Development of the Respiratory System

Question 1

What germ layer forms the respiratory system

Answer 1

• The endoderm

Question 2

the larynx & trachea are developed in the ___________ of development

Answer 2

• 4th week

Question 3

Where does the laryngotracheal groove appear

Answer 3

• At the caudal end on the ventral wall of the primitive pharynx

Question 4

Explain the development of the trachea

Answer 4

• Endodermal lining of laryngotracheal tube → tracheal & pulmonary epithelium & glands (Wnt2/2b, FGF10, BMP4)
• Splanchnic mesenchyme around the LT tube → cartilage, connective tissue & muscles of the trachea
• LT groove → LT diverticulum → LT tube
• Tracheoesophageal septum separates the LT tube from the pharynx (now esophagus)

Question 5

What does the tracheoesophageal septum do?

Answer 5

• It divides the cranial part of the fore gut into the ventral respiratory passageways (NKX2.1,Shh, Wnt7b) and the dorsal digestive passageways (SOX2, BMP7) with BARX1 at the septum

Question 6

Explain the development of the bronchi

Answer 6

• A respiratory bud appears at the caudal end of the laryngotracheal diverticulum in the 4th week of development
• The bud divides = 2 primary bronchial buds which then grow laterally → L&R primary bronchi → 2L&3R secondary bronchi → 8-9L&10R tertiary bronchi = 24 orders of branching
• developing bronchi & lungs grow laterally into the intraembryonic coelom = pleural cavity (houses the lungs)

Question 7

What are the 4 stages of lung development

Answer 7

• Pseudoglandular stage
• Canalicular stage
• Terminal sac stage
• Alveolar stage

Question 8

What happens in the pseudoglandular stage

Answer 8

• “The stage p.t. a false gland” (looks like an exocrine gland)
• This is between weeks 5-17
• Respiration does not take place (bcos alveoli haven’t been developed)

Question 9

What happens in the canalicular stage

Answer 9

• This overlaps the pseudoglandular stage (weeks 16-26) bcos its a cranial → caudal development
• the lumina of the bronchi & bronchioles increase
• 24 weeks: terminal bronchioles → respiratory bronchioles, alveolar ducts & terminal sacs
• lung tissue becomes vascular, respiration may be possible

Question 10

What happens in the terminal sac stage

Answer 10

• From 26 weeks to birth
• Terminal sacs develop at the end of the alveolar ducts which capillaries bulging into then = alveoli
  2 types of alveolar cells
• Type I = squamous shaped
• Type II produce pulmonary proteins & fats which decrease surface tension (collapsing alveoli)

Question 11

What happens in the alveolar stage

Answer 11

• from 32 weeks to 8 years
• Alveocapillary membrane thins bcos capillaries bulge more into the terminal sacs
• 95% of alveoli form postnatally
• fetus practices breathing in the uterus

Question 12

Explain the developent of the diaphragm

Answer 12

• The diaphragm separates the thoracic & abdominal cavities
  develops from 4 sources
• septum transversum (central tendon of diaphragm)
• pleuroperitoneal membranes (become a smaller component)
• dorsal mesentery of the esophagus
• lateral body walls → peripheral part of the diaphragm

Question 13

What is congenital diaphragmatic hernia

Answer 13

• It results from failure of the pleuroperitoneal membranes fusing with the other components of the diaphragm (usually on the left)
• Causes the abdominal organs to enter the thoracic cavity = defective heart & lung development

Question 14

Explain the development of the larynx

Answer 14

• Larynx develops at the cranial, distal end for the laryngotracheal tube
• The cartilage of the 4th and 6th pharyngeal arches form the cartilages of the larynx
• Cranial LT tube mesenchyme → arytenoid swellings that grow towards the tongue → glottis
• Laryngeal ventricles from epithelial recanalization in week 10
• Epiglottis formed from hypo pharyngeal eminence

Question 15

What can cause esophageal atresia

Answer 15

• Defects in the Shh, Wnt, or BMP pathway or mutations of Sox2 or NKX2.1 (dorsoventral patterning)