Surfactant is particularly important for survival of the premature infant. When surfactant is insufficient, the air-water (blood) surface membrane tension becomes high, bringing great risk that alveoli will collapse during expiration. As a result, respiratory distress syndrome (RDS) develops. This is a common cause of death in the premature infant. In these cases the partially collapsed alveoli contain a fluid with a high protein content, many hyaline membranes, and lamellar bodies, probably derived from the surfactant layer. RDS, which is therefore also known as hyaline membrane disease, accounts for approximately 20% of deaths among newborns. Recent development of artificial surfactant and treatment of premature babies with glucocorticoids to stimulate surfactant production have reduced the mortality associated with RDS and allowed survival of some babies as young as 5.5 months of gestation.
Although many abnormalities of the lung and bronchial tree have been described (e.g., blind-ending trachea with absence of lungs and agenesis of one lung), most of these gross abnormalities are rare. Abnormal divisions of the bronchial tree are more common; some result in supernumerary lobules. These variations of the bronchial tree have little functional significance, but they may cause unexpected difficulties during bronchoscopies.
More interesting are ectopic lung lobes arising from the trachea or esophagus. It is believed that these lobes are formed from additional respiratory buds of the foregut that develop independently of the main respiratory system.
Most important clinically are congenital cysts of the lung, which are formed by dilation of terminal or larger bronchi. These cysts may be small and multiple, giving the lung a honeycomb appearance on radiograph, or they may be restricted to one or more larger ones. Cystic structures of the lung usually drain poorly and frequently cause chronic infections.
Summary rThe respiratory system is an outgrowth of the ventral wall of the foregut, and the epithelium of the larynx, trachea, bronchi, and alveoli originates in the endoderm. The cartilaginous, muscular, and connective tissue components arise in the mesoderm. In the fourth week of development, the tracheoesophageal septum separates the trachea from the foregut, dividing the foregut into the lung bud anteriorly and the esophagus posteriorly. Contact between the two is maintained through the larynx, which is formed by tissue of the fourth and sixth pharyngeal arches. The lung bud develops into two main bronchi: the right forms three secondary bronchi and three lobes; the left forms two secondary bronchi and two lobes. Faulty partitioning of the foregut by the tracheoesophageal septum causes esophageal atresias and tracheoesophageal fistulas (Fig. 12.3).
After a pseudoglandular (5-16 weeks) and canalicular (16-26 weeks) phase, cells of the cuboidal lined bronchioles change into thin, flat cells, type I alveolar epithelial cells, intimately associated with blood and lymph capillaries. In the seventh month, gas exchange between the blood and air in the primitive alveoli is possible. Before birth the lungs are filled with fluid with little protein, some mucus, and surfactant, which is produced by type II alveolar epithelial cells and which forms a phospholipid coat on the alveolar membranes. At the beginning of respiration the lung fluid is resorbed except for the surfactant coat, which prevents the collapse of the alveoli during expiration by reducing the surface tension at the air-blood capillary interface. Absent or insufficient surfactant in the premature baby causes respiratory distress syndrome (RDS) because of collapse of the primitive alveoli (hyaline membrane disease).
Growth of the lungs after birth is primarily due to an increase in the number of respiratory bronchioles and alveoli and not to an increase in the size of the alveoli. New alveoli are formed during the first 10 years of postnatal life.
1. A prenatal ultrasound revealed polyhydramnios, and at birth the baby had excessive fluids in its mouth. What type of birth defect might be present, and what is its embryological origin? Would you examine the child carefully for other birth defects? Why?
2. A baby born at 6 months gestation is having trouble breathing. Why?
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