Platelet-specific Clec1b-deficient (Clec1b^ΔPLT) mice show mild Clec1b^−/−-like lung abnormalities. (A) Clec-2 staining (magenta) with CD41 (green, platelets, and megakaryocytes) or Ela2 (green, neutrophils) and DAPI (blue) in E17.5 wild-type lungs. Arrowheads indicate megakaryocytes (white) and neutrophils (black). Clec-2 expression entirely overlapped with CD41 expression, but not with Ela2. (B) Ventral views of Clec1b^fl/fl (wild-type), Clec1b^ΔPLT, and Clec1b^−/− fetal lungs at E17.5. (C) Neonatal lethality of Clec1b^fl/fl and Clec1b^ΔPLT neonates. *P = .00127, Fisher’s exact test. (D) Immunostaining of α-SMA (green) and DAPI (blue) in E17.5 distal lung. AL, alveolar sac; M, lung mesothelium. (E) Quantification of α-SMA expression in Clec1b^fl/fl and Clec1b^ΔPLT distal lung at E17.5; mean ± SD, n = 3 each. *P = .0211; **P = .0033, Holm-Sidak test. (F) Whole-mount immunohistochemistry of Wt1 (magenta) and Pdpn (green) in E17.5 luMCs. Arrowheads: Wt1-negative luMCs. (G) Quantification of luMC density; mean ± SD, n = 3 each. *P = .0281, Student t test. (H) Quantification of Wt1-negative rate in luMCs; mean ± SD, n = 3 each. N. S., not significant; P = .062, Student t test. (I) Platelet counts in E17.5 Clec1b^+/+, Clec1b^−/−, Clec1b^fl/fl, and Clec1b^ΔPLT fetal blood; mean ± SD, n = 6, 6, 3, and 4, respectively. N.S., not significant, Tukey’s test. Scale bars: 25 μm (A,D,F); 1 mm (B).