Fig. 1: Measurement of alveolarization by Chord Length, Mean Linear Intercept and Radial Alveolar Counts.

Chord Length – a test line is placed (dashed orange line with arrows) and the distance between alveolar structures that cross the line is measured, intercepts can span one alveolus (A), or cross the alveolar duct (D). Vessels are not counted. Mean Linear Intercept – 100 µm grid placed over lung fields (representative black lines), lung structures in the vertical and horizontal that intercept grid are counted (examples marked with asterisks), and the average linear distance between structures is calculated for the entire grid. Radial Alveolar Counts – a perpendicular line is drawn from a terminal bronchiole ending as a respiratory bronchiole to the lung interlobar septa (blue line) and the number of bisected saccules is counted (black numbers). Alveolar Counts – Alveoli structures are counted in sequential sections (not shown) within a given high-powered field with alveoli included (black X) if touching the inclusion line (green line) or excluded if touching the exclusion line (red line). For alveoli opening into alveolar ducts, a counting event is the presence of a bridge connecting the free edges of alveolar septae in the sampling section but not the sequential look-up section.