Stress ball morphogenesis: how the lizard builds its lung

Lung function is closely coupled to its structural anatomy, which varies greatly across vertebrates. Although architecturally simple, a complex pattern of airflow is thought to be achieved in the lizard lung due to its cavernous central lumen and honeycomb-shaped wall. We find that the wall of the lizard lung is generated from an initially smooth epithelial sheet, which is pushed through holes in a hexagonal smooth muscle meshwork by forces from fluid pressure, similar to a stress ball. By combining next-generation sequencing with timelapse imaging, we reveal that the hexagonal smooth muscle geometry self-assembles in response to circumferential and axial stresses downstream of pressure. A quantitative computational model predicts the pressure-driven changes in epithelial topology, which we replicate using a 3D-printed engineered tissue model of optogenetically-driven smooth muscle contraction. These results reveal the physical principles used to sculpt the unusual architecture of the lizard lung, which could be exploited as a novel strategy to engineer tissues. Overall design: Lungs were dissected from anole embryos at embryonic day 5 and 6 and dissociated in dispase. Cells were processed in the 10x Genomics Chromium system. The tissues we isolated contain all expected cell types of embryonic lungs, including mesenchymal cells at each stage of differentiation into smooth muscle.