Respiratory Tract Development Flashcards
Describe formation of the lung bud (respiratory diverticulum)
- Develops from ventral aspect of foregut endoderm
- Initially in contact with primitive foregut
- Growth of tracheo-oesaphageal ridges towards one another forming a septum
- Separates into trachea ventrally and osephagus dorsally
What are tracheo-oesophageal fistulas/
- Fault partitioning of trachea and oesophagus - structures become connected
- Can sometimes form with atresia of the oesophagus
- Upper oesophagus blind ending, lower oesophagus forms fistula
What would be the clinical presentation of tracheo-oesophageal fistulas?
- Cough
- Risk for aspiration
- Feeding difficulties
- Choking
- Cyanosis
Describe the pseudoglandular phase.
- Occurs from week 5 up to around week 16
- Characterised by rapid branching (aka branching morphogenesis)
What are the basic steps of the pseudoglandular phase?
- Week 4 - lung bud forms trachea and 2 bronchial buds
- Week 5 - bronchial buds branch to form left and right primary bronchi
- Branch into secondary bronchi (2 lefft and 3 right)
- Week 16 - forms terminal bronchioles
Describe pleural development during the pseudoglandular phase.
- As lung buds branch, expand into pericardioperitoneal canals on each side of foregut caudally and laterally
- Punch into visceral mesoderm - forms visceral pleura
- Surrounding parietal mesoderm becomes parietal pleura
- Space in between is pleural cavity
Describe control of branching.
- Regulated by interactions between epithelium and mesenchyme (here is surrounding visceral mesoderm)
- SHH pathway - separation of trachea and oesophagus, patterning of cartilage in respiratory tubes
- FGF, Wnt involved in early branching
What process is the pseudoglandular phase characterised by?
- Differentiation of respiratory epithelium e.g basal cell/multiciliated cell/secretory cell development
When does the canalicular phase and what is it characterised by?
- Week 16 up to around week 26
- Formation of respiratory bronchioles and terminal sacs
What are the basic steps of the canalicular phase?
- Further division of terminal bronchioles. Forms respiratory bronchioles
- These divide into 3-6 alveolar ducts
- Ducts end in alveolar sacs i.e primitive alveoli. Epithelium is mostly cuboidal at this stage
When does the saccular phase occur and what is it characterised by?
- Between weeks 26 and 26
- Specialisation
What occurs to the cells surrounding the alveolar sac during the saccular phase?
- Differentiate and specialise
- Increase in number of respiratory bronchioles and alveolar sacs
Describe the Type I and II pneumocytes during the saccular phase.
- TYPE I - cuboidal cells become squamous. Surrounding capillaries protrude towards these cells - cause further gas exchange
- TYPE II - secrete phospholipid-rich surfactant - lowers surface tension of air-alveolar interface. Secretion increases closer to birth
When does the alveolar phase occur and what is it characterised by?
- Begins at 36 weeks and continues after birth (up to 3 years old)
- Characterised by alveolar maturation
What are the basic steps of the alveolar phase?
- Increase in number of primitive alveoli
- Subdivision of primitive alveoli (development of septae within sacs)
- Allows increased surface for gas exchange
