Magnitude of folding. Villi
evaginations of mucosa, w/ core of lamina propria, project into lumen
distinguishing characteristic of small intestine (not found in large intestine)
shape of villus varies at different levels:
in duodenum - broad, short and leaf-like
in jejunum - tongue-like projections
in ileum - tall, narrow, finger-like projections
3rd folding layer.
3. plicae circulares
circular folds of entire mucosa , w/ core of submucosa
permanent projections into lumen
begin in proximal duodenum
maximized in distal duodenum & proximal jejunum
diminish until virtually absent in distal half of ileum
plicae consists of multiple villi
Gland types in the duodenum.
dependent upon secretions from 3 gland types:
pancreas and liver (accessory organs)
duodenal glands (Brunner’s glands)
found only in submucosa of duodenum
secrete mucous (resemble pyloric gastric glands)
intestinal crypts (or mucosal glands)
epithelium invaginating into lamina propria
present in small and large intestine
similar to gastric glands
Simple columnar epithelium in the small intestine. Types?
surface epithelium replaced every 3 to 7 days
more than one type of cell is present
absorptive columnar cells (with microvillous border)
goblet cells (lining surface and dipping into crypts)
lymphocytes (visible migrating from lamina propria to lumen
What are products and characteristics of enteroendocrine cells?
enteroendocrine cells (in stomach and intestine)
found mainly in crypts, occasionally (rarely) in villus
secrete product into vascular channels in lamina propria
incretins (K-cells) (hunger)
serotonin (enterochromaffin cells)
motilin (gut sound in PCM!)
these cells secrete their contents into
the blood stream – why?
found in base of intestinal crypts
contain large acidophilic granules
function is not well defined
secrete enzyme lysozyme & peptide defensins
degrade components of bacterial cell wall
Secrete into lumen not blood.
M cells (immune cells)
specialized cells in epithelium
have limited, small microfolds
(compared to regular gut epithelial cells)
part of innate and classic immune systems
actively capture/transport Ag’s to APCs & B cells in lamina propria
hard to identify
Where are B cells in the Small Intestine.
lamina propria (contains mainly immune cells – B cells)
loose CT, prone to lymphoid infiltration
contains large numbers of plasma cells (in villus and between crypts)
produce secretory IgA
transported into intestinal lumen
interacts with bacteria, antigen, virus, etc.
other typical CT cell types also present
begin blindly in villi
known as lacteals
lymphatics originate here
Discuss lymph nodules in the mucosa of SI.
become more numerous in distal small bowel
macroscopic aggregates of lymphoid tissue
known as Peyer’s patches
especially prominent in ileum
may appear in lower jejunum
M cells more prevalent in these areas
Peyer’s patch also in appendix
Decribe the submucosa and the serosa in the SI.
projects into plicae circulares
(plicae present in duodenum and jejunum)
infiltrated with lymphocytes in region of Peyers patches
submucosal glands found only in duodenum (known as Brunner’s glands)
follows general plan
duodenum has serosa on anterior surface only
organ is retroperitoneal
jejunum and ileum have serosa
continuous w/ supporting mesentery
What is the ileocecal junction?
site where ileum joins large intestine
formed by folds of the mucosa and submucosa
supported by mass of circular smooth muscle
from muscularis externa
Cecum- small blind pouch of large intestine structurally identical to colon.
What is the appendix?
slender, blind diverticulum of cecum
small, irregular lumen often containing cellular debris
villi are absent
some Paneth cells
What is the histology of the appendix?
simple columnar epithelium w/ goblet cells
lymphoid tissue occupies lamina propria
muscularis mucosae often incomplete
submucosa is thick
thin muscularis externa
but usual 2 layers present
Does the large intestine have villi?
No, less surface amplification as a result.
Discuss the intestinal crypts of the LI
intestinal crypts (crypts of Lieberkuhn)
longer & more closely packed than in small intestine
occasional enteroendocrine cells present
Paneth cells absent
Are digestive enzymes secreted by the LI
surface epithelium identical to small intestines
i.e. simple columnar epithelium, but goblet cells more numerous
NO digestive enzymes secreted by large intestine
Colon lymph nodes, and muscle layers.
plica circulares not present in large intestine
in colon, plica semilunares present
incomplete (half-moon) folds in intestinal wall
(do not form complete circular fold, as plicae in small intestine do)
produce characteristic sacculation (haustra)
scattered lymphatic nodules throughout CT
muscularis externa of colon is “incomplete”
inner layer completely encircles submucosa
outer longitudinal layer in three longitudinal bands
bands known as taeniae coli
What do lacteles do?
They carry fats directly to the heart to be mixed.
Where is the serosa present on the rectum?
muscularis externa of rectum is complete
taeniae coli expand to reform outer muscle layer
(i.e. NO taeniae coli on rectum)
upper 1/3 (intraperitoneal portion)
anterior surface in middle 1/3 (retroperitoneal portion)
lower rectum/anal canal (extraperitoneal portion)
Where does the anal canal begin?
What happens at the anal junction?
at rectum / anal canal junction
intestinal crypts decrease in number
eventually disappear entirely
mucous membrane forms longitudinal folds
known as rectal columns
What happens at the pectinate line?
at pectinate line
epithelium abruptly changes:
simple columnar becomes strat. squam. noncorn.
becomes continuous with epithelium of skin
What glands are present in the anal canal?
submucosal CT contains:
subepithelial circumanal glands
longitudinal thin-walled veins
convolution/dilation causes internal hemorrhoids
Decribe the muscularis externa in the anal canal.
internal layer thickens to form internal anal sphincter
external sphincter formed by skeletal muscle
surrounds anal canal
What happens at the anal verge?
caudal portion of anal canal (anal verge)
changes to stratified squamous epithelium, keratinized
(normal thin skin)
find: sweat glands