2. • The endoderm forms the epithelial layer of the digestive and respiratory systems
• The endoderm primarily influences formation of the notochord, blood
vessels, heart, and part of the mesoderm
• At also lines the tubes in the body (digestive and respiratory)
• The endoderm comes from two sources:
• The definitive endoderm
• The visceral endoderm
• The definitive endoderm cells migrate through the primitive groove to
replace visceral endoderm
3. Regulation of
Endoderm
• Sox17 seems to be the transcription factor marking the
endoderm.
• Without Sox17, endoderm will not form and will be
mesoderm cells.
• Mesoderm cells express the gene for brachyury
transcription factor.
• Whether Sox 17 or Brachyury transcription factors
form depends on nodal (secreted from the visceral
endoderm).
• High levels of Nodal induce Sox17
• The anterior-posterior axis is determined by Wnt, FGFs,
and BMPs which are found posteriorly.
• These form the precursors for the foregut, midgut, and
hindgut
4.
5. The Pharynx
• The pharynx forms the anterior portion
of the digestive and respiratory tubes
• The pharynx in the embryo contain four
pairs of pharyngeal pouches. Between
those pouches are four pharyngeal
arches
• 1st pouch- auditory cavities of the
middle ear and Eustachian tubes
• 2nd pouch- tonsils
• 3rd pouch- thymus and half of the
parathyroid
• 4th pouch- the other half of the
parathyroid gland
6. Digestive Tubes and its
Derivatives
• The lining of the digestive tube is from
endodermal cells
• However, mesenchyme cells from the
splanchnic lateral plate mesoderm wrap
around the tube to provide the superficial
layers (connective tissue and muscle)
• Sox17 is an important transcription factor for
this
7. Accessory
Organs
• The endoderm forms the lining of three
accessory organs of the digestive system
• A liver bud (hepatic diverticulum) forms off
the foregut into the surrounding
mesenchyme.
• Where the liver bud connects to the
foregut will be the future common bile
duct
• Another pouch will form off this hepatic
diverticulum to form the gallbladder
• This is called the cystic diverticulum
8. continued
• There are two more buds growing
off the foregut:
• The ventral pancreatic bud
forms just below the cystic
diverticulum
• There is also a dorsal
pancreatic duct.
• These two buds will be
brought together when
the stomach rotates
9. Liver formation
• This is influenced by
cardiogenic mesoderm
• The developing heart is
probably because of FGFs
secreted by the heart and BMP
from the lateral place
mesoderm
• The ectoderm and
notochord, however, block
liver formation
10. Pancreatic
Formation
• With the pancreas, it is the opposite of the
liver.
• The developing heart blocks its formation
whereas the the notochord promotes it
• It is probably the case that this section of
the gut tube has the ability to form
either the liver or pancreas
• The notochord activates pancreatic
development by suppressing Shh expression
in the endoderm
11. The Respiratory Tube
The lungs are a derivative of the
digestive tube
A laryngotracheal groove develops
between the 4th pair of pharyngeal
pouches
It bifurcates (splits) to form the
bronchi and eventually the lungs.
