02-11-22 – Development of the Respiratory System Flashcards
Learning outcomes
- Describe again the parts of the respiratory system
- Describe the embryonic development of the trachea and bronchi
- Describe the embryonic development of the lungs and pleura
- Describe the embryonic development of the diaphragm
- Be able to discuss the congenital abnormalities that may affect the respiratory system and diaphragm
Bilaminar disk
Bilaminar disk
Trilaminar embryo. What does the neural crest form?
What does the paraxial mesoderm form?
What does intermediate mesoderm form?
What does parietal (somatic) lateral plate mesoderm form?
What does the intraembryonic coelom form?
What does the visceral (splanchnic) mesoderm form?
- Neural crest forms the endocardial cushions
- Paraxial mesoderm forms somites
- Intermediate mesoderm forms genitourinal (GU) systems
- Parietal (somatic) lateral plate mesoderm forms the body walls
- The intraembryonic coelom forms body cavities (thoracic, abdominal, pelvic) in between the visceral and parietal lateral plate mesoderm
- The visceral (splanchnic) lateral plate mesoderm forms serous membranes e.g pleural membranes, serous pericardium, peritoneum
What do the 2 layers of the lateral plate mesoderm cover/fuse with during lateral folding?
What movement occurs during lateral folding?
What ends up being pinched?
What lines the yolk sac?
What does this form?
What 3 things will the primitive yolk sack form during development?
During lateral folding, the parietal layer of lateral plate mesoderm fuses with the ectoderm to form the surface ectoderm
* The visceral layer of lateral plate mesoderm coves the endoderm
* During lateral folding, the lateral parts of the tri-laminar disk start to move anteriorly and medially towards each other, with them both eventually meeting and fusing In the midline
* This results in the primitive yolk sack being pinched (the yolk sac is lined by endoderm)
* This process forms a gut tube from the endoderm/yolk sac, and a connection between the gut tube and the yolk sac called the vitelline duct
- During development, the primitive yolk sack will form:
1) The gut tube
2) GU systems
3) Respiratory systems
When is lateral folding complete?
What has happened to the surface ectoderm?
What will the CNS develop from?
What will the surface endoderm end up forming?
What does the parietal layer of lateral plate mesoderm form?
What does the intraembryonic coelom form?
Where level is the diagram?
- Lateral folding is complete when the lateral foldings have met anteriorly in the midline and fused
- The surface ectoderm has detached itself from the neural groove to become the neural tube
- The CNS will develop from the neural tube
- The surface ectoderm will eventually form the serous membranes e.g pericardium, pleura, peritoneum
- The parietal layer of lateral plate mesoderm will fuse with the ectoderm to form the thoracic wall/body walls (and parietal pleura)
- The intraembryonic coelom forms body cavities (thoracic, abdominal, pelvic) in between the visceral and parietal lateral plate mesoderm
- The diagram has been takin from a transverse section above the vitelline duct, so it can’t be seen
When does cranio-caudal folding also take place?
How does it occur?
What would happen if we didn’t have this folding?
- Cranio-caudal folding also take place during lateral folding
- Cranio-caudal folding takes place on the sagittal plane, with the cranial and caudal regions folding towards each other
- If we didn’t have this folding, 2 sacs would develop into 2 tubes
What does the visceral layer of lateral plate mesoderm surround?
What is the dorsal mesentery?
What does the dorsal mesentery do?
What is inside the dorsal mesentery?
- The visceral layer of lateral plate mesoderm covers all sides of the gut tube, so it eventually becomes intra-peritoneal
- The dorsal mesentery is part of the visceral lateral plate mesoderm that extends between the posterior abdominal wall and the gut tube
- The dorsal mesentery serves as the root of gut tube, and suspends the gut tube through attachment to the posterior abdominal wall
- In the dorsal mesentery, we have the arteries that supply various parts of the gut tube
What is the diverticulum?
When does is appear?
What does the diverticulum develop into?
When do the tracheo-oesophageal ridges/folds grow?
How do they move as they grow?
What does the respiratory diverticulum divide into?
How is the gut tube divided up? What does the foregut form?
The diverticulum is a growth that appears anterior to the gut tube in the midline
* It appears at about week 4
* The diverticulum develops into part of the respiratory system
* The tracheo-oesophageal ridges/folds grow as the diverticulum grows
* As these ridges/folds grow, they start to move towards each other, with the aim being to separate the gut tube from the respiratory diverticulum (with the exception of the laryngeal orifice – comes later)
* The respiratory diverticulum will divide into 2 lung buds, which will grow, expand, and dilate to become part of the lungs
* The gut tube is divided into the foregut, midgut, and hindgut
* The foregut developed into the oesophagus, stomach, and part of the duodenum
When do the pharyngeal arches form?
What do they consist of?
How many pharyngeal arches do we have?
Why is this?
What 3 things does each pharyngeal arch have?
- At about weeks 4-5, dilations occur called the pharyngeal arches
- Each pharyngeal arch consists of a pouch separated by a cleft
- We have 5 pharyngeal arches – 1, 2, 3, 4, 6, labelled from top to bottom
- There is no 5 in humans, as originally, pharyngeal arches were found in fish that have arches 1 – 6
- 3 things each pharyngeal arch have:
1) Cartilaginous element – may be cartilage or bone
2) An artery (aortic arch artery)
3) A nerve (cranial nerve)
What is the only thing the trachea-oesophageal ridge/fold not close?
What 2 things do we need to have a connection between?
How big is the laryngeal orifice initially?
Where is the laryngeal orifice found?
What 2 things do the cartilaginous elements of the 4th pharyngeal arch become?
What 4 things do the cartilaginous elements of the 6th pharyngeal arch become?
What does the laryngeal orifice become?
What boundaries does it have?
- The only thing the trachea-oesophageal ridge/fold does not close is the laryngeal orifice
- For air to pass to from the pharynx to the trachea, we need to have a connection called the laryngeal orifice, which will eventually become the laryngeal inlet
- The laryngeal orifice is originally just a slit
- The laryngeal orifice is found on part of the 4th pharyngeal arch called the epiglottal swelling and the anterior part 6th pharyngeal arch called the laryngeal swelling
- The cartilaginous elements of the 4th pharyngeal arch will form the:
1) Thyroid cartilage
2) Epiglottis - The cartilaginous elements of the 6th pharyngeal arch will form the
1) Cricoid
2) Arytenoid
3) Corniculate
4) Cuneiform cartilages - The laryngeal orifice becomes the laryngeal inlet, with the epiglottis superiorly, and the arytenoids swellings and aryepiglottic folds laterally and inferiorly
Laryngeal inlet diagram
Laryngeal inlet diagram
As the lungs develop, how many cavities are there?
What does the endoderm form in relation to the respiratory system?
What 3 things does the mesoderm form in relation to the respiratory system?
What do the developing left and right primary bronchi form?
Where does it move as these are forming?
- At the point the lungs are developing, the body the embryo only has 1 cavity that extends from the neck till the pelvis
- The endoderm forms the respiratory epithelium that forms the inner lining of the trachea
- In relation to the respiratory system, the mesoderm forms:
1) Cartilage
2) Trachealis muscle
3) Vessels - The developing left and right primary bronchi will form the tracheobronchial tree
- As this forms, the right and left primary bronchi will start pushing itself into the visceral layer of the lateral plate mesoderm, which will become the visceral pleura
What are the 4 overlapping stages of lung development?
Why can embryos not survive in the first 2 stages of development?
When does surfactant start to be produced?
What is it produced by?
From what time onwards can the foetus survive?
Why can some babies that are delivered between 20-28 weeks survive while others cannot?
- 4 overlapping stages of lung development (stages can occur at the same time):
1) Pseudoglandular
2) Canalicular
3) Terminal Saccular
4) Alveolar - Embryos can’t survive during the first 2 stages of development since surfactant is required for breathing and has not been produced during these stages
- From 20 weeks, type 2 pneumocytes (type II alveolar cells) begin to secrete surfactant
- From 28 weeks and onwards the foetus can survive, as there is now enough surfactant to support life
- Between 20 – 28 weeks, some babies delivered during this period can survive depending on the amount of surfactant produced, which comes down to anatomical variation and will differ from case to case
What does the diaphragm separate?
What nerve roots supply motor fibres to the diaphragm?
What 2 structures supply sensory fibres to the diaphragm?
Why is the diaphragm innervated by some cervical nerves, yet is in the thorax/abdomen?
What are the 4 sources of origin of the diaphragm?
- Diaphragm separates the thoracic from abdominal cavities
- The phrenic nerve (C3-5) is the sole motor innervation of the diaphragm
- The central tendon of the diaphragm receives sensory innervation from the phrenic nerve, and the muscular periphery of the diaphragm receives sensory innervation from intercostal nerves
- The diaphragm was partially developed in the cervical region before migrating to the thorax/abdomen, taking its nerve innervation with it, which is why it is innervated by some cervical nerves
- 4 sources of origin of the diaphragm:
1) Pleuroperitoneal fold
2) Septum transversum
3) Mesentery of the oesophagus
4) Myoblasts from the body wall
What part of the diaphragm does the pleuroperitoneal fold form?
What does it fuse with?
What does it contain? What are myoblasts and somites?
- The pleuroperitoneal folds form the posterior and lateral parts of the diaphragm
- Pleura-peritoneal folds fuse with septum transversum (2)
- Pleura-peritoneal folds contains myoblasts from the somites in C3,4,5
- Phrenic nerve is also from C3-5
- Myoblasts are precursors of myocytes – muscle cells
- Somites are precursor populations of cells that form many structures such as bones, skin, muscles, tendons, and ligaments
Where is the diaphragm located in the cervical region?
What 2 parts of the diaphragm does the septum transversum form?
- The diaphragm is located in the septum transversum in the cervical region
- The septum transversum forms the anterior part of the diaphragm and the central tendon of the diaphragm
What part of the diaphragm does the mesentery of the oesophagus form?
What part of the diaphragm do myoblasts from the body wall form?
What is this part of the diaphragm innervated by?
- The mesentery of the oesophagus forms crura of the diaphragm
- Myoblasts from the body wall form the muscular periphery of diaphragm
- The muscular periphery is innervated by intercostal nerves
What is a fistula?
What is the most common tracheooesophageal defect?
What are tracheooesophageal fistulas caused by?
What will happen to the trachea with the most common defect?
What will happen if the baby tries to feed?
What are other common fistulas?
- A fistula is an abnormal connection between two body parts, such as an organ or blood vessel and another structure
- The most common tracheooesophageal defect is a tracheooesophageal fistula, where there is no connection to the proximal gut tube (oesophagus), and an abnormal connection between the distal gut tube and the trachea (A on diagram)
- This type of fistula accounts for 90% of tracheooesophageal defects
- Tracheooesophageal fistulas are formed by the failure of the fusion of the tracheooesophageal ridge while separating the trachea from the gut tube
- With the most common defect, contents can move back up into the trachea, causing chemical pneumonitis, as the acid from the stomach will damage the respiratory epithelium lining the trachea
- If the baby tries to feed, it will reflux/vomit the food back up
What are 4 other tracheooesophageal fistulas?
- 4 other tracheooesophageal fistulas
1) B on diagram
* The gut tube and trachea are separated, but there is no connection between the proximal and distal oesophagus
2) C on diagram
* Connection between the oesophagus and trachea
3) D on diagram
* Connection between proximal oesophagus and trachea, no connection to distal oesophagus
4) E on diagram
* Connection between proximal oesophagus and trachea, and distal oesophagus and trachea
How might we be able to tell that there is a tracheoesophageal fistula/oesophageal atresia (oesophagus doesn’t form properly) before the baby is born?
- We might be able to tell that there is a tracheoesophageal fistula/oesophageal atresia (oesophagus doesn’t properly) before the baby is born due to the size of the mother’s stomach
- There will be more amniotic fluid present (Polyhydramnios), which will make the mothers abdomen larger than usual
What is a hernia?
When can diaphragmatic hernias occur?
What can happen to abdominal organs?
What are the 3 different types of diaphragmatic hernia?
Which is the most common?
- A hernia occurs when an internal part of the body pushes through a weakness in the muscle or surrounding tissue wall
- Diaphragmatic hernias can occur if any of the 4 things that make up the diaphragm
- These defects can usually allow abdominal organs to pass into the thoracic cavity
- 3 different types of diaphragmatic hernia:
1) Bochdalek hernia
* Posterolateral Defect in fusion of pleura-peritoneal folds
* Most common diaphragmatic tumour, accounting for 95% of cases
2) Morgani hernia
* Anterior diaphragm (septum transversum forms anterior part of diaphragm and central tendon)
* Account for 2% of cases
3) Central hernia
* Septum transversum forms anterior part of diaphragm and central tendon
What does hypoplasia mean?
- Hypoplasia is the incomplete development or underdevelopment of an organ or tissue.
- Hence, organs or tissue affected by hypoplasia have a below average number of cells.
