Respiratory System Development
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Direct answer
The respiratory system begins in week four as a laryngotracheal diverticulum budding from the ventral wall of the foregut, separated from the oesophagus by the tracheo-oesophageal folds that fuse into a septum. The bud divides into bronchial buds and undergoes successive branching through the pseudoglandular, canalicular, saccular and alveolar stages; the epithelium is foregut endoderm, while cartilage, muscle and connective tissue come from the surrounding splanchnic mesoderm. Surfactant production maturing near term decides the fate of a preterm infant.
What you must remember
- Separation from foregut: tracheo-oesophageal folds fuse as the tracheo-oesophageal septum; imperfect fusion gives oesophageal atresia with or without tracheo-oesophageal fistula — the commonest type has a blind upper oesophagus with the distal oesophagus joined to the trachea at its lower end, presenting with choking, coughing and polyhydramnios.
- Stages of lung development: pseudoglandular (weeks five to sixteen, all segmental bronchi formed), canalicular (weeks sixteen to 26, bronchioles and vascular channels; the limit of viability is about 22 to 24 weeks), saccular or terminal sac (weeks 26 to birth, thin air-blood barriers appear) and alveolar (from late foetal life to about eight years of age, alveoli multiply).
- Surfactant: type II pneumocytes begin making it from weeks 20 to 24 and it matures by about weeks 34 to 35; deficiency in premature infants causes neonatal respiratory distress syndrome (hyaline membrane disease); maternal corticosteroids accelerate maturation.
- Larynx: from the fourth and sixth pharyngeal arch cartilages with the sixth arch muscles; the recurrent laryngeal nerves ascend with the developing trachea, explaining their different courses on the two sides.
- Pleural cavities: the lungs expand into the pericardioperitoneal canals, splitting body-wall mesoderm into parietal pleura, the visceral pleura being splanchnic mesoderm.
- Congenital anomalies: agenesis or hypoplasia (with diaphragmatic hernia and oligohydramnios), congenital lobar emphysema, bronchogenic cysts and sequestration; diaphragmatic hernia on the left side compresses the lung and causes pulmonary hypoplasia.
Common confusion
Students mix the canalicular and saccular stages: the canalicular stage builds the respiratory bronchioles and sets the viability limit, the saccular stage thins the barrier and starts true gas exchange. The fistula type is another trap — in the common variety the fistula joins the distal oesophagus to the trachea, so the abdomen fills with air and feeds cause choking, not simple obstruction. Finally, alveoli continue to form after birth to about eight years; at birth the lung is structurally complete but far from alveolar completion.
Exam-focused takeaway
Theory questions ask for the origin of the respiratory diverticulum and its cartilage, the stages of lung development with weeks, the formation of the tracheo-oesophageal septum and the types of fistula, and the significance of surfactant with the basis of antenatal steroid therapy. In the viva, correlate the stages with viability and hyaline membrane disease. Practicals include a tracheo-oesophageal fistula specimen.
Frequently asked questions
When does the respiratory diverticulum appear?
In the fourth week, on the ventral aspect of the foregut just caudal to the pharyngeal pouches; it is soon separated from the oesophagus by the tracheo-oesophageal septum.
Which is the commonest tracheo-oesophageal fistula?
Oesophageal atresia with the distal oesophagus communicating with the posterior trachea, giving polyhydramnios before birth and choking with the first feed after birth; it associates with the VACTERL group.
Why does a preterm infant develop hyaline membrane disease?
Because type II pneumocytes have not made enough surfactant before about 34 to 35 weeks, so alveoli collapse with each breath, causing neonatal respiratory distress with ground-glass radiographic opacities.
What is the embryological origin of the lung?
The epithelial lining of all airways and alveoli is foregut endoderm; cartilage, smooth muscle, connective tissue and visceral pleura are splanchnic mesoderm derivatives.