Study of rat lung alveoli using corrosion casting and freeze fracture methods coupled with digital image analysis

Relative areas and volumes can be estimated from vascular corrosion casts of rat lung alveoli using a calibration obtained from bulk frozen hydrated tissue. These morphometric measurements are roughly independent of the shrinkage and distortion artifacts known to arise in the corrosion casting procedure. Digital image processing of the SEM micrographs is employed to facilitate the measurement of casts and frozen tissue. The vascular corrosion casting technique is modified also to permit successful casting of alveolar air passages. The modified technique produces faithful casts of dead-ended luminal structures where continuous perfusion of casting medium into the tissue is not possible. The casts of alveolar air passages and their corresponding vasculature are compared to determine the volume of a single alveolus. By utilizing the calibrated measurements of lung areas and volumes, an estimate of lung vascular surface area per unit volume is obtained. This number, when multiplied by the respiratory tidal volume of the rat, may represent the total lung vascular surface are available for physiological gas exchange during normal respiration.