Architecture, elastic fiber, and collagen in the distal air portion of the lung of the 18-month-old rat.

Background: The increase in the distal air spaces which takes place with age is the only sufficiently documented datum for differentiation between the senile and the adult lung. There are other pulmonary components which may be modified as the lung enters the phase of old age, but they have not as yet been sufficiently studied. The purpose of this study is to make a morphometric comparison between the wall thickness of the distal air spaces, elastic fiber, and collagen in lungs of rats of between 5 and 18 months of age.

Methods: The left lung of each rat was histologically processed for light microscopy. The sections were contrasted using methylene blue, resorcinfuchsin and Sirius red. Systematic randomized sampling was used for the selection of the histological fields studied. Morphometric variables were studied, and were systematized into three groups, namely: variables related with the alveolar architecture, variables which quantify elastic fiber, and those which quantify collagen.

Results: The old animals exhibited significant differences (p < 0.05) in the following variables: 1. In relation with the alveolar architecture, mean linear intercept, alveolar chord and wall thickness increased, whereas internal alveolar perimeter and tissue density decreased. 2. The relation elastic fiber density/lung tissue density increased. None of the variables which quantify collagen displayed significant differences.

Conclusion: Our data lead us to consider that the lungs of the old animals displayed, not only an enlargement of the distal air spaces, but also a thickening of the alveolar wall and an increase in elastic fiber in relation to the rest of the lung tissue.