Immunostaining of the alveoli in the <i>Pdpn</i>^KO1st mice for podoplanin (PDPN) and thyroid transcription factor-1 (TTF-1).

<p>In the wild type <i>Pdpn</i>^+/+ and <i>Pdpn</i>^+/- mice, there are the PDPN-positive type I alveolar epithelial cells among the TTF-1-positive type II alveolar epithelial cells. The PDPN expression area of the type I alveolar epithelial cells (arrowheads) is fewer in the <i>Pdpn</i>^+/- mice than in the <i>Pdpn</i>^+/+ mice. In the <i>Pdpn</i>^-/- mice, the terminal ends of the respiratory tree consist of the alveolar ducts and respiratory bronchioles with only TTF-1-positive type II alveolar epithelial cells (arrows), lacking alveolar sacs with PDPN-positive type I alveolar epithelial cells. Bar: 10μm.