A MORPHOMETRIC QUANTITATION OF DEVELOPMENTAL-CHANGES IN ELASTIC FIBERS IN RAT LUNG PARENCHYMA - VARIABILITY WITH LUNG REGION AND POSTNATAL AGE

Elastin has long been thought to play a pivotal role in alveolar septal development by providing the structural framework around which new alveoli develop inthe immature lung. This theory was derived from observations that the dramatic increase in lung elastin and alveolar septal development occurred during the same time period, days 4 to 13 in the rat. Using stereologic techniques, we evaluated volume density of respiratory tissue in parenchyma and total length of parenchymal elastic fibers in the lungs of neonatal rats ranging in age from 4 to 26 days. Lung tissue was obtained from each of the four right lung lobes and from the upper, middle, and lower regions of the left lung. When changes in these two parameters in the lung as a whole were considered by averaging values for each of the seven regions, a continuous increase was observed in both the volume of parenchymal tissue in parenchyma and the total length of elastic fibers from days 4 to 22. A significant correlation was found between the volume of respiratory tissue in parenchyma and the length of parenchymal elastic fibers from days 4 to 26, supporting the existing theory that a causal relationship exists between these two parameters. A comparison of the ratios of elastic fiber length to volume density of respiratory tissue in parenchyma (L(v)/V(rp)) from each of the seven lung regions sampled indicated a significant variability in L(v)/V(rp) within different regions of the same lung. In addition, the effects of postnatal age and lung region on L(v)/V(rp) were interdependent; the lung regions that differed with respect to the length density of elastic fibers varied with postnatal age. Although elastic fiber length and volume of parenchymal tissue in parenchyma in the whole lung increase in a parallel fashion during alveolar development, the observed variability among lung regions suggests that the relationship between alveolar septal development and elastic fibers is more complex than has been previously appreciated.