Visualization and stereological characterization of individual rat lung acini by high-resolution X-ray tomographic microscopy

The small trees of gas-exchanging pulmonary airways, which are fed by the most distal purely conducting airways, are called acini and represent the functional gas-exchanging units. The three-dimensional architecture of the acini has a strong influence on ventilation and particle deposition. Due to the difficulty in identifying individual acini on microscopic lung sections, the knowledge about the number of acini and their biological parameters, like volume, surface area, and number of alveoli per acinus, are limited. We developed a method to extract individual acini from lungs imaged by high-resolution synchrotron radiation-based X-ray tomographic microscopy and estimated their volume, surface area, and number of alveoli. Rat acini were isolated by semiautomatically closing the airways at the transition from conducting to gas-exchanging airways. We estimated a mean internal acinar volume of 1.148 mm(3), a mean acinar surface area of 73.9 mm(2), and a mean of 8,470 alveoli/acinus. Assuming that the acini are similarly sized throughout different regions of the lung, we calculated that a rat lung contains 5,470 +/- 833 acini. We conclude that our novel approach is well suited for the fast and reliable characterization of a large number of individual acini in healthy, diseased, or transgenic lungs of different species, including humans.