12.3 - Organization and Embryology of the GI System Flashcards
Describe the characteristics of the esophagus, including epithelium, glands, tissue layers, or any other important structures
- beginning after the upper esophageal sphincter (UES) is lined with SSNK epithelium
- contains sparse esophageal submucosal glands (esophageal glands proper) in lamina propria
- contains even sparser esophageal cardiac glands in the lamina propria
- Contains 2 sphincters
1) Upper Esophageal Sphincter (UES)
2) Lower Esophageal Sphincter (LES) - mucularis mucosae layer is incomplete near larynx
- the muscularis externa has skeletal muscle fibers in it (more of them near larynx)
What are the two glands of the esophagus, and what tissue layer are they located in?
- Glands are located in lamina propria layer
1) Esophageal Submucosal glands = esophageal glands proper - sparse
2) Esophageal cardiac glands - even sparser
What are the two sphincters associated with the esophagus
1) Upper Esophageal Sphincter (UES)
2) Lower Esophageal Sphincter (IES)
- clinically important
- physiological sphincter
- formed by ICL near the juncture with the stomach
Describe the muscular tissue layers in the esophagus
1) Muscularis Mucosae
- is of varying thickness
- incomplete near the larynx
- is innervated by the submucosal (Meissner’s) plexus - (as is the mucosa in general)
2) Muscularis Externa
- novelly includes skeletal muscle fibers (more near the larynx)
- innervated by myenteric (Auerbach’s) Plexus - like the rest of the GI system
What are the 3 histologically distinct regions of the stomach
1) cardiac stomach
- near LES
2) fundic stomach
- forming the majority of the organ
3) Pyloric stomach
- near the pyloric sphincter, the junction with the duodenum
What are gastric glands
= long, simple tubular glands
- characteristic of the body of the stomach
- contain large numbers of parietal cells (produce hydrochloric acid and gastric intrinsic factor)
- also contain large numbers of chief cells (produce enzyme precursors - i.e. pepsinogen)
- in cardiac and pyloric regions - the cardiac and pyloric glands are shorter and contain primarily mucous neck cells
What are parietal cells
- in the gastric glands in large numbers
- produce hydrochloric acid and gastric intrinsic factor
What are chief cells
- in the gastric glands in large numbers
- produce enzyme precursors (zymogens) such as pepsinogen
what are Rugae
= the anatomical temporary folds of the surface of the stomach
What are the gastric pits
= nearly microscopic invaginations in the surface of the stomach
- lined entirely with surface mucous cells in all regions
Describe the muscularis externa of the stomach
- consists of three layers
befitting an organ that is more a bag than a tube
What are the 3 distinct anatomical segments of the small intestine
1) Duodenum
2) Jejunum
3) Ileum
What are intestinal villi
= the chief novel characteristic of the small intestine
= macroscopic epithelial evaginations
Describe the two most common cells in the intestinal epithelium
1) Enterocytes
- have a striated border - consists of dense microvilli and a glycocalyx
2) Goblet cells
- produce mucus
What are the significance of the intestinal crypts
- contain Paneth Cells at their bases
- Paneth cells secrete zymogens
What are enteroendocrine cells (EEC)
= also called enterochromaffin cells - because most are reactive for chromium salts
- are found scattered throughout the simple columnar epithelia of the digestive system
What are central lacteals
- in the core of the intestinal villi
= lymphatic vessels specialized to carry dietary lipids in the form of chylomicrons
Describe the regional specializations within the small intestine
1) Brunner’s Glands
- in the submucosa of the duodenum
2) Peyer’s Patches
3) Gut-associated lymphoid tissue (GALT)
- contain lymphoid follicles
- within the ileum
What are M-cells
- interact with lymphocytes of the GALT
= specialized cells for transcytotic transport of antigens
What are plicae circulares
= permanent, circular mucosal folds
- most prominent in the lower duodenum + upper jejunum
What are the 5 anatomical portions of the large intestine
1) Cecum
2) Ascending Colon
3) Transverse Colon
4) Desccending Colon
5) Rectum (12cm straight section of colon which pierces the peritoneal wall)
- not distinguished from each other histologically except for cecal appendix (vermiform appendix)
- all regions of large intestine lack villi, but possess crypts
- ratio of goblet cells increases as move toward rectum
What is significant about the cecal appendix and what is its other name
= vermiform appendix
- contains significant amounts of lymphatic tissue + lymphoid follicles
What are taenae coli
= the discrete bands (up to 3) of the Outer longitudinal layer (OLL) of the muclaris externa,
- the muscularis externa is discontinuous in the large intestine
Describe the important characteristics of the anal canal
- approximately 4cm of length
- contains the transition between the simple columnar epithelium of the colon and the SSK epithelium of the skin (goes to SSNK first)
- submucosa = specialized with longitudinal folds called anal columns (columns of Morgagni)
- Has both internal and external anal sphincters
What is the pectinate line
= the first transition within the anal canal
- from simple columnar epithelium of the colon to the SSNK
- is an abrupt transition
Describe the specialization of the submucosa of the anal canal
- has longitudinal folds = anal columns = columns of Morgagni
Describe the sphincters of the anal canal
- the ILL of the mucularis externa terminates in the internal anal sphincter
- an external anal sphincter of skeletal muscle is also present
Describe the body folding occurring in the 4th week of development and on WRT the GI system
- when body folding is complete the endoderm layer forms the gut tube with 3 regions
1) Foregut
2) Midgut
3) Hindgut - enddermal lining of the gut tube is surrounded by lateral plate mesoderm
- endoderm gives rise to the specific cells of glands associated with the GI tract:
A) liver
B) Gall Bladder
C) Pancreas
What is the role of the Splanchnic LPM in the GI development
- gives rise to the remainder of the wall of the GI tract including the stroma of
- glands
- muscle
- CT
- Peritoneal lining of the gut tube
What are the Major glands of the GI system
- salivary glands
- liver
- gall bladder
- pancreas
What are the histological layers of the GI from lumen inwards
1) Mucosa
= wet lining surface composed of 3 layers
1A - Epithelium - primarily SS or simple columnar
1B - Lamina Propria - well vascularized loose CT (can be quite cellular and may contain glands
1C - Muscularis Mucosae - thin ICL and OLL of smooth muscle, modifies shape of the mucosa
2) Submucosa
- Dense irregular (usually) CT with larger blood vessels, nerves, occasional glands
3) Musclaris Externa
- thicker than the muscularis mucosae
- ICL and OLL are helically arranged (spiral)
- FXNs to move material through the GI tract
4) Adentitia or Serosa
- surrounding layer of supporting CT
- may be either continuous with that of an adjacent organ or body wall (adventitia) or free in a body cavity and covered with a mesothelium (serosa)
What are the five main functions of the GI tract that is responsible for the alterations seen in the histological layers of each organ/section of the tract
1) Mechanical Propulsion
- the peristaltic and segmental movement of ingested material through the tube
2) Digestion
- both physical breakdown and chemical degradation
3) Absorption
- the uptake into the body of needed substances
4) Secretion
- both lubrication and delivery of digestive enzymes, hormones, antibodies into the lumen
5) Protection
- the defenses that prevent/limit invasion by ingested viruses/bacteria and which, in some areas of the GI tract, protect against self-digestion
