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Transcript
Digestive System
• Primitive gut (PG) develops I0 from endoderm
– PG ⇒ digestive system
• Dorsal part of the yolk sac is incorporated into embryo as the PG
due to formation of head, tail & lateral folds during 4th week
– Endoderm gives rise to most of epithelium of the
digestive tract & parenchyma of its associated glands
• Liver & pancreas
– Ectoderm of stomodeum (primitive mouth)⇒ epithelium
at the superior end of digestive tract
– Ectoderm of proctodeum (anal pit)⇒ epithelium at the
inferior end of the digestive tract
Digestive tract
• Connective tissue & muscles in the wall of
the digestive tract are derived from
splanchnic mesenchyme that surrounds
the endodermal primitive gut (PG)
• PG is divided into four parts
– Pharynx
– Foregut
– Midgut
– hindgut
Pharynx
• Supported by pharyngeal/branchial arches
– Series of paired sacculations in lateral walls
⇒ Pharyngeal pouches
• By the end of the 4th week five pairs have formed
Pharyngeal Pouches
• First pharyngeal pouch
– Enlarges & develops into a tubotympanic recess
• This will become the auditory tube & tympanic
cavity (middle ear)
• Second pharyngeal pouch
– Remains small & is largely obliterated as
palatine tonsil develops
• Endodermal cells form surface epithelium of tonsil &
lining of its crypts
• Mesenchyme around this pouch differentiates into
lymphoid tissue
Pharyngeal Pouches (cont.)
• Third pharyngeal pouch
– Dorsal endoderm ⇒ inferior parathyroid glands
– Ventral endoderm unite to form thymus
– Both glands lose connection with pharynx & migrate in a
caudal & medial direction
• Fourth pharyngeal pouch
– Dorsal endoderm ⇒ superior parathyroid glands
• Fifth pharyngeal pouch (part of fouth)
– Ultimobranchial body which incorporates into thyroid
gland
• These cells give rise to parafollicular/C cells & secrete calcitonin
Foregut
• Lies caudal to pharyx & extends as far
back as the liver outgrowth
• At about 4 weeks a small diverticulum
appears in ventral wall at caudal border
– Tracheobonchiole/respiratory diverticulum
• Gradually separates from foregut dividing foregut
– Dorsal esophagus
– Ventral respiratory primordium
Foregut
• Esophagus
– Tube extending from pharynx to stomach
• Initially short but elongates rapidly keeping pace with
differentiating neck & descending heart & lungs
Foregut
• Stomach
– Fusiform dilatation in 4th week of development
– During the following weeks appearance &
position changes (descends)
• Increases in length
• Dorsal border grows faster than ventral wall + 90o
CW rotation along its long axis
– Posterior faces left, Anterior faces right
– Convex greater curvature (GC) ⇒ lies on left
– Concave lesser curvature (LC) ⇒ lies on right
– Epithelium differentiates into gastric glands &
surrounding mesenchyme
• Three incomplete layers of the muscular coat
Foregut
• Duodenum
– Develops early in 4th week from caudal part of
foregut & cephalic part of midgut
• Junction of two parts directly distal to origin of liver
bud
– Takes form of C shape loop & rotates to right
as stomach rotates
• Duodenal rotation along with rapid growth of head of
the pancreas causes swing to left side of abdomen
– During 2nd month the lumen is obliterated by
cell proliferation but recanalized shortly after
Foregut
• Liver & gallbladder
– Liver primordium appears in middle of 3rd
week as hepatic diverticulum (HD) or liver bud
at distal end of foregut
• Hepatic diverticulum rapidly enlarges due to cell
proliferation dividing into a large & small part
– Large cranial part ⇒ liver primordium
– Small caudal part ⇒ gallbladder primordium
– Hepatic cells continue to divide, the
connection between the HD & duodenum
narrows ⇒ bile duct
Liver (cont.)
• Epithelial hepatic cell cords intermingle
with vitelline & umbilical veins ⇒ Hepatic
sinusoids
• Liver grows rapidly & fills most of
abdominal cavity
– Initially right & left lobe are = size
• Right lobe becomes larger & subdivides into
caudate & quadrant lobes
– At 9th week 10% of fetal body weight
– At full term 5% of fetal body weight
Gallbladder & Pancreas
• Gallbladder primordium
– Distal end expands ⇒ gallbladder
– Proximal part stays narrow ⇒ cystic duct
• Pancreas formed by two buds originating from
endodermal lining of duodenum
– Buds arise on opposite sides at end of 4th week
• Dorsal pancreas ⇒ forms most of the gland
– Pushes out from dorsal wall slightly above hepatic diverticulum
• Ventral pancreas ⇒ head & uncinate process
– Develops near entry of bile duct
– Main pancreatic duct
• Formed by distal part of dorsal duct + entire ventral duct
Midgut
• In a 4-5 week old embryo midgut is
suspended from dorsal abdominal wall by
a short mesentery & communicates with
the yolk sac via vitelline duct
• Will give rise to:
– Part of duodenum & rest of the small intestine
• Distal to where the bile duct enters
– Ceacum
– Appendix
– Ascending colon
– Proximal 2/3 of transverse colon
Midgut (cont.)
• All structures of midgut supplied by superior
mesenteric artery
• Development is characterized by a rapid
growth in the length of the gut ⇒ formation
of primary intestinal loop
– Cranial limb
• Part of duodenum, jejunum, part of ileum
– Caudal limb
• Balance of ileum to proximal 2/3 of transverse colon
– Vitelline duct
• At junction of cranial & caudal limb
• If it persists in adult ⇒ Meckel’s diverticulum
Midgut (cont.)
• As the intestinal loop elongates rapidly &
liver enlarges, the abdominal cavity is
temporally too small causing intestinal
loop to project into the umbilical cord ⇒
physiological umbilical hernia (starting
around the 6th week)
– By about the end of the 3rd month the
herniated loops begin to return to the
abdominal cavity
Hindgut
• Gives rise to:
–
–
–
–
–
distal 1/3 of transverse colon
the descending colon
the sigmoid colon
the rectum
upper part of anal canal
• All hindgut derivatives are supplied by the
inferior mesenteric artery
• The terminal portion = cloaca
– Cloacal membrane
• lined with endodermal cells internally
• Lined with ectodermal cells externally
Congenital Malformations
•
•
•
•
•
•
•
•
Branchial Anomalies
Esophageal Atresia
Esophageal stenosis
Short esophagus
Intestinal stenosis & atresia
Omphalocoele
Umbilical hernia
Meckel’s Diverticulum
Branchial Anomalies
• Most of the abnormalities of the branchial
region are represented by remnants of the
branchial structures that normally disappear
– Branchial fistula
• An abnormal opening on the side of the neck
– Usually the result of persistance of parts of 2nd branchial
groove & 2nd branchial pouch
– Agenesis of thyroid & parathyroid
•
•
•
•
DiGeorge Syndrome
Failure of 3rd & 4th pharyngeal pouches to differentiate
May be due to teratogen action
No known genetic cause
Esophageal Atresia
• Usually occurs with tracheo-esophageal
fistula (TEF)
• Several types
– Lower esophagus communicates with back of
trachea & upper esophagus ends in a blind
pouch ( most common)
– Discontinuous esophagus with no tracheoesophageal fistula (rare)
• Possibly genetic
• Possible maternal vitamin A deficiency
Congenital Esophageal Stenosis
• Rarer than esophageal atresia with TEF
• Little known etiology
• Symptoms
– Regurgitation of food
– Failure to gain weight
– Dilation of esophagus above level of
obstruction may press against the trachea &
bronchi ⇒ produce wheezing sounds
Congenital Malformations (cont.)
• Short esophagus
– Causes partial thoracic stomach
• Small portion of stomach above diaphragm
• Gastroesophageal incompetence is a common
cause of vomiting (often projectile) during infancy
• Intestinal stenosis & atresia
– Occurs commonly in duodenum & ileum
– May be due to interruption of blood supply to
a loop of fetal intestine
• Results in necrosis, which later becomes fibrous
cord connecting proximal & distal ends
Congenital Malformations (cont.)
• Omphalocoele
– Results from failure of intestine to return to
abdomen during 10th week
– Hernia may contain a single loop of intestine
or most of the intestine
• Herniated mass is covered by a thin transparent
membrane composed of peritoneum & amnion
– Associated with chromosome abnormalities &
other severe defects
• Umbilical hernia
– Differs from omphalocoele
• herniated mass is covered by skin & subcutaneous
tissue
Congenital Malformations (cont.)
• Gastroschisis
– Herniation of abdominal contents through the
body wall directly into the amniotic cavity
• Defect occurs lateral to umbilicus, usually on the
right, through a region weakened by regression of
the right umbilical vein which normally disappears
– Viscera are not covered by peritoneum or
amnion
• Bowel may be damaged by exposure to amniotic
fluid
– 1 in 10,000 births (frequency is ⇑)
• May be related to cocaine use
Congenital Malformations (cont.)
• Meckel’s Diverticulum
– Ileal diverticulum is common
– Higher frequency among males
– Represents remnant of the vitelline duct
– Appears as finger-like pouch arising from ileum
– 3-6 cm long & may remain connected to
umbilicus by a fibrous cord or fistula
– Wall of diverticulum contains all tissue layers of
the ileum
• May include gastric & pancreatic tissue
• Gastric secretion often produces ulceration