Embryology 3

Question 1

What happens during the folding of the embryo?

Answer 1

The parietal mesoderm folds down to surround the visceral mesoderm of the intraembryonic body cavity (coelom).
The yolk sac becomes absorbed into the primitive gut.

Question 2

What does the primitive gut tube form?

Answer 2

Formed from the endoderm.
Forms the foregut, midgut and hindgut.

Question 3

How are the trachea and lung buds formed?

Answer 3

At 4 weeks, trachea and lung buds bud out from the ventral wall of the primitive foregut.
They are surrounded by the lateral plate splanchnic mesoderm.
The trachea lies posterior to the oesophagus.

Question 4

How do the trachea and lung buds separate from the oesophagus?

Answer 4

An oesophagotracheal septum develops between the oesophagus and trachea.
The lung buds enlarge and form the lungs.

Question 5

What happens if the trachea and lung buds don’t separate?

Answer 5

A tracheoesophageal fistula forms.
A common congenital abnormality, where there is an abnormal passage between the two tubes.

An oesophageal atresia forms.
The oesophagus is closed or absent.

Question 6

How do the pleura develop in an embryo?

Answer 6

Each lung bud invaginates the wall of the coelomic cavity and grows to fill the space.

The original coelomic cavity is reduced to the pleural space, due to the growth of the lung.

Question 7

How do the lung buds become surrounded by pleura?

Answer 7

Lung buds push the splanchnic mesoderm towards the somatic mesoderm and become completely surrounded by both layers.

Question 8

What mesoderm layers form the pleura?

Answer 8

Splanchnic - visceral.
Somatic - parietal.

Question 9

Why do the lungs get surrounded in a closed space?

Answer 9

The canal is sealed off, to maintain the correct ventilatory pleural cavity pressures.

Question 10

What is the pericardioperitoneal canal?

Answer 10

The space between the somatic and splanchnic mesoderm around the lungs.
It is initially continuous with the pericardial and peritoneal cavities.

Question 11

How is the canal sealed off?

Answer 11

Pleuropericardial folds are formed from the body wall and lined with somatic mesoderm.
These folds seal the canal, and the canal around the lungs becomes the pleural cavity.

Question 12

What is the pseudoglandular stage?

Answer 12

Foetal period.
7-17 weeks.
Lungs develop.
No respiratory portion or gas exchange.

Question 13

What is the canalicular stage?

Answer 13

Foetal period.
17-27 weeks.
The respiratory portion develops.
Alveolar ducts form.
Epithelium differentiates into Type I and Type II pneumocytes (still no gas exchange).

Question 14

What is the saccular stage?

Answer 14

Foetal period.
27-40 weeks.
Alveolar sacs, primitive alveoli, and surfactant develop (gas exchange begins).

Question 15

What is the alveolar stage?

Answer 15

Postnatal period.
32 weeks to 8 years.
Proliferation of alveoli from alveolar sacs.
~95% of adult alveoli forms postnatally.

Question 16

What is IRDS?

Answer 16

Infant Respiratory Distress Syndrome.
Caused by surfactant deficiency.
Alveoli partially collapse.
Debris consisting of blood elements accumulate (hyaline cartilage = glassy appearance).

Question 17

What causes lung agenesis?

Answer 17

Failure of lung bud development.
Insufficient mesoderm formation.
Teratogens.

Question 18

What is pulmonary hypoplasia?

Answer 18

Incomplete development of the lungs, resulting in a low number of bronchopulmonary segments or alveoli.

Question 19

What four structures form the diaphragm?

Answer 19

Septum transversum.
Pleuroperitoneal folds.
Skeletal muscle.
Dorsal mesentery of the oesophagus.

Question 20

What do the pleuroperitoneal folds do in the formation of the diaphragm?

Answer 20

Grows in from the peripheral body wall to fuse with the septum transversum.
Closes off the pleuroperitoneal canals.
Folds carry the lower intercostal nerves to supply the diaphragm periphery.

Question 21

What happens if the pleuroperitoneal folds do not form successfully?

Answer 21

Herniation of abdominal contents into the thorax occurs.

Question 22

What does skeletal muscle do in the formation of the diaphragm?

Answer 22

Covers the pleuroperitoneal folds.
Grows from the peripheral body wall.
Forms the muscular part of the diaphragm.
From somites C3-C5 (carries the phrenic nerve to the muscular part of the diaphragm).

Question 23

What does the dorsal mesentery do in the formation of the diaphragm?

Answer 23

Attaches the embryological oesophagus to the posterior thoracic wall.
Skeletal muscle from the periphery covers the dorsal mesentery to form crura.

Question 24

What is the only structure in place in early diaphragm development?

Answer 24

The septum transversum.
Forms the central tendon.

Question 25

What are hernias that can occur in the diaphragm?

Answer 25

Hiatus hernia -
A weak oesophageal hiatus.

Diaphragmatic hernia -
A weak foramen of Bochdalek / Morgagni.
Failure of the diaphragm to completely close during development.