Human lung growth in late gestation and in the neonate

We studied the lungs of 42 infants dying of obstetrical accidents, acute infections, and trauma, who ranged in age from 19 wk gestation to 3 wk postpartum at 42 wk gestation. The earliest lungs (19 to 20 wk gestation) had smooth-walled respiratory channels lined by cuboidal epithelium. Between 22 and 24 wk gestation with further development of the acinus, these channels took on a wavy internal configuration. After 28 wk gestation the air spaces (saccules) of the developing acinus became subdivided by secondary crests. Alveolar development began in some infants as early as 32 wk and was uniformly present by 36 wk. Respiratory bronchioles were not present in any of our cases, including those in the immediate postnatal period. Quantitative studies showed a rapid increase in lung volume and alveolar surface area beginning at about 28 wk gestation (crown-rump length, 25 cm) coinciding with increasing complexity of the saccules. Surface area, lung volume, and total number of alveoli showed an exponential relationship to gestational age and crown-rump length. Air-space wall thickness showed a rapid decline starting at 28 wk gestation and is best related exponentially to age and crown-rump length. Volume proportions of air spaces increased steadily throughout gestation. The increase in bronchial and bronchiolar air and air-space dimensions correlated relatively poorly with age, and nonparenchyma remained a relatively constant volume proportion, regardless of age.(ABSTRACT TRUNCATED AT 250 WORDS)