The respiratory system starts to develop with an inconspicuous laryngo-tracheal groove (stage 10, ca. 28 days, 10 ) that forms in the ventral midline at the lower end of the pharynx. The caudal part of this groove develops into the lung anlage in that it divides into right and left lung buds (stage 12, ca. 30 days, 12). The upper part of this groove remains undivided and becomes the trachea.
The two lung buds extend and form the main bronchus (stage 14, ca. 33 days, 14). These subdivide further and form 2 (on the left side) and 3 (on the right) lobular buds and thus the lobular bronchi. The whole lung development takes place in five main phases that partially overlap (4); followed by a maturation stage:
- Embryonic phase (3rd - 8th week)
- Pseudoglandular phase (5th - 17th week)
- Canalicular phase (16th - 26th week)
- Saccular phase (24th week up to birth)
- Alveolar phase (36th week up to the 18th month post partum)
- Stage of the microvascular maturation (from birth to 2 - 3 years)