Proliferative activity in the murine pulmonary vasculature in response to hypobaric hypoxia. Frozen lung sections double immunolabeled for Ki67 and α-smooth muscle actin were used for the detection of proliferating cell within the walls of intrapulmonary vessels. Nuclei of individual cells were visualized by staining with DAPI. Exemplary immune histochemistries are demonstrated in (A). The results of a quantitative analysis of the number of proliferating cells/vessel depending on time of exposure to hypobaric hypoxia is given in (B). In the boxplots the middle horizontal line indicates the median, the top and bottom of each box identifies the upper and lower quartiles of the distribution and the top and bottom horizontal line gives the total distribution (n = number of animals. ** p ≤ 0.01).