Morphometric estimation of regional differences in the rat lung.

To provide a safe basis for the sampling of tissue in future morphometric investigations of the rat lung, we searched for quantitative regional differences in pulmonary structure at light microscopic (LM) and electron microscopic (EM) levels. The lungs of 11 male rats about 6 weeks of age were fixed by standard intratracheal instillation of glutaraldehyde in the supine position and embedded either in paraffin for LM or in epoxy resin for EM investigation. Sampling of tissue was designed to test for differences between lobes and between central and peripheral lung parenchyma. LM morphometry was performed by manual point counting and by using a version of an improved automated image analyzer, Quantimet 720. EM morphometric results were obtained by manual point and intersection counting only. LM point counting showed that the proportion of parenchyma was highly constant in all lobes, varying only between 79.9% and 81.5%. In the left lung, which was partitioned into two equal halves, the amount of parenchyma was significantly lower in the apical region (mean values, 72.6% compared to 83.1%; p less than 0.002), which regularly contained the hilum. Quantimet analysis of central and subpleural lung portions revealed intralobar differences. The volume density of interalveolar septa and the air space surface density were significantly decreased in subpleural compared to central lung regions (by 7% and 4.6%, respectively). EM morphometry demonstrated that the interalveolar septa were evenly structured in all lobes except for the harmonic mean thickness of the air-blood barrier, which was lower in upper lobes. In addition, the volume density of interstitial cells was found to be significantly increased in central compared to peripheral parenchyma. The results indicate that for quantitative LM analysis the smallest possible sampling unit is an entire lobe. For EM morphometry, the often practiced approach to consider information drawn from one lobe representative for the whole lung seems to be appropriate for most parameters. In view of the structural differences between central and peripheral lung parenchyma, however, attention has to be paid to applying a properly weighted sampling procedure. Depending on the size of the lobe, the peripheral mantle (2 mm thick) can represent up to 75% of the lobar volume.