embryology of the respiratory system and clinical implications

Question 1

gastrulation [ respiratory diverticulum]

Answer 1

the respiratory diverticulum [lung bud] forms as a blind-ended outgrowth from the ventral wall of the foregut. occurs at 4 weeks.
therefore epithelial internal lining in larynx, trachea, bronchi and lungs is of endodermal origin

Question 2

development of the trachea

Answer 2

oesophagotracheal ridges fuse to form oesophagotracheal septum, during 4th week

Question 3

splanchnic mesoderm

Answer 3

gives rise to cartilage, CT and muscles

Question 4

endoderm

Answer 4

gives rise to epithelium and glands of trachea and pulmonary epithelium

Question 5

stages of lung maturation

Answer 5

pseudoglandular stage –> canalicular period –> terminal sac period –> alveolar period

Question 6

pseudoglandular stage

Answer 6

5-16 weeks.
terminal bronchioles form. by the end of this period, all the major components of the lung have formed, except those required for gas exchange

Question 7

formation of the pleural cavities

Answer 7

the pericardioperitoneal canals become separated from the pericardial cavity by the pleuropericardial folds.
as the bronchi grow, they remain is pushed right up against the paritetal pleura.

Question 8

canalicular period

Answer 8

16-26 weeks. lumens of the bronchi and terminal bronchioles enlarge. the tubes of the walls become thinner. mesodermal tissue becomes vascularised. by 24 weeks, each terminal bronchiole divides into 2 or more respiratory bronchioles. at the end of this period, the first terminal sacs form at the end of the respiratory bronchioles.

endoderm gives rise to mucosal lining of the bronchi and alveoli. muscle and cartilage ate from splachopleuric mesoderm. blood vessels develop by anginogenesis and start to come into close apposition with the lung epithelium

Question 9

terminal sac period [saccular phase]

Answer 9

26 weeks - birth.

further thinning of the tubes. numerous saccule formed. lined by type I and type II alveolar cells [produce surfactant]

Question 10

formation of the diaphragm: the 4 embryonic components

Answer 10

• transverse septum
• • Pleuroperitoneal membranes
• Dorsal mesentery of oesophagus
• Muscular ingrowth from lateral body walls

Question 11

transverse septum

Answer 11

• forms in early development [day 22]
• thickened bar of mesoderm
• grows dorsally from the ventrolateral body wall.
• caudal to pericardial cavity
• primordium of central tendon of diaphragm
• liver right below it
• septum transversum grows dorsally.
• pleuroperitoneal membranes grow medioventrally to meet up
• seal off the pleural and peritoneal cavities
• innervated by the phrenic nerve

Question 12

pleuroperitoneal membranes

Answer 12

• form lateral wall of pleural and peritoneal cavities.
• appear from start of 5th week.
• form posterior and lateral parts of the diaphragm.
• fuse during the 7th week.
• innervated by the phrenic nerve

Question 13

dorsal mesentery of oesophagus

Answer 13

• will form the medial regions of the diaphragm
• forms muscle bundle anterior to the aorta
• derived from myoblasts originating on the vertebral column.

Question 14

muscular ingrowth from lateral body walls

Answer 14

• contributes muscle to peripheral region of the diaphragm.
• happens by week 12
• myoblasts from myotomes of neighbouring somites.
• this area is innervated by spinal nerves from T7-T12

Question 15

mature diaphragm

Answer 15

• central diaphragm mainly tendinous . you can see the muscular part around the aorta from the oesophageal mesoderm

Question 16

alveolar period

Answer 16

8 months- term.

• increased surfactant production
• mature alveoli develop [but only around 5% are formed before birth]
• primordial alveoli increase in size, Type 1 epithelial cells thin further and capillaries become even closer

Question 17

changes in lung before birth

Answer 17

• ramping up of surfactant production in the last 2 weeks
• breathing movements occur before birth to stimulate lung development and respiratory muscles.
• amniotic fluid is aspirated.

Question 18

changes in lung at birth

Answer 18

• at birth, the lungs are partially filled with fluid
• this is removed by: pressure on the thorax during delivery, expelling fluid through mouth and nose. absorption into the pulmonary circulation. absorption into the lymphatics.
• this leaves a thin coating of surfactant lining the alveolar cell membranes

Question 19

problems of lung development

Answer 19

• trachea and oesophagus don’t separate correctly
• the lung doesnt develop normally
• the diaphragm doesnt develop normally

Question 20

oesophageal atresia and tracheoesophageal fistula

Answer 20

• fluid in the mouth cant go down oesophagus
• back into mouth and down trachea
• and then into lungs
• causes coughing and choking
• and then pneumonia

Question 21

lung development

Answer 21

• reduced number of lung segments
• reduced number of terminal air sacs
• loss of node ciliary functions affects determination of left-right axis, causes situs inversus
• lung tissue develops but not attached to pulmonary circulation

Question 22

diaphragm development

Answer 22

• gaps in the diaphragm
• gut may push into thoracic cavity
• 80% on the left side
• usually posterilaterally
• may have underdeveloped lung [pulmonary hypoplasia]
• may push the heart across
• usually failure of fusion of all parts posterolaterally
• clinically called Bochdalek hernias
• those in the centre are called Morgagni hernias

Question 23

prematurity

Answer 23

• may not have enough surfactant
• surface tension will be high at the air-water interface
• risk of alveoli collapsing during expiration
• causes respiratory distress syndrome

Question 24

respiratory distress syndrome

Answer 24

premature babies:
- gasp
- are cyanosed
- rapid breathing
2% of newborns affected. causes 20% deaths in newborns.
treat with artificial surfactant and glucocorticoids [steroids] to stimulate surfactant production