Question

Development of Bronchi and Lungs

Answer 1

It has already been mentioned that the laryngotracheal tube divides into two bronchial buds at its distal end. Each bud is surrounded by a con­densation of the splanchnic mesen-chyme. Early in the 5th week each .bronchial bud enlarges to form the pri-mordium of the right and left principal or (main) bronchi. The right principal bronchus divides further into three sec­ondary bronchi, whereas the left prin­cipal bronchus divides into two secon­dary bronchi, indicating that the right lung will consist of three lobes and the left lung will have only two lobes.
The secondary or lobar bronchi divide repeatedly in a dichotomous manner. As this occurs, the mesen-chyme surrounding the main bronchi
also divides. By the end of the 7th week ten tertiary or segmental bron­chi have been formed in the develop­ing right lung, while eight segmental bronchi have appeared in the left lung. The dichotomous division of the bron­chial tree continues and about 17 gen­erations of subdivisions have been formed by the 24th week and respira­tory bronchioles have developed. An additional 6 orders of airways form after birth.
The developing lungs grow lat­erally into the pericardioperitoneal canals, one of which lies on each side of the foregut. As described in chapter 4, formation of the pleuropericardial and pleuroperitoneal membranes sepa­rates the pericardioperitoneal canals from the pericardial and peritoneal cavities, respectively. Henceforth, the pericardioperitoneal canals are called primitive pleura! cavities,
The rapidly enlarging lungs soon fill up the space available within the primitive pleural cavities. The splanch­nic mesoderm covering the outer sur­face of each lung develops into a se­rous membrane called visceral pleura, whereas the somatic mesoderm of the thoracic body wall gives rise to the pa­rietal pleura. The narrow space be­tween the visceral and parietal pleura is called pleural cavity. With further expansion in lung volume, the lungs gradually grow caudally and burrow into the mesoderm of the body wall. This burrowing splits off more and more pericardium from the thoracic wall
and allows the lungs to flank the heart on each side.
The endoderm of the bronchial buds gives rise to the lining epithelium of the bronchi, bronchioles and alveoli. The splanchnic mesenchyme surround­ing the bronchial tree differentiates*into the cartilaginous plates, smooth muscle and connective tissue of the air tubes. This mesenchyme also forms the sup­porting connective tissue of the alveo­lar sacs. The blood vessels of the lung also develop from this mesenchyme.