Transition from prealveolar macrophage intermediate to mature alveolar macrophage requires LPL. (A) During embryogenesis, fetal macrophages (not shown) derived from yolk-sac precursors are present in lung tissue. Around embryonic day 16, fetal monocytes migrate from the fetal liver to seed the lungs, presumably via the bloodstream.^10,11  During the final days of fetal development, these monocytes downregulate Ly6C and upregulate CD11c, leading to their designation as prealveolar macrophages. Following birth, prealveolar macrophages upregulate SiglecF and are then identified as mature alveolar macrophages, which appear in the alveolar space around PND1.¹⁰  (B) Representative flow cytometry demonstrating gating scheme to identify fetal macrophages, fetal monocytes, prealveolar macrophages, and mature alveolar macrophages. Whole lung homogenates from WT or LPL^−/− neonatal mice at indicated ages are shown. Percentages of each gate are given within each flow plot. Flow cytometric gates for each experiment were established using samples obtained from contemporaneous adult WT control animals (supplemental Figure 3A). (C) Quantification of percentage of CD45⁺ cells that were mature alveolar macrophages, prealveolar macrophages, fetal monocytes, or fetal macrophages from whole lung homogenates isolated from WT or LPL^−/− pups. Age indicated as DOB, PND. Data for each age combined from at least 2 independent experiments; n of each sample given on the x-axis.