small intestine - monday Flashcards
what does the intestine do
continues digestion and absorption of food and fluids
what makes up the small intestine
- duodenum
- jejunum
- ileum
what are the 4 general histological layers of the GI tract
- mucosa
- submucosa
- muscularis externa
- serosa/adventitia
what type of epithelium is in the SI
simple columnar epithelium
what are plicae circulares
fold of mucosa and submucosa encircling lumen, grossly viewable
unlike rugae of the stomach, plicae circulares are ____ folds
permanent
what are villi
fingerlike projections of mucosal layer only into lumen of gut
what type of epithelium are villi lined by
simple columnar
what are the 3 cell types that villi consist of
- enterocytes
- goblet cells
- enteroendocrine
what are enterocytes
absorptive cells
on the apical surface of villi and contain thousands of microvilli which form a brush border
what do microvilli contain
- actin core
- enzymes - lactase, maltase, sucrase
what are the enzymes in microvilli involved in
the terminal digestion of carbs and proteins
what triggers the movement of microvilli
submucosal (meissner’s) plexus
what are the two functions of the brush border
- increase surface area for absorption
- increase surface area for enzymes that perform final stages of extracellular digestion, preparing them for transport across the cell membrane
what is the purpose of maximizing surface area
digestion and absorption require mixing of ingesta with bile and enzymes, retention of chyme, with exposure to adequate SA
what are lacteals
blind-ended lymphatic vessel
what are goblet cells
produce mucus to protect the lumen from abrasion and bacterial invasion
what are enteroendocrine cells
cells that secrete their contents basally
what is within the core of each villus (in the lamina propria)
4 things
- capillar plexus
- lacteal
- thin strands of muscularis mucosae
- lymphocytes, eosinophils, macrophages, etc
where are crypts located
between villi - invaginations of the mucosa
what are the 5 things that are in crypts
- simple tubular glands
2. stem cells - enteroendocrine cells
4. paneth cells - sometimes goblet cells
what are paneth cells
protect lining of gut from microorganims such as bacteria & parasites
in SI only
what do paneth cells secrete
lysozymes and defensins
what animals are paneth cells absent in
only some animals
dogs, cats, pigs, and racoons
where are paneth cells located
base of crypt
where do secretions from paneth cells enter
lumen directly - do NOT enter the bloodstream
what cells are responsible for the rapid turnover of the intestinal epithelium
stem cells
what is the first part of the SI
duodenum
what is the unique feature of the duodenum
brunner’s glands in submucosa
what do brunner’s glands produce
mucus and bicarbonate to offset acidic chyme expelled from stomach
what does the duodenum contain
- brunner’s glands
- villi: short, broad, leaf-like with scattered enteroendocrine cells
- retroperitoneal
- houses bile and pancreatic ducts
- paneth cells at base of crypts
what types of nervous systems does the enterogastric reflex involve
CNS and ENS, endo/paracrine signals
what is the enterogastrin reflex
- afferent fibers of the vagus nerve receive stimuli in duodeum
- these stimuli block vagus-induced stomach empyting
what is released from enteroendocrine cells in the duodeum
enterogastric reflex
CCK and secretin gets secreted into the bloodstream
what do CCK and secretin suppress
enterogastric reflex
gastric empyting
what is the second part of the SI
jejunum
what are the characteristics of the jejunum
- longest villi
- abundant paneth cells in crypts
- no glands in submucosa
what is the third part of the SI
ileum
what are the characteristics of the ileum
- shorter villi
- paneth cells
- peyer’s patches in submucosa
what are peyer’s patches
specialized lymphatic nodules
what are M cells
microfold cells
specialized epithelial cells that contain small microfolds on their lateral edges
what do M cells do
- trap antigens from the lumen of the gut via endocytosis
- antigens released beneath M cells and taken up by APC (DC) in underlying dome
- antigens presented to T cells to initate immune response
