Numerical density of alveoli in the human lung--measurement using serial thin slices as three-dimensional probes

The numerical density of alveoli was estimated in a surgical lung specimen taken from a female, aged 51, for adenocarcinoma of the left upper lobe. The specimen was fixed by intrabronchial infusion with a mixture of polyethylene glycol and formalin. Using a special slicer, 36 serial slices were prepared at a thickness of 0.2 mm; in these, a rectangular 3-D sampling probe, 2.0 mm x 2.0 mm x 0.2 mm, was set, and the number of alveoli contained in it was counted under a stereo microscope. After the serial slice were subjected to computer-assisted 3-D reconstruction of the acini and supplying airways, two acini, with a volume of 140 and 104 mm3, respectively, were selected, in each of which five sampling probes were set, two in the centriacinar and three in the peripheral region. Counting, performed according to the disector rule, gave a mean alveolar numerical density of 121/mm3 (SD = 5.6). No significant difference in density proved to exist not only between the two acini but between the central and peripheral portions of each acinus. This justifies the premise, on which the conventional stereological methods have been based, that the numerical density of alveoli of the lung is uniformly distributed over the whole organ.