Changes occurring with increasing age in the rat lung: Morphometrical study

Background: It was hypothesized that the evolution towards the senile lung is progressive, being initiated in the adult stage; and for this reason changes similar to those described in the senile lung can be detected in the lungs of middle-aged rats. To test the hypothesis, the following design was used. The lungs of two groups of rats, adult (mean age of 16 weeks) and middle-aged (mean age of 56 weeks) were morphometrically compared. Methods: Thirty-one Wistar rats were used for the study; their lungs were processed histologically. The microscopic fields were analysed in a computer, and 20 variables were quantified. These were grouped into (a) variables which describe the shape and size of the distal airspace, (b) variables which describe the distal lung tissue, and (c) variables which describe elastic fibers. The results were statistically compared: correlation tests were carried out, and the specificity, sensitivity, and misclassification indices were calculated. Results: All the results of the variables which define the size of the air-spaces were found to be significantly higher (P<0.0001) in the middle-aged animals; the results obtained when the lung tissue was quantified directly from the histological section suggested a loss of tissue in the middle-aged animals. However, when these data were converted into absolute values, no loss was indicated in the total lung tissue. The values of the variables which describe the elastic fiber were found to have increased significantly (P<0.0001) in the middle-aged animals. The misclassification index was found to be lower than 10% in six variables and between 10% and 20% in four. Conclusion: The low misclassification indices found lead us to consider that our morphometric method is ideal for distringuishing the lungs of the two groups of animals used. The results of the quantification of the variables show that the middle-aged animals exhibit simple enlargement of the distal airspaces, without tissue loss, which coincides with the current definition of the senile lung. © 1994 Wiley-Liss, Inc.