Lung microvascular adaptation in infants with chronic lung disease

Microvascular development is critical for normal lung maturation. The aims of this study were (1) to quantitatively and qualitatively assess lung microvascular growth in the human fetus, from 22 to 40 weeks' gestation, and (2) to compare development in these infants to those with mild, moderate and severe chronic lung disease (CLD). Using 1- and 4-microm thick sections and electron microscopy, lungs were morphometrically assessed for surface density of distal air spaces; volume density of parenchymal vessels having an air-blood barrier (ABB); percent of distal air space wall having an ABB, and capillary loading, defined as ABB/mm2 of epithelial surface area. The percent of vessels with ABB increased in controls during development in parallel with increasing lung parenchyma. Infants with severe CLD had fewer ABBs and less capillary loading than controls up to 34 weeks' post-conceptional age (PCA), but by 36-40 weeks, showed catch-up growth. Microvasculature vessel diameter, septal thickness, and air sac diameter at 36-40 weeks' PCA were increased with severe CLD, and vessels were more distant from the air surface. We conclude that infants with severe CLD have both stunted secondary septation and microvascular development, but over time, the primary septal wall adapts by thinning and increasing the number of ABBs, thereby taking on the function of secondary septa.