Development of the Respiratory System

Question 1

Describe some of the embryological development that is common to all organs.

Answer 1

1) TRILAMINAR EMBRYO (ENDODERM, MESODERM, ECTODERM)

◙ If we move further along the development, the ectoderm will develop a neural groove (the tips of the walls of this groove are called the neural crest)
◙ Meanwhile the lateral plate mesoderm will divide into two layers: parietal (somatic) mesoderm (will line body walls) and visceral (splanchnic) mesoderm (will surround the organs) which will become the serous membranes.

2) LATERAL FOLDING OF THE TRILAMINAR EMBRYO (TO FORM BODY CAVITIES)
◙ As development progresses, the parietal layer of the lateral plate mesoderm will “merge” with the ectoderm in the ventral midline.
◙ In the meantime 2 things take place
1) The primitive yolk sac becomes “pinched”. Primitive yolk sac will become the digestive tract or the gut tube. While this takes place, the visceral layer of the lateral plate mesoderm will envelope the future gut tube. Eventually the connection between the gut tube and the secondary yolk sac will become the Vitelline duct.
2) The leading edge of the ectoderm and the parietal layer of the lateral plate mesoderm will move antero-medially.
◙ At this stage we have
1) a cavity (the coelom) which will become the thoracic, abdominal and pelvic cavity
− The cavities are continuous via the pericardio-peritoneal canals until the diaphragm forms
2) A body wall (the thoracic wall) made out of a surface ectoderm and parietal layer of lateral plate mesoderm deep to it.
3) The gut tube, suspended by visceral layer of lateral plate mesoderm
◙ The parietal layer of lateral plate mesoderm became the parietal pleura

3) WHILE LATERAL FOLDING TAKES PLACE
◙ Cranio-caudal (Cephalo-caudal) folding also takes place (head and tail folds meet with 2 lateral folds at the umbilicus)

After this (Ennis skipped some of the other steps), the real stuff begins (development of the laryngeal inlet, the trachea, the bronchial tree and the lung)

Question 2

The development of which structures is significant in the development of the respiratory system ?

Answer 2

Lung buds
Pharyngeal arches
Laryngeal orifice
Lungs
Diaphragm

Question 3

Describe the embryological formation of lung buds.

Answer 3

♠ When the embryo is about 4 weeks old, In the cervical portion of the gut tube, in the ventral midline, the respiratory diverticulum starts to appear
♠ The respiratory diverticulum expands ventrally and towards the chest, in front of the gut tube.
♠ Meanwhile, tracheo-esophageal ridges (folds) grow towards each other and finally separate the respiratory diverticulum from the gut tube
♠ The diverticulum bifurcates into two branches. Later these two branches expand or dilate. Terminal ends of these dilatations are called lung (bronchial) buds that will become the lungs

Question 4

Describe the embryological formation of pharyngeal arches.

Answer 4

♠ The gut tube will develop into digestive system, from the mouth to the anus
♠ In the 4th-5th weeks of development pharyngeal arches start to appear. These arches are separated from each other by pharyngeal clefts
– The 5 pharyngeal arches (numbered as 1-2-3-4-6) have a cartilaginous element, an artery (an aortic arch), and a nerve (cranial nerve)
♠ Respiratory diverticulum is anterior to them

Question 5

Describe the embryological formation of the laryngeal orifice.

Answer 5

♦ Even though the tracheo-esophageal septum separates the gut tube from the respiratory tract, laryngeal opening is maintained
♦ The laryngeal orifice surrounded by laryngeal and epiglottic swellings
♦ Upper end of the respiratory diverticulum will become the larynx, middle section will become the trachea and lower end will become tertiary bronchi, bronchioles and the alveoli

Question 6

Describe the embryological formation of the lungs.

Answer 6

♥ The respiratory diverticulum develops in front of the gut tube, derived from the endoderm, surrounded by the visceral layer of lateral plate mesoderm.
♥ When the bronchial tree grows and expands, it pushes the visceral layer of lateral plate mesoderm
– visceral layer of lateral plate mesoderm will develop into visceral pleura
♥ Lungs develop in 4 overlapping stages
1. Pseudoglandular
2. Canalicular
3. Terminal Saccular
4. Alveolar
♥ Since sections involved in gas exchange has not developed by that time, first 2 stages are not compatible with life
– From 20 weeks type 2 pneumocytes begin to secrete surfactant
– From 28 weeks and onwards foetus can survive

Question 7

Which layers of trilaminar embryo form the respiratory system ?

Answer 7

The epithelium of the respiratory system is derived from endoderm, everything else (cartilage, trachealis muscle, etc) is derived from mesoderm

Question 8

Describe the embryological formation of the diaphragm.

Answer 8

► The diaphragm has 4 sources of origin

• Pleura-peritoneal folds (1)
• Pleura-peritoneal folds fuse with septum transversum (2)
– the septum transversum forms the central tendon of the diaphragm
– contains myoblasts from the somites in C3, 4, 5
– “During the descent of the septum, the phrenic nerve is carried along and assumes its descending pathway”
• Crura will develop from mesentery of the oesophagus (3)
• Muscular in-growth from the body wall (4)

Question 9

What is the result of failure of normal development of either source of origin of the diaphragm or failure of fusion results in what ?

Answer 9

Diaphragmatic Hernia

Question 10

What is the cause of diaphragmatic hernia ?

Answer 10

Failure of normal development from any source of origin of the diaphragm, or failure of fusion.

Question 11

Identify congenital abnormalities that may affect the respiratory system and diaphragm.

Answer 11

1) Tracheo-esophageal fistula
2) Esophageal atresia
3) Diaphragmatic hernias
4) Respiratory Distress Syndrome (RDS)
5) Abnormal Lung Development
6) Pulmonary Hypoplasia

Question 12

Explain what tracheo-esophageal fistulas are, and how they arise.

Answer 12

♣ Tracheo- esophageal folds do not fuse, resulting in communication between the trachea and esophagus (i.e. tracheo-esophageal fistula)
♣ Usually accompanied by esophageal atresia
♣ Usually (90% of the time), trachea connected to the distal part of esophagus (rather than the entire esophagus or the proximal part of the esophagus). In that case, the proximal part of the esophagus is blind ended.
♣ Consequences:
-After birth, when the baby attempts to feed, milk enters the trachea, causing choking and possible development of pneumonitis and pneumonia.
-Also the possibility of Retrograde passage of gastric acid from the GI tract into the respiratory system.

Question 13

Explain what esophageal atresias are.

Answer 13

Failure of the full esophageal lumen to develop. Often associated with tracheo-esophageal fistulas.
Consequences: During a normal pregnancy, the foetus swallows amniotic fluid which is resorbed from the gut and returned to the maternal circulation. If oesophageal atresia develops, this circulation of fluid is prevented and polyhydramnios develops (excess amniotic fluid).

Question 14

Explain what diaphragmatic hernias are.

Answer 14

Failure of normal development from any source of origin of the diaphragm, or failure of fusion. Intra-abdominal organs can pass into thorax and compress the lungs.

Usually Bochdalek hernia, posterolateral defect in fusion of pleura-peritoneal folds (may also be anterior (aka Morgagni hernias, sternal origin of diaphragm does not fuse properly with the remainder) or central).

Question 15

Define and describe Respiratory Distress Syndrome (RDS). Also identify treatment for it.

Answer 15

AKA hyaline membrane disease.
Insufficient surfactant secretion results in the collapse of the alveolar wall during expiration

Treatment: artificial surfactant + glucocorticoids to stimulate surfactant secretion.

Question 16

Define pulmonary hypoplasia.

Answer 16

Incomplete development of the lungs, resulting in an abnormally low number or size of bronchopulmonary segments or alveoli.