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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.

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 


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

    products include:

    incretins (K-cells) (hunger)

    GIP (L-cells)

  serotonin (enterochromaffin cells)


  motilin (gut sound in PCM!)




    substance P

    endorphins (opiates)

these cells secrete their contents into

the blood stream – why?


Paneth cells?

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.


M cells

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

  lymphatic capillaries

  begin blindly in villi
  known as lacteals

lymphatics originate here


Discuss lymph nodules in the mucosa of SI.

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


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)

muscularis externa

  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?


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

  intestinal crypts
  enteroendocrine cells
  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

  typical serosa


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)


  present on:

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

  absent from:

    lower rectum/anal canal (extraperitoneal portion)


Where does the anal canal begin?

Rectoanal junction. 


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

    hemorrhoidal veins

    convolution/dilation causes internal hemorrhoids


Decribe the muscularis externa in the anal canal.

muscularis externa

  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

  hair follicles

  sebaceous glands