Histology Lecture 2 -- Small and Large Intestines Flashcards
Give the proportions of the small intestine proper occupied by the jejunum and the ileum
Jejunum = proximal 2/5ths
Ileum = distal 3/5ths
Length of duodenum
20 - 25 cm long
3 divisions of the duodenum and their characteristic features
- First third = continuous with pylorus (duodenal cap/bulb)
- Middle third = contains opening of bile and pancreatic ducts
- Last third = continuous with jejunum
Describe the outermost layer of the duodenum
Retroperitoneal, so outer layer is both serosa (front) and adventitia (back)
Where does the duodenum connect to the jejunum?
Ligament of Treitz
Function of ligament of Treitz
Anchor the duodenal-jejunal flexure to the posterior abdominal wall
Describe the folds of the intestinal wall
Permanent semicircular folds that are at right angles to the long axis of the gut (plicae circularis, valves of Kerckring, or valvulae conniventes).
Most pronounced in jejunum, less in ileum.
Histological characteristics of the valves of Kerckring
Core of submucosa with a mucosal lining
How do the plicae circularis react when the gut is distended
They fold against the intestinal lumen (rather than stretch out like the esophagus and the stomach)
Two types of structural modifications from the “surface” in the small intestine
- Villi
- Crypts of Lieberkühn
Describe the length of the villi in the different portions of the small intestine
Longest in jejunum –> progressivley shorter toward ileum
Location of lamina propria in the small intestine mucosa
- Core of the villus
- Narrow spaces between the crypts
Where do the crypts in the small intestine end?
Just above the muscularis mucosa
Five cell types present in the small intestine villi and crypt
- Striated border cell (enterocyte or absorptive cell)
- Oligomucous cell (precursor to the goblet cell)
- Enteroendocrine cell
- Paneth cell
- Crypt base columnar (stem) cell
Where can ALL cell types of the small intestine mucosa be found? Why?
Crypts because stem cells resides there and they give rise to all of the other cell types
Which cell types of the small intestine mucosa are restricted to the crypt base?
- Paneth cells
- Stem cells
3 small intestine mucosa cell types that occur in the epithelium of the villus
- Striated border cell
- Goblet cell
- Entereoendocrine cell
Describe how the cells of the small intestine mucosa migrate
Cells of the crypt migrate to the villus and continue to migrate continuously to the villus tip (extrusion zone). Cells become compressed and undergo cell death, break their junctional complexes and become detached from the basement membrane and are lost into the lumen
2 major and 2 minor cell types lining the surface of both small and large bowels
Major:
- Enterocytes
- Goblet cells
Minor
- Endocrine cells
- Intraepithelial migrating T lymphocytes (mostly T-suppressor cells)
Describe the distribution of goblet cells in the bowels
Progressively becomes more frequent distally
Describe the distribution of Paneth cells in the bowels
Become less frequent distally and can be found up until the hepatic flexure of the large bowel.
Appear in the distal bowel ONLY in conditions of chronic injury
2 components of the small intestine lamina propria
- Lacteals = blind-ended lymphatic vessels
- Migratory cell population involved in immune function and inflammation (major)
Purpose of lacteals in the small intestine lamina propria
Fat absorption
4 components of the migratory cell population in the lamina propria of the small intestine
- Lymphocytes (mostly T-helper cells and plasma cells making mostly IgA)
- Macrophages (mostly apical, for phagocytosis and APC)
- Eosinophils
- Mast cells
Describe the muscularis mucosa of the small intestine
Well developed with an inner circular and outer longitudinal layer of smooth muscle (relatively thin)
Describe the tunica muscularis of the small intestine
Two muscle layers (inner circular and outer longitudinal) that form a rather thick coat
Define Brunner’s glands
Tubuloalveolar mucous glands that resemble and are a continuation of the pyloric glands in the stomach, found predominantly in the submucosa with some extension into the mucosa of the duodenum
Where do Brunner’s glands empty into?
The bottom of duodenal crypts
Describe the distrbution of lymphatic nodules in the small intestine
Progressively more frequent toward the terminal ileum, where they aggregate into large patches (Peyer’s patches) of lymphoid tissue
Effect of Peyer’s patches on the mucosa and submucosa of the intestines
Distend the mucosa and extend into the submucosa –> bulging on the outer surface of the gut
Where are Peyer’s patches concentrated
Antimesenteric border
Describe the histological features of enterocytes
- Tall columnar
- Oval elongated nucleus at base
- Prominent striated border in apical region (microvilli)
- Glycoprotein coat around microvilli
4 enzymes contained by the glycocalyx of the microvilli
- Disaccharidases
- Dipeptidases
- Alkaline phosphatase
- Enterokinase (in duodenum)
Describe the histology of microvilli
Microvillus core filled with actin filaments that are inserted into a dense patch at the tip that contains alpha actinin. Actin filaments are cross-linked to the lateral cell membrane and extend into the cell apex at right angles to mingle with intermediate filaments (in cell web). Myosin is among the cell web material and interacts with actin.
Epithelium held together by junctional complexes just below striated border.
Describe the junctional complexes between the striated border cells of the intestines
Consists of tight and loose zonular junctions and desmosomes.
Where are extensive tight junctions found in the small intestine and what is their purpose?
Found between absorptive/striated border and goblet cells –> intestinal fluid is kept from moving between epithelial cells
What event occurs at the base of microvilli?
Active pinocytosis
What enzymes contribute to the breaking down of triglycerides into fatty acids and monoglycerides in the small intestine? Where do they come from?
- Lipase (from pancreas)
- Bile salts (from gall bladder)
Describe the function of the absorptive cells
- Fatty acids and monoglycrides diffuse across cell membrane as micelles complexed to bile salts
- Enterocyte produces chylomicrons
- SER in the apex has enzymes that esterify micelles beck to TG –> golgi region (become membrane-bound, visible chylomicrons)
- Leave cell at basolateral cells membrane –> lacteal in the lamina propria
Function of goblet cells in small intestine
Produce acid and neutral mucins stored in mucinogen droplets accumulated in the apical side of the cell cytoplasm prior to secretion. (Basal nucleus often compressed by the droplets)
Where do enteroendocrine cells most frequently occur in the gut
Lower half of the epithelium
Number of the different types of enteroendocrine cells in the intestine
At least 16
4 hormones from enteroendocrine cells concentrated toward the proximal small gut
- Cholecystokinin
- Secretin
- Gastric inhibitory polypeptide
- Motilin
3 hormones from enteroendocrine cells concentrated more distally in the gut
- Enteroglucagon
- Substance P
- Neurotensin
2 hormones from enteroendocrine cells that are more evenly distributed in the gut
- Serotonin
- Somatostatin
2 histological types of endocrine cells found in the gut
- Open (extend to the lumen)
- Closed (do not reach the lumen; covered by other cell types)
3 divisions of the intestinal epithelium crypts
- Crypt base
- Mid crypt
- Crypt top
2 main cells of the crypt base
- Crypt base columnar cell
- Paneth cell
Function of crypt base columnar cells
Stem cell for unitarian theory of intestinal renewal. Undifferentiated, squeezed into narrow spaces between Paneth cells. Divides by mitosis to produce:
- 1 stem cell to remain in crypt base
- 1 committed cell that migrates up the wall of the crypt as it differentiates
4 cell lines produced by crypt base columnar cell mitosis
- Paneth cell
- Endocrine cell
- Goblet cell
- Enterocyte/absorptive/striated border cell
Length of time for villous lining cells to differentiate, migrate and be lost from villous tip
56 days
Turnover for Paneth cells
3 weeks
Describe the histology of Paneth cells
- Large pyramidal (resemble serous acinar cells)
- Basophilic base due to large amount of ER
- Round and basal nucleus
- Apex filled with large eosinophilic Paneth granules (are often sensitive to fixation and dissolved to leave empty vacuoles in apex)
Describe the Paneth granules under electron microscopy
- Electron dense core
- Lighter outer cortex
- Membrane-bound
- Contains some enzymes, but mostly lysozyme
3 substances present in Paneth cells
- Lysozymes
- Immunoglobulins
- Epidermal growth factors
Describe the production of the endocrine granule in a maturing enteroendocrine cell
Young columnar enteroendocrine cells being to produce endocrine granule toward the base and the cell reverses polarity to secrete from the base
Functions in mid crypt region
Young differentiating cells from the crypt base columnar cell mitoses undergo further mitotic division to increase the number of cell produced
Describe the distribution of enterocytes and goblet cells in the intestines
High number of striated border cells in proximal small and large bowel and decreases in favor of the goblet cells distally in the intestine
How is the transition from rectum to anus defined?
Pectinate line = abrupt transition from simple columnar epithelium of the colon to the stratified squamous non-keratinized epithelium lining anal canal
When does the epithelium become keratinized after the GI tract?
At the anal orifice
Describe the mucosa of the colon
No villi; only crypts, which have:
- Short columnar striated border cells/enterocytes
- Numerous goblet cells
- Few enteroendocrine cells
- Paneth cells normally only found up to the hepatic flexure
How is the epithelium of the crypts and surface renewed in the colon?
Division of crypt base columnar cells
How are cells destroyed at the surface of the colon epithelium?
Apoptosis
Describe the lamina propria of the colon
Contains frequent, but usually isolated, lymphoid follicles which often extend into the submucosa
Describe the tunica muscularis of the colon
- Uniformly thick layer of inner circular smooth muscle
- Modified outer longitudinal layer (forms three thickened bands that run along the length of the colon)
Define the taenia coli
“Worms of the colon” = thickened bands of the outer longitudinal layer of the colon’s tunica muscularis
Function of the taenia coli
Strengthen longitudinal contractions and help move fecal material along the colon
Function of the colon
Dessiccation and storage of feces, water, and electrolyte absorption
Describe the mucosa of the appendix
Similar to the colon except that high numbers of often confluent lymphatic follicles resembles the Peyer’s pathces in the terminal ileum, often extending into the submucosa and interrupting the muscularis mucosa
Describe the tunica muscularis of the appendix
Internal circular and external longitudinal, but much thinner than any other part of the bowels
Potential role of appendix
Some role in immunity that is not well understood, inferred by the density of lymphoid tissue
