3. SMALL INTESTINE = completes digestion, absorbs nutrients, secretes gut
hormones. Consists of 3 regions: Duodenum, Jejunum, Ileum.
A) Plicae = mucosal folds with submucosal core, act to increase surface area for
absorption
B) Villi = fingerlike mucosal projections with lamina propria core, also increase
surface area for absorption
C) Microvilli = fingerlike projections of cell membranes, make up "brush
border" on epithelial membrane; further increase surface area for absorption
D) Crypts of Lieberkuhn = mucosal glands of intestine, extend downward
between villi; produce mucus, enzymes, hormones
1. Paneth Cells = stain vivid red in H&E, secrete lysosomes that degrade
bacteria, proteins and mucopolysaccharides; also have phagocytic role; function to
regulate intestinal bacterial flora
2. Enteroendocrine Cells = secrete hormones involved in gastric secretion,
intestinal motility, pancreatic secretion and gall bladder contraction; same hormones
as from enteroendocrine cells in gastric glands
3. Goblet Cells = secrete mucus
4. Germinative zone in Crypts occurs at crypt base columnar cells = serve as stem
cells for all other crypt cell types
- Submucosal Brunner's Glands are present in the duodenum, secrete alkaline
mucus that protects epithelium by neutralizing acid from stomach
- Lamina Propria has numerous lymph nodules which may also extend into
submucosa; large masses of nodules exist in the L. propria of the ileum = Peyer's Patches
4. LARGE INTESTINE = resorbs water from intestinal contents, secretes mucus for
lubrication, no digestive enzymes are secreted by the large intestine
- Component Sections (anterior to posterior) = Ileocecal valve/cecum (appendix =
diverticulum from cecum) -- colon -- rectum -- anus
- No plicae or villi present in large intestine, but short microvilli present on lumenal surface
- Intestinal Crypts are deeper than in small intestine, contain:
1) Goblet Cells = numerous as mucus is necessary to aid passage of more solid
waste down tract
2) Columnar Cells = same as mucosal surface cells
3) Enteroendocrine Cells
4) Basal Stem Cells
- Outer Longitudinal layer of Muscularis Externa is aggregated into 3 longitudinal
bands called taenia coli
APPENDIX = diverticulum from anterior part of large intestine
- Contains prominent lymphatic nodules in L. propria which extend deep into the
submucosa; these function to combat microbial or parasite antigens from small
intestine and from the large intestine where bacterial content is much higher than in
other parts of the digestive tract.
-Structurally, the appendix is similar to the large intestine; appendix crypts contain a
few Paneth cells and goblet cells and numerous enteroendocrine cells
III. DIGESTIVE GLANDS = develop as outgrowths of gut epithelium
1) Pancreas = contains exocrine (empties to duodenum) and endocrine portions
Exocrine Secretion = alkaline serous secretion containing enzymes that break down
proteins (trypsin, chymotrypsinogen, carboxypeptidase), fats (lipase) and carbohydrates
(amylase). The proteolytic enzymes are released as inactive forms that are activated
by other enzymes after entering duodenum
- Exocrine portion = compound tubuloalveolar gland
- Centroacinar Cells = one or more pale-staining cells in middle of acini,
represent terminal cells of intercalated ducts. These cells distinguish pancreas from
salivary glands.
Endocrine Secretion = Islets of Langerhans (pale-staining spherical regions of polygonal
glandular cells in reticular CT, highly vascularized); Cell Types:
a) Alpha-cells = found mainly at periphery of islets, have pinkish cytoplasm
with Masson trichrome; secrete glucagon (causes glycogenolysis, lipolysis,
gluconeogenesis, thereby increasing blood glucose levels)
b) Beta-cells = found on interior of islets, produce insulin (causes glycogenesis
in liver and skeletal muscle and cellular uptake of glucose which decreases blood
glucose levels)
c) D-cells = resemble alpha-cells, but larger, bluish cytoplasm with Masson
trichrome; secrete somatostatin which inhibits secretion of insulin and glucagon
d) C-cells = small unstained cells in interior, not present in humans, possibly
represent reserve cells
2) Liver = the largest compound gland (compound tubular) in the body
FUNCTIONS:
a) Metabolic interconversion and synthesis
b) Detoxification
c) Phagocytosis/Waste removal (e.g., bilirubin from Hemoglobin breakdown)
d) Secretion of bile - contains bilirubin, IgA (Antibody activity, forms immunological
barrier against microbes), bile salts (emulsify fats)
STRUCTURE:
a) Dense irregular CT capsule covered by mesothelium
b) Divided into lobes, lobes into lobules (4 lobes in humans)
- Basic unit of structure = Liver Lobule (hexagonal in shape)
1. Central Vein = located at center of lobule (middle of hexagon)
2. Sinusoids = thin-walled blood vessels between cords of hepatocytes
3. Portal Canal = contains portal vein, hepatic artery, bile duct, lymphatic, nerve
4. Hepatocytes = epithelial secretory cells, secrete bile, stacked into cords radiating
outward from central vein
- Bile secreted from hepatocytes into bile canaliculi --- bile ductules --- bile duct ---
hepatic duct --- common bile duct (empties to duodenum) - SEE HANDOUT
- Sinusoids supplied by venules from portal canal capillaries (these capillaries empty
the hepatic artery) and from portal vein - SEE HANDOUT
- Hepatocytes bordering sinusoids have microvilli (increase S.A. for absorption and
secretion)
- Space of Disse = microvilli project into this space between hepatocytes and sinusoidal
endothelium (no basal lamina present)
- Sinusoids lined by:
a) Endothelial Cells = squamous cells forming fenestrated membrane (gaps exist
between cells)
b) Kupffer Cells = fixed macrophages, look similar to monocyte
3) Gall Bladder = thin-walled elongate sac, stores and concentrates bile
- Mucosa when empty is highly folded, tall columnar epithelium
- No muscularis mucosae or submucosa
- Drained by cystic duct, joins hepatic duct to form common bile duct
- Following a meal, gall bladder stimulated to contract by duodenal hormones and
parasympathetic stimulation of Sphincter of Oddi (surrounds entrance of common bile
duct into duodenum)
- Bile enters gall bladder by backing up from hepatic duct up through the cystic duct -
SEE HANDOUT
To Lecture 16
