THE DEVELOPMENT OF THE RESPIRATORY APPARATUS
The development of the nose was considered in the previous chapter in relation to the development of the mouth and face (p. 141). The larynx, trachea, and lungs find a common origin in a ventral outgrowth from the foregut, directly caudal to the second of the two swellings that form the tongue (Figure 4-29).
The primordium extends caudally as a (tracheobronchial) groove in the pharyngoesophageal floor; the groove is later converted into a tube by infolding and fusion of its lips. Fusion commences caudally and extends forward until the esophagus and pharynx are divided from the respiratory tract, except for a small cranial opening that persists as the entrance to the larynx. The fact that the initial development has the form of a groove rather than a tube is important because it explains the wide variety of communications between the esophagus and trachea that may occur as congenital anomalies when the process of division has been locally unsuccessful.The further differentiation of the larynx includes the appearance of the separate cartilages and muscles by condensation and differentiation of the mesoderm of the neighboring pharyngeal arches. The epiglottis has a somewhat different origin, developing as a caudal division of the second of the two median swellings that give rise to the tongue.
After separation from the esophagus, the caudal end of the respiratory tract grows down the neck and comes to lie in the median mesoderm that intervenes between the two forward-pointing extensions of the celom that become the pleural cavities. The apex of the tract splits into two lung buds (Figure 4-29, B), whose further splitting first reproduces the pattern of the bronchial tree and then creates the smaller respiratory passages that succeed the bronchi. In babies about 18 divisions succeed the stem bronchi by the time of birth; however, the process is not yet complete, and further divisions are added during infancy. The branches of the lung buds become invested by the splanchnic mesoderm into which they thrust, and it is this mesoderm that forms the tissues of the respiratory organs other than the lining epithelium (which is, of course, supplied by the foregut endoderm). The histological development of the lungs encompasses three phases named after the dominant microscopic characters: the first (glandular) phase establishes the bronchial pattern, the second (canalicular) phase establishes the respiratory portion of the lung, and the third and final (alveolar) phase is concerned with the development of the alveoli.
The production of surfactant, a substance secreted by certain alveolar cells that reduces the surface tension to allow alveolar expansion when breathing commences, is of rather late occurrence. The respiratory distress syndrome of the newborn is associated with immaturity of this feature of development.