtches
This is referred to as Gut Associated Lymphoid tissue (GALT)
What is this?
Myenteric plexus AKA
Auerbach’s plexus
Myenteric plexus
As in other regions of the GI tract, components of the enteric nervous system can be found in the muscularis region
the myenteric plexus originates from neural crest cells and forms between the inner and outer layers of smooth muscle
What is this?
Large intestine
The colon or large intestine is a large diameter tube that absorbs about 4/5 of the water, Na+ Cl- from the fluid presemmted to it from the ileum concentrating and solidifying it to form feces
Mucosa of the large intestine
The mucosa lacks villi and except in the rectum has no major folds
The mucosa consist of simple columnar epithelial cells which regularly invaginate to form tubular intestinal glands
A majority of the surface cells and cells of the glands are goblet cells
Undifferentiated stem cells are present within the bases of the glands and migrate upward to form the other cell types
Dispersed enteroendocrine cells may also be found within the mucosal epithelium
IDentify large intestine section
Muscularis of the large intestine
The muscularis of the colon has inner longitudinal and outer circular layers but differs from that of the small intestine, with fibers of the outer layer gathered into three separate longitudinal bands called teniae coli.
Intraperitoneal portions of the colon are covered by serosa which is characterized by small pendulous protuberances of adipose tissue called appendix epiploica
Appendix
The appedix is a small blind-ended diverticulum of the cecum with layers similar to those of the large intestine
the characteristic features of the appendix are the accumulations of lymphoid tissue resent within the lamina propria
Identify
