Elastic Fibers Define Embryonic Tissue Stiffness to Enable Buckling Morphogenesis of the Small Intestine
During embryonic development, tissues must possess precise material properties to ensure that cell-generated forces give rise to the stereotyped morphologies of developing organs. However, the question of how material properties are established and regulated during development remains understudied. Here, we aim to address these broader questions through the study of intestinal looping, a process by which the initially straight intestinal tube buckles into loops, permitting ordered packing within the body cavity. Looping results from elongation of the tube against the constraint of an attached tissue, the dorsal mesentery, which is elastically stretched by the elongating tube to nearly triple its length. This elastic energy storage allows the mesentery to provide stable compressive forces that ultimately buckle the tube into loops. Beginning with a transcriptomic analysis of the mesentery, we identified widespread upregulation of extracellular matrix related genes during looping, including genes related to elastic fiber deposition. Combining molecular and mechanical analyses, we conclude that elastin confers tensile stiffness to the mesentery, enabling its mechanical role in organizing the developing small intestine. These results shed light on the role of elastin as a driver of morphogenesis that extends beyond its more established role in resisting cyclic deformation in adult tissues. Overall design: Looping morphogenesis of the small intestine is driven by geometric and mechanical properties of the mesentery.In the absence of a strong contextual understanding of mesentery biology, we opted to perform a transcriptomic analysis of the tissue throughout the stages of looping, on the grounds that genes related to regulation of mesentery growth and stiffness are likely differentially regulated before, during, and at the end of looping. We performed bulk RNA-Seq analysis on mesentery tissue collected from chick embryos at embryonic day 8 (E8)  when looping begins, E12 when looping is underway, and E16 when looping is complete.