Fig. 3.

Microvascular maturation/maturation of the alveolar septa. Immature septa contain a double layered capillary network (red) and a central sheet of interstitial tissue (green: a, c). To increase the efficiency of the lungs the septa mature. This includes a reduction from a double to a single layered capillary network. The central septum of connective tissue (green) is reduced to a septum interwoven with the capillary network (b, d). a, b Transmission electron micrographs of human lungs postnatal day 26 (a) and adult (b). c, d Scanning electron micrographs of vascular casts (Mercox^Ⓡ) of rat lungs at postnatal days 4 (c) and 44 (d). Bars, a, b 10 µm; c, d 25 µm.

From Schittny (2017) and Woods and Schittny (2016) by courtesy of Springer Nature Switzerland, Basel