A three-dimensional study of alveologenesis in XXXX lung.

@entity1 @entity20421 @entity3113 @entity20616 @entity1353 @entity19 @entity871 @entity2107 @entity358
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 @entity1 diagnosed with @entity2107 ( @entity2107 ), a prevalent @entity358 that is often associated with lifelong @entity1353 . 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, @entity20616 and lipid droplets in normal as well as @entity2107 -like @entity19 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 ( @entity20421 ) and @entity3113 ( @entity3113 )-expressing myofibroblasts, as well as the @entity20616 fibers that they produce. Accordingly in 3D, both @entity3113 and @entity20616 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. @entity20421 -expressing cells are only @entity3113 -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 @entity20616 fibers are already present in the alveolar region prior to alveologenesis, suggesting that during alveologenesis, there is not only new @entity20616 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 @entity871 -induced models of @entity2107 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 @entity20421 -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 @entity2107 .
