Distribution and number of Clara cells in the normal and disturbed development of the human fetal lung.

In this study the numerical growth and topological distribution of Clara cells were investigated in normal and hypoplastic lungs of fetuses ranging in age from the 10th to the 24th gestational week. In addition, the lungs of premature infants suffering from hyaline membrane syndrome (HMS) and bronchopulmonary dysplasia (BPD) were used as a model of disturbed lung growth in the early postnatal phase. Clara cells were observed to appear in the airway epithelium of fetuses of the 15th gestational week. After the 15th week of gestation the Clara cell number increased monotonously with increasing gestational age, reaching 5.4% Clara cells in the bronchial epithelium and 11.2% in the bronchiolar epithelium at the 24th gestational week. In the investigated period of gestation the Clara cell number was significantly higher in the bronchiolar epithelium compared to the bronchial epithelium. Hypoplastic lungs showed no difference in number and distribution of Clara cells compared to normal age-matched controls. This finding suggests the growth of Clara cells to be relatively accelerated compared to the decreased maturation of the lung parenchyma. The HMS/BPD cases showed normal Clara cell counts in the bronchial epithelium, whereas in the bronchiolar epithelium this value was decreased. This finding is caused by the extreme turnover of the airway epithelium in HMS/BPD; the local distribution of the epithelial damage is speculated to be caused by the physicochemical properties of inhaled oxygen.