Adult lung MKs are localized to the alveolar interstitium of the distal lung, and fetal lung MKs are found intra- and extravascularly. (A-C) Immunofluorescent staining of 5- to 6-µm fixed-frozen week-10 (WK10) adult mouse lung sections stained for markers of MKs (CD41a and CD42d), lung epithelial cells (NKX2.1), alveolar type 1 epithelial cells (PDPN), and nuclei (Hoechst). Lung borders and an alveolar airspace are outlined with gray dashes. Multinucleated adult lung MKs were preferentially localized to the interstitium of the alveolar septum between neighboring alveoli. (D-E) Immunofluorescent staining of 5- to 6-µm fixed-frozen E13 fetal mouse lung sections stained for VE-Cadherin (endothelial cell marker), CD41a, NKX2.1, and Hoechst. Fetal lung MKs were identified in the developing lung in extravascular (D) and intravascular (E) spaces. Platelets were notably less prevalent in the fetal lung. White and red arrows highlight platelets and MKs, respectively.