ABSTRACT.: To quantitatively follow the progressive capillarization of the fetal airway epithelium, we examined human lung tissue from nine fetuses ranging in gestational age from 18-26 wk. Our goals were to 1) determine the initial time of appearance of the air blood barrier (ABB) in the fetus; 2) follow the increase in the number of ABB per total epithelial airway surface (capillary load) with gestational age; and 3) measure the thickness of the ABB. Our results, obtained by using light and electron microscopy and an interactive computerized morphometry system, show that ABB first appear at 19 wk. Increasing gestation is accompanied by an exponential increase in the number of ABB (r = 0.96) and the total surface area that the ABB contribute to the total surface area of airway epithelium (r = 0.93). ABB thickness is comparable to the dimensions of minimal barrier thickness of the adult ABB. The structural development that we describe may be one of the factors determining preterm viability.
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