L7 Embryology Development of the Respiratory System

Question 1

What sections are the gut tube split into?

Answer 1

Foregut, midgut and handout.
The primitive gut tube develops during week 3-4 by incorporating the yolk sac during cranio-caudal and lateral folding fo the embryo. each section will do onto form different structures.

Question 2

At what stage does the respiratory diverticulum form?

Answer 2

Day 22 - week 4

This is a ventral outpounching of the gut tube. This then grows in a ventral-caudal direction.

Question 3

What are trachea-oesphageal ridges?

Answer 3

These ridges separate the oesophagus and the trachea. These ridges eventually grow closer together until they fuse to form a trachea-oesphageal septum which separates the respiratory diverticulum and the gut tube.

Question 4

What is a trachea-oesphageal fistula?

Answer 4

An abnormal connection between the trachea and the oesophagus.

Question 5

What is the most common trachea-oesphageal fistula? What is the consequence of this defect?

Answer 5

85-90% are a trachea-oesphageal fistula with an associated atresia.

• When the baby feeds the milk will be regurgitated due to the closure of the oesophagus.
• Milk could also spill into the trachea causing an infection and other respiratory problems.
• Stomach contents could also be brought up and spill into the trachea. This includes enzymes and stomach acid.
• The stomach can become distended as it fills with air as the air from the trachea spills into the stomach

Question 6

What other defects are fistula associated with?

Answer 6

Using the acronym VACTERL or VATER (less common):

V - vertical defects e.g. spina bifida
A - Anal atresia i.e. the anal canal is a blind ended tube
(C) - Cardiac defects - congenital malformations are most commonly associated with cardiac defects
T - Trachea-oesphageal fistula
E - Oesphageal atresia
R - Renal abnormalities
(L) - Limb defects

Question 7

What occurs at week 5?

Answer 7

There is further growth and differentiation of the lung buds to form primary bronchi which go onto form secondary bronchi. At this point there are 3 buds on the right and 2 on the left.

Question 8

What occurs at week 6?

Answer 8

Tertiary bronchi buds are developed. These go onto form segments of the lung. This is known as broncho-pulmonary segments. This is a very regulated process and occur due to communication between the endoderm and the mesoderm. At this point there are 10 buds on the right and 8 on the left.

Question 9

What occurs at week 16?

Answer 9

Terminal bronchioles are formed. At this point respiration still cannot occur.

Question 10

What occurs at week 26?

Answer 10

Respiratory buds start to form. Gaseous exchange can occur, but it is not efficient.

Question 11

What occurs at week 36?

Answer 11

The formation of alveolar sacs. Formation continues up until 8 years of age.

Question 12

Which part of the respiratory system does the endoderm and mesoderm form?

Answer 12

The endoderm gives rise to the underlying epithelia and the mesoderm gives rise to the connective structures such as the cartilage and the vasculature.

Question 13

How is the visceral and parietal pleura formed?

Answer 13

The lateral plate pleura comes together in the folding fo the embryo. This creates a visceral mesoderm layer and a parietal mesoderm layer which give rise to the visceral and parietal pleura respectively. The visceral pleura surrounds the gut tube and the parietal pleura forms the parietal cavity.

Question 14

What is pulmonary agenesis?

Answer 14

Pulmonary agenesis is the failure to form the lungs. This can be unilateral where only one lung fails to form or bilateral whereby no lungs are form. Bilateral pulmonary agenesis is incompatible with life. Unilateral pulmonary agenesis usually presents when the individual is in respiratory distress as the demand on sth lung is further increased . The demand is already high when there is only one lung. This is synonymous with other congenital malformations.

Question 15

What is pulmonary hyperplasia? Give an example.

Answer 15

This is failure of the lungs to develop properly. There are varying degrees fo severity from mild to fatal. An example is in congenital diaphragmatic hernia. The diaphragm can develop an opening and the intestinal contents can herniate through. If they take up a large amount of space development, this means the lungs will not develop fully.

Question 16

What is a supernumerary lobe?

Answer 16

An extra lobe that can develop on the lung such as an azygous lobe. This is an accessory lobe at the apex of the lung due to the migration of the azygous vein. There is little functional significance of the lobe.

Question 17

Name the 4 stages of the histogenesis ion the lung including the timeframes.

Answer 17

1. Psuedo-glandular stage - Week 5-17
2. Cannicular stage Week 16-24
3. Terminal sac stage Week 24-Birth
4. Alveolar stage Week 36 - 8 years

Question 18

What occurs in the psuedo-glandular stage?

Answer 18

Endodermal lung buds undergo branching only if they are exposed to bronchial mesoderm. Branching of the respiratory tree cause by the endodermal, mesodermal signalling. The signals between stimulate the branching morphology and the formation of terminal bronchioles. The terminal sac is primitive as this is where the sac will develop from. Terminal bronchioles are part of the conducting potion of the respiratory system and so gaseous exchange cannot occur. If a baby is born in this period, they will not be able to survive.
In the mesoderm, blood vessels form.

Question 19

What is the significance of the cannicular stage?

Answer 19

The canalicular stage takes place between the 16th and 25th week. At this time the gas exchanging portion of the lung is formed and vascularized. There is a decrease of interstitial tissue and growth of the capillary network. By 20 weeks there is differentiation of the type I pneumocyte. The type I pneumocyte is the primary structural cell of the alveolus, and gas exchange will occur across these very thin, membrane-like cells. Capillaries begin to grow in absolutely close proximity to the distal surface of the alveolar cells (if the potential alveolar space is considered proximal). At about the same time, there is the appearance of lamellar bodies, also called inclusion bodies, in type II alveolar cells. The lamellar body is the site of surfactant storage, prior to its release into the alveolar space.
Toward the end of the period, there is a better prognosis for birth. Gaseous exchange can start to occur, not very efficient. This is as we have started to develop respiratory bronchioles and terminal sacs at the end (these are highly vascularised). The later we leave that, the greater the prognosis of survival. 16-18 weeks is 2%, week 25-26 weeks 60% survival.

Question 20

What is the significance of the terminal sac stage?

Answer 20

The terminal sac, or saccular stage encompasses the period from 26 weeks until term. During this stage, there is a decrease in interstitial tissue, and a thinning of the alveolar walls. As this stage progresses, there are recognizable Type I and Type II cells. Further development of the sacs. At the end of the sacs, is where we develop primitive alveoli for efficient gaseous exchange. The epithelium derived from the endoderm, begins to thin so gases can diffuse across it easier. The capillaries develop further and become more numerous. They are now in closer proximity to the now thinner epithelium.

Question 21

What is the significance of the alveolar stage?

Answer 21

Development of alveoli.

Question 22

What are type I pneumocytes?

Answer 22

The type I pneumocytes form part of the barrier across which gas exchange occurs. They can be identified as thin, squamous cells whose most obvious feature is their nuclei.

Question 23

What are type II pneumocytes?

Answer 23

The cell responsible for the production and secretion of surfactant.

Question 24

What is Infant Respiratory Distress Syndrome (IRDS)?

Answer 24

Infantile respiratory distress syndrome (IRDS), is a syndrome in premature infants caused by developmental insufficiency of pulmonary surfactant production. A deficiency in surfactant means that surface tension increases as the alveoli struggle to inflate. This leads to laboured breathing and the threat of asphyxiation.

Question 25

How can medical intervention cause IRDS?

Answer 25

Here there is laboured breathing due to inefficient gaseous exchange. This requires more effort as the infants are threatened with immediate asphyxiation. This means there is an increased rate of breathing. You may have to ventilate the baby. If you are putting at a high pressure into the lungs, this can damage the delicate tissues. The linings have become damage leading to leaking of the serine proteins into the spaces. This can lead to detachment of the alveolar lining due to ventilation.

Question 26

How can you prevent IRDS?

Answer 26

Can be prevented by giving glucocorticoids prior to birth. You can give a regulated amount a before birth as this stimulates the production of surfactant.