Embrylogy notes made by Jón Kolbeinn Guðmundsson for University of Debrecen Medicine and Dentistry students.

1. Embryology part 6
Jón Kolbeinn Guðmundsson

2. Folding of the embryo and formation of the gut tube.
Lateral folding of the embryo gives us the gut tube. The gut tube is now dividied into 3
areas: foregut, midgut and hindgut. Each part giving rise to a separate structure.
The liver, gallbladder, pancreas and spleen all arise from the distal end of the foregut.

3. Transverse septum and the future diaphragm
Between the yolk stalk and the pericaridium there is a septum which
separates the abdomen from the thorax region. It is a transverse
mesodermal plate called the transverse septum, and will eventually form the
diaphragm.
Transverse septum

4. The development of the liver, biliary
ducts and gallbladder &
The development of the pancreas
Topic 22 & 23

5. LIVER - The liver bud (hepatic diverticulum)
At around 3rd week after fertilization a liver bud will start growing out from the distal
end of the forgut.
The liver bud starts to grow ventrally from the foregut. As the bud grows in size and
penetrates the septum, the connection between it and the foregut narrows forming
the bile duct.
From the liver bud the gallbladder emerges as a diverticulum.

6. The liver bud forks into left and right lobes. These grow in the cephalic direction until it
meets the transverse septum, forming hepatic cords as they go.
During further development the hepatic cords intermingle with the vitelline and
umbilical veins, which form the hepatic sinusoids (the venous system of the liver, which
drains into the inferior vena cava).
The hepatic cords differentiate into parenchyma (hepatocytes) and the lining of biliary
ducts.

7. Circulation of the fetal liver and the anatomical remnants in the adult
Two thirds of the oxygen rich blood from the placenta passes through the liver during
development. The rest passes directly to the Inferior vena cava via a shunt called the
ductus venosus. Once the baby is born, the ductus venosus closes and becomes the
ligamentum venosum.
The same thing occurs with the
umbilical vein. After birth the
umbilical cord is cut and the
umbilical vein degenerates into
the round ligament (Ligamentum
teres)

8. ** Clinical correlation ** The remnant veins in the round ligament can
reopen during portal hypertension, this will result in caput medusae and is
an example of porta-caval anastomosis

9. As the liver grows it will press against the transverse septum (diaphragm).
Where the liver is pressed against the future diaphragm, this spot will become the
bare area of the liver, no visceral mesoderm covers the liver there, hence “bare area”.

10. The liver grows fast and starts taking a great amount of space. Due to the rotation of
the stomach, the liver moves towards the right side and starts to fill up alot of space in
the abdominal cavity.
In the fetus, the liver is twice the relative size compared to an adult. This is because
during development, the liver is the main site of hematopoesis.

11. Liver in relation to mesoderm
The developing liver is covered by mesoderm of the septum transversum.
It connects to the foregut (stomach & duodenum) via the lesser omentum and
to the ventral abdominal wall via the falciform ligament.
** remember that the falciform ligament contains the umbilical vein in the
developing fetus.

12. PANCREAS - The pancreatic buds
The soon after the liver bud and the gallbladder start to form, a dorsal pancreatic bud
appears at the dorsal end of the foregut, and a smaller ventral pancreatic bud at the
ventral end in close proximity to the gallbladder.
The two pancreatic buds
(dorsal and ventral) will
eventually join and fuse
together. This occurs due to
the rotation of the stomach
and the foregut.

13. Common
bile duct
Hepatic
duct
Gallbladder and biliary ducts
Cystic duct
Ventral
pancreatic bud
Dorsal
pancreatic bud
As the liver grows larger, the hepatic
duct forms along with the cystic duct.
Together the cystic duct and hepatic
duct drain into the common bile duct
which is in close proximity to the
ventral pancreatic bud.

14. The hepatocytes in the fetal
liver start to produce bile
around 12th week of
development, where the bile is
stored in the gallbladder and is
secreted into the duodenum.
* The first stool that the new-
born baby passes is called
meconium and is basically
green coloured due to the bile.
Common
hepatic duct
Left hepatic ductRight hepatic duct
Cystic duct
Common bile
duct

15. 90 degree clockwise
rotation

16. Fusion of the ventral and dorsal pancreatic buds
Due to the 90 degree rotation of
the foregut (stomach mostly), the
ventral pancreatic bud swings to
the dorsal side and is now
situated below the dorsal bud.
Clearly the common bile duct
does so as well and is now
situated behind the duodenum
and the pancreatic buds.
The duodenum swings towards
the right side of the abdomen due
to the rotation of the foregut and
take on it’s characteristic “C”
shape.
The dorsal pancreatic bud becomes the head, neck, body and tail of the pancreas.
The ventral pancreatic bud becomes the uncinate process.

18. The pancreatic ducts
The parenchyma of the ventral and dorsal buds fuse, along with their ducts.
The duct from the dorsal bud joins the duct from the ventral bud forming the main
pancreatic duct (of Wirsung). Notice that the common bile duct and the main pancreatic
duct join together and open into the duodenum at the major papilla (Ampulla of Vater)
Main pancreatic duct
(of Wirsung)
Major papilla
(Ampulla of Vater)
*Accessory pancreatic
duct (of Santorini)
* Minor papilla
* The proximal part of the dorsal pancreatic duct either
disappears or persists as an accessory pancreatic duct.
There can be quite a bit of anatomic variation.*

19. Pancreas in relation to mesoderm.
Once the pancreas has formed in the dorsal mesogastrium. It will be pushed by
the growing viscera towards the dorsal abdominal wall and will fuse with it,
making it along with the duodenum retroperitoneal, the exception is the tail of
the pancreas which will be intraperitoneal.

21. The development of the
respiratory tract
Topic 8

22. LUNG – formation of the lung bud
Once folding of the embryo has occurred and the gut tube has
formed, at around 22 day (3rd week) a lung bud forms ventrally
from the upper part of the foregut (oesophagus region)
The lung bud grows ventrally and
caudally (downwards).
Small ridges start to pinch the lung
bud off from the oesophagus.
The lung bud is made from
endoderm and will become the
epithelial lining of the respiratory
tract, the surrounding lung
parenchyma and vasculature will
derive from mesoderm.
* The lung bud is sometimes called
respiratory diverticulum *

23. After separation, the developing trachea lies ventral to the oesophagus.
The left and right lung buds form.

24. Branching
Continued branching of the primary bronchi gives rise to secondary bronchi, 3 on
the right and 2 on the left.
During 6th week further branches result in the formation of tertiary branches. These
will each supply a bronchopulmonary segment.

26. Histological differentiation of the lungs
Maturation of the lungs is divided into 4 periods.
1. Pseudoglandular: 5-17 weeks
2. Canalicular: 16-25 weeks
3. Terminal sac: 24 weeks until birth
4. Alveolar: birth till about 8 years old.
Note that the lung continues to develop into long into childhood.
These maturation periods are important clinically, as they determine the
likelihood of survival of premature babies.
For embryology we are mostly concerned with the first 3 maturation periods

27. 1. Pseudoglandular period
From weeks 5-17. Branching of the respiratory tree has occurred from
terminal bronchioles.
Respiration is not possible at this stage, therefore the fetus cannot survive if
born prematurely.

28. 2. Canalicular period
16-25 weeks. Terminal bronchioles give rise to respiratory bronchioles, which in turn give
rise to respiratory bronchioles.
The surrounding mesodermal tissue becomes highly vascularized with capillaries.
Respiration is just possible towards the end of the canalicular period as some terminal
sacs (primitive alveoli) have developed at the ends of the respiratory bronchioles.
*the reason for this is because maturation starts slightly earlier in the cranial part of the
lungs*

29. 3. Terminal sac period
24 weeks until birth. Further terminal sacs (primitive alveoli) develop. The
epithelium starts to thin and become squamous. Capillaries come into contact
with epithelium and the blood-air barrier is formed.
At this stage we have two types of pneumocytes, type 1 and type 2.
Type 1 is involved in gas exchange and type 2 produces surfactant.
Surfactant is necessary to reduce surface
tension in the alveoli and keep them open,
if surfactant is lacking there is a risk of the
alveoli collapsing. That is called Infant
respiratory distress syndrome (IRDS)

30. The formation of the pleural cavity.

31. Once the lung bud has branched into two separate lung buds and surrounded by
mesoderm. The pleuropericardial cavity begins to divide into two separate cavities.
The pleural and pericardial cavity.
This happens when pleuropericardial fold begin to move towards each other in the
midline.

32. Eventually the pleuropericardial folds have met in the midline and
fused, making the pleural and pericardial cavity completely separate
Later still, as the lungs develop and grow they begin to wrap around
the pericardial cavity.