Supplemental Video

One video file in Quicktime format.

Files in this Data Supplement:

• Video E1 - Local duplication of the capillary network at the basis of a newly forming septum. The newly forming septum shown in figure 5 a+b is also shown in video A1 (LungCap-day21.mov). Vascular casts of 21 days old rat lungs were obtained using Mercox®. Inside of one alveolus (Fig. 5a) an up-folding of the single layered capillary network was observed. This is indicative for the formation of a new septum. By turning the sample we were able to look at the backside of this location. We observed a groove in the capillary network (white line) representing the up-folding seen in the first alveolus. The groove is already partly covered by a capillary duplication and a second duplication is just forming (arrow in figure 5b).