Fig. 5.

Corrosion casts of new alveolar septum formation. a In the midline of the alveolar cavity, an elevated vessel (dotted red line) starts to upfold, whereby another double-layered vessel crosses under it. b The septal vessel continues to elevate tightened dumbbell-shaped by two vessel at the end (white arrows). c A septal ridge (red dotted line) can be seen in the middle of the alveolar cavity whose local influxes (white arrows) are increasing by pillar formation (yellow stars) and vascular duplications. d Upfolding of a new alveolar septum (red dotted line) starts towards the alveolar entrance ring (AER). A second double-layered ring (orange dotted line) is developing by intussusceptive angiogenesis (yellow stars). e In the developed new alveoli with AERs (red dotted lines), an elevation and formation of new septa (red lines) is present in the midline. f Vascular remodeling modifies and expands the alveolar basket by the occurrence of pillars, typical hallmarks of intussusceptive angiogenesis (circle)