Scanning electron micrographs of Mercox casts. (a) Fetal chicken lung microvasculature. (b) Rat lung microvasculature at postnatal day 44. The small holes indicated by arrows have diameters of about 2 µM. The holes correspond to tissue pillars that extend across the capillary lumina. Scale bars: (a) 12 and (b) 20 µM. Used with permission from Wiley-Blackwell: Djonov, Kurz, and Burri (2003) [35].