Embryology of the Respiratory System and clinical implications

Question 1

Stages of early respiratory system development

Answer 1

formation of trachea and bronchi
lung development
formation of pleural cavities
diaphragm development

Question 2

What is the respiratory diverticulum?

Answer 2

lung bud

Question 3

When and where does the respiratory diverticulum form?

Answer 3

around the 4th week
forms as blind-ending outgrowth from ventral wall of foregut
endodermal origin

Question 4

When does the trachea develop?

Answer 4

during the 4th week

Question 5

What happens for the trachea to form?

Answer 5

oesophagotracheal ridges fuse to form oesophagotracheal septum
laryngotracheal tube and oesophagus is separate

Question 6

What does the laryngotracheal tube develop into?

Answer 6

larynx and trachea

Question 7

Describe the structure of the laryngotracheal tube

Answer 7

inner lining: endoderm

outer lining: splanchnic mesoderm

Question 8

What does the endoderm of the laryngotracheal tube give rise to?

Answer 8

epithelium and glands of trachea and pulmonary epithelium

Question 9

What does the splanchnic mesoderm of the laryngotracheal tube give rise to?

Answer 9

cartilage, connective tissue and muscles

Question 10

Stages of lung maturation

Answer 10

1. pseudoglandular stage
2. canalicular period
3. terminal sac period
4. alveolar period

Question 11

When is the pseudoglandular stage?

Answer 11

5-16 weeks

Question 12

What occurs in the pseudoglandular stage?

Answer 12

terminal bronchioles form
by the end: all major components of the lung has formed, except those needed for gas exchange
development of pleural cavities

Question 13

Development of pleural cavities

Answer 13

pericardioperitoneal canals are connected to pericardial (primitive thoracic) and peritoneal (primitive abdominal) cavities
pericardioperitoneal canals become separated from the pericardial cavity by pleuropericardial folds

Question 14

When do the pericardioperitoneal canals become completely separated?

Answer 14

remain connected to peritoneal (abdominal) cavity until closed by fusion of the peritoneal folds during formation of the diaphragm

Question 15

When is the canalicular period?

Answer 15

16-26 weeks

Question 16

What happens in the canalicular period?

Answer 16

lumens of bronchi and terminal bronchi enlarge
tissue becomes vascularised
by 24 weeks: each terminal bronchiole has formed 2 or more respiratory bronchioles
towards end: 1st terminal sacs form at end of resp. bronchioles

Question 17

What are respiratory bronchioles characterised by?

Answer 17

cuboidal epithelium

Question 18

When is the terminal sac period?

Answer 18

26 weeks - birth

Question 19

What happens in the terminal sac period?

Answer 19

many terminal sacs form (primordial alveoli)
type 1 alveolar epithelial cells form
capillaries come into close contact with flat epithelial cells - start to blge into primordial alveoli
type 2 alveolar cells begin to form

Question 20

Type 1 alveolar cells

Answer 20

epithelial cells of the terminal sacs become flat and thin squamos cells from cuboidal
line alveolar sacs
allows efficient gas exchange

Question 21

Type 2 alveolar epithelial cells

Answer 21

rounded, secretory epithelial cells
line alveolar sacs
formed from end of 6th month
produce surfactant

Question 22

What is surfactant

Answer 22

produced by type II alveolar epithelial cells
phospholipid-rich fluid
forms monomolecular film over internal walls of the terminal sace and mature alveoli
lowers surface tension at the air-alveolar interface so alveoli don’t collapse due to change in pressure

Question 23

When is the alveolar period?

Answer 23

8 months - childhood

Question 24

What happens in the alveolar period?

Answer 24

increased production of surfactant
primordial alveoli increase in size
type I cells thinner and capillaries even closer association
postnatal increase in lung size: increased divisions to form repiratory bronchioles and continued prmordial alveoli production

Question 25

Changes in lungs before birth

Answer 25

amount of surfactant produced increases, mostly in last two weeks breathing movements occur to stimulate lung development and respiratory muscles amniotic fluid aspirated

Question 26

Changes in lungs at birth

Answer 26

lungs half-filled with liquid, removed by: 1. pressure on thorax during delivery 2. absorbed into circulation via pulmonary circulation 3. absorbed into lymphatics thin coating of surfactant left lining alveolar cell membranes

Question 27

Lungs of a stillborn

Answer 27

1st breath not taken, lungs have no air and are full of fluid and baby will sink in water

Question 28

What is the diaphragm?

Answer 28

musculotendinous dome-shaped partition that separates the thoracic and abdominal cavities

Question 29

What are the embryonic components of the formation of the diaphragm?

Answer 29

transverse septum pleuroperitoneal membranes dorsal mesentery of oesophagus muscular ingrowth from lateral body walls

Question 30

What is the transverse septum?

Answer 30

sheet of non-continuous tissue (mesodermal in origin) | primordium of central tendon of diaphragm (non-muscular)

Question 31

Where and when does the transverse septum grow?

Answer 31

4th week grows dorsally from ventrolateral body wall forming liver is embedded in tissue caudal to pericardial cavity, partially separating it from peritoneal cavity ventral and lateral, not posterior

Question 32

When and where do the pleuroperitoneal membranes form?

Answer 32

5th week | form from the lateral wall of pleural and peritoneal cavities

Question 33

What do the pleuroperitoneal membranes form

Answer 33

forms posterior and lateral parts of the diaphragm | fuses with the transverse septum and dorsal mesentery in the 7th week

Question 34

What does the dorsal mesentery form?

Answer 34

medial region of the diaphragm muscle bundles anterior to the aorta (in adults: Crura of the diaphragm) not yet mature/muscularised

Question 35

What is the dorsal mesentery derived from?

Answer 35

myoblasts that have previously migrated into the dorsal mesentery of oesophagus

Question 36

Formation of the mature diaphragm

Answer 36

muscular ingrowth from lateral body walls contributes muscle to peripheral region of diaphragm external to the region that is derived from the pleuroperitoneal membranes by 12th week

Question 37

Give examples of defects and problems of the lungs and diaphragm

Answer 37

premature and Respiratory Distress Syndrome oesophageal atresia and tracheoesophageal fistula congenital cysts of the lung congenital diaphragmatic hernia

Question 38

Premature baby

Answer 38

not enough surfactant high surface tension at blood-air interface risk of alveoli collapsing during expiration RDS

Question 39

RDS

Answer 39

2% of newborns rapid, laboured breathing treatments (if known in advance): artificial surfactant and treatment with glucocorticoids to stimulate surfactant production

Question 40

Oesophageal atresia and tracheoesophageal fistula

Answer 40

abnormal separation of the oesophagus and trachea by the oesophagotracheal septum more common in males associated with other defects like congenital heart disease most common: proximal blind end of oesophagus

Question 41

Atresia meaning

Answer 41

narrowing/withering away

Question 42

Fistula meaning

Answer 42

abnormal opening or passage

Question 43

Problem of most common oesophageal atresia and tracheoesophageal fistula

Answer 43

XS amounts of fluid in mouth/upper RT, gastric contents may reflux - into trachea and lungs coughing and choking pneumonia

Question 44

Congenital cysts of the lungs

Answer 44

terminal bronchi abnormally dilated, usually at periphery small and numerous/few and large problems: drain poorly, chronic lung infections

Question 45

Congenital diaphragmatic hernia

Answer 45

congenital absence of large area of diaphragm due to: failure of fusion of pleuroperitoneal membrane with other components, posterolateral defect problems: abdominal contents hemiated, hypoplastic lung, pressure on heart