Image Analysis Quantitation of Type 2 Cells and Alveolar Walls Part I: Influence of Time on the Developing Mouse Lung

Abstract

Quantitative measurements of the lungs of Swiss-Webster male mice at 6, 10, and 16 weeks of age were obtained using image analysis. The measurements included numbers and area of type 2 cells, and the area, perimeters, and linear intercepts of alveolar walls. In addition, type 2 cell:alveolar wall ratios were used to compare cell and alveolar wall interrelationships with time. The most outstanding of the findings was a 21.8% increase in mean type 2 cell area with time (6 weeks vs. 16 weeks of age; P < 0.05), with only a relatively slight increase (6.3%) in the number of type 2 cells. Type 2 cell (≥ 12 μm) number also increased at 16 weeks of age (P < 0.06). Accompanying the increases in cell size and number was an increase in alveolar wall area (25.6%, or 21.4% with the type 2 cell field area excluded). Since the wall area increase was less than that for type 2 cell field area or type 2 cell mean area but greater than the type 2 cell number increase, there were disparate trends in the ratios comparing cell area and cell number to alveolar wall area. Significantly, type 2 cell numbers, mean type 2 cell area, and alveolar wall area had not plateaued at 16 weeks of age. Thus, there is the implication that the type 2 cell population and the alveolar walls are still undergoing changes at least up to the age of 10 weeks and possibly beyond 16 weeks of age. This is a longer period of lung development than expected from prior literature reports and raises the question of a correspondingly greater period of increased susceptibility to noxious air pollutants. An influence of an ambient level of NO₂ exposure on the developing mouse lung is the subject of Part II of this investigation.

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