Histology of the Intestine

Question 2

Magnitude of folding. Villi

Answer 2

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

Question 3

3rd folding layer.

Answer 3

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

Question 4

Gland types in the duodenum.

Answer 4

digestive function
dependent upon secretions from 3 gland types:

exocrine glands
pancreas and liver (accessory organs)

  submucosal glands
  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

Question 5

Simple columnar epithelium in the small intestine. Types?

Answer 5

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

Question 6

What are products and characteristics of enteroendocrine cells?

Answer 6

enteroendocrine cells (in stomach and intestine)

found mainly in crypts, occasionally (rarely) in villus

secrete product into vascular channels in lamina propria

products include:

incretins (K-cells) (hunger)

GIP (L-cells)

serotonin (enterochromaffin cells)

somatostatin

motilin (gut sound in PCM!)

cholecystokinin

neurotensin

VIP

substance P

endorphins (opiates)

these cells secrete their contents into

the blood stream – why?

Question 7

Paneth cells?

Answer 7

Paneth cells
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.

Question 8

M cells

Answer 8

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

Question 9

Where are B cells in the Small Intestine.

Answer 9

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

lymphatic capillaries

begin blindly in villi
known as lacteals

lymphatics originate here

Question 10

Discuss lymph nodules in the mucosa of SI.

Answer 10

lamina propria
lymph nodules
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

Question 11

Decribe the submucosa and the serosa in the SI.

Answer 11

submucosa

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)

muscularis externa

follows general plan

serosa

duodenum has serosa on anterior surface only

organ is retroperitoneal

jejunum and ileum have serosa

continuous w/ supporting mesentery

Question 12

What is the ileocecal junction?

Cecum?

Answer 12

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.

Question 13

What is the appendix?

Answer 13

appendix

slender, blind diverticulum of cecum

small, irregular lumen often containing cellular debris

villi are absent

intestinal crypts
enteroendocrine cells
some Paneth cells

Question 14

What is the histology of the appendix?

Answer 14

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

typical serosa

Question 15

Does the large intestine have villi?

Answer 15

No, less surface amplification as a result.

Question 16

Discuss the intestinal crypts of the LI

Answer 16

intestinal crypts (crypts of Lieberkuhn)
  longer & more closely packed than in small intestine

occasional enteroendocrine cells present
Paneth cells absent

Question 17

Are digestive enzymes secreted by the LI

Answer 17

surface epithelium identical to small intestines

i.e. simple columnar epithelium, but goblet cells more numerous

NO digestive enzymes secreted by large intestine

Question 18

Colon lymph nodes, and muscle layers.

Answer 18

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

Question 19

What do lacteles do?

Answer 19

They carry fats directly to the heart to be mixed.

Question 20

Where is the serosa present on the rectum?

Answer 20

muscularis externa of rectum is complete

taeniae coli expand to reform outer muscle layer

(i.e.  NO taeniae coli on rectum)

serosa

present on:

upper 1/3 (intraperitoneal portion)
anterior surface in middle 1/3 (retroperitoneal portion)

absent from:

lower rectum/anal canal (extraperitoneal portion)

Question 21

Where does the anal canal begin?

Answer 21

Rectoanal junction.

Question 22

What happens at the anal junction?

Answer 22

at rectum / anal canal junction

intestinal crypts decrease in number

eventually disappear entirely

mucous membrane forms longitudinal folds

known as rectal columns

Question 23

What happens at the pectinate line?

Answer 23

at pectinate line
epithelium abruptly changes:
simple columnar becomes strat. squam. noncorn.

becomes continuous with epithelium of skin

Question 24

What glands are present in the anal canal?

Answer 24

submucosal CT contains:

subepithelial circumanal glands

longitudinal thin-walled veins

hemorrhoidal veins

convolution/dilation causes internal hemorrhoids

Question 25

Decribe the muscularis externa in the anal canal.

Answer 25

muscularis externa

internal layer thickens to form internal anal sphincter

external sphincter formed by skeletal muscle

surrounds anal canal

Question 26

What happens at the anal verge?

Answer 26

caudal portion of anal canal (anal verge)

changes to stratified squamous epithelium, keratinized

(normal thin skin)

find: sweat glands

hair follicles

sebaceous glands