title: A three-dimensional study of alveologenesis in mouse lung.
abstract: Alveologenesis is the final step of lung maturation, which subdivides the alveolar region of the lung into smaller units called alveoli. Each of the nascent dividers serves as a new gas-exchange surface, and collectively they drastically increase the surface area for breathing. Disruption of alveologenesis results in simplification of alveoli, as is seen in premature infants diagnosed with bronchopulmonary dysplasia (BPD), a prevalent lung disease that is often associated with lifelong breathing deficiencies. To date, a majority of studies of alveologenesis rely on two-dimensional (2D) analysis of tissue sections. Given that an overarching theme of alveologenesis is thinning and extension of the epithelium and mesenchyme to facilitate gas exchange, often only a small portion of a cell or a cellular structure is represented in a single 2D plane. Here, we use a three-dimensional (3D) approach to examine the structural architecture and cellular composition of myofibroblasts, alveolar type 2 cells, elastin and lipid droplets in normal as well as BPD-like mouse lung. We found that 2D finger-like septal crests, commonly used to depict growing alveolar septae, are often artifacts of sectioning through fully established alveolar walls. Instead, a more accurate representation of growing septae are 3D ridges that are lined by platelet-derived growth factor receptor alpha (PDGFRA) and alpha smooth muscle actin (α-SMA)-expressing myofibroblasts, as well as the elastin fibers that they produce. Accordingly in 3D, both α-SMA and elastin were each found in connected networks underlying the 3D septal ridges rather than as isolated dots at the tip of 2D septal crests. Analysis through representative stages of alveologenesis revealed unappreciated dynamic changes in these patterns. PDGFRA-expressing cells are only α-SMA-positive during the first phase of alveologenesis, but not in the second phase, suggesting that the two phases of septae formation may be driven by distinct mechanisms. Thin elastin fibers are already present in the alveolar region prior to alveologenesis, suggesting that during alveologenesis, there is not only new elastin deposition, but also extensive remodeling to transform thin and uniformly distributed fibers into thick cables that rim the nascent septae. Analysis of several genetic as well as hyperoxia-induced models of BPD revealed that the myofibroblast organization is perturbed in all, regardless of whether the origin of defect is epithelial, mesenchymal, endothelial or environmental. Finally, analysis of relative position of PDGFRA-positive cells and alveolar type 2 cells reveal that during alveologenesis, these two cell types are not always adjacent to one another. This result suggests that the niche and progenitor relationship afforded by their close juxtaposition in the adult lung may be a later acquired property. These insights revealed by 3D reconstruction of the septae set the foundation for future investigations of the mechanisms driving normal alveologenesis, as well as causes of alveolar simplification in BPD.
authors.affiliation: Laboratory of Genetics, University of Wisconsin-Madison Madison, WI 52706, United States.
authors.forename: Kelsey
authors.initials: K
authors.lastname: Branchfield
authors.affiliation: Laboratory of Genetics, University of Wisconsin-Madison Madison, WI 52706, United States.
authors.forename: Rongbo
authors.initials: R
authors.lastname: Li
authors.affiliation: Department of Surgery, University of Wisconsin-Madison Madison, WI 53706, United States.
authors.forename: Vlasta
authors.initials: V
authors.lastname: Lungova
authors.affiliation: Laboratory of Genetics, University of Wisconsin-Madison Madison, WI 52706, United States.
authors.forename: Jamie M
authors.initials: JM
authors.lastname: Verheyden
authors.affiliation: Department of Pediatrics University of Wisconsin-Madison Madison, WI 53706, United States.
authors.forename: David
authors.initials: D
authors.lastname: McCulley
authors.affiliation: Laboratory of Genetics, University of Wisconsin-Madison Madison, WI 52706, United States. Electronic address: xsun@wisc.edu.
authors.forename: Xin
authors.initials: X
authors.lastname: Sun
mesh_terms: D000199:Actins / Q000378:metabolism; D000818:Animals; D016477:Artifacts; D004549:Elastin / Q000378:metabolism; D049452:Green Fluorescent Proteins / Q000378:metabolism; D021621:Imaging, Three-Dimensional; D008055:Lipids / Q000737:chemistry; D051379:Mice; D023421:Models, Animal; D008954:Models, Biological; D058628:Myofibroblasts / Q000166:cytology / Q000378:metabolism; D020286:Pericytes / Q000378:metabolism; D011650:Pulmonary Alveoli / Q000166:cytology / Q000254:growth & development*; D020796:Receptor, Platelet-Derived Growth Factor alpha / Q000378:metabolism; D013312:Stress, Physiological
chemical_list: D000199:Actins; D008055:Lipids; C544013:alpha-smooth muscle actin, mouse; D049452:Green Fluorescent Proteins; D004549:Elastin; D020796:Receptor, Platelet-Derived Growth Factor alpha
keywords: Alveologenesis; Development; Lung; Mouse