Fluid secretion and luminal pressure control lateral branching morphogenesis in the embryonic avian lung.

Abstract

During lung development, the embryonic airway originates as a wishbone-shaped epithelial tube, which undergoes a series of branching events to build the bronchial tree. This process depends crucially on cell proliferation and is thought to involve distinct branching modes: lateral branching, wherein daughter branches emerge along the length of a parent branch, and bifurcations, wherein the tip of a parent branch splits to form two new daughter branches. The developing airway is fluid-filled, and previous studies have shown that altered luminal pressure can influence rates of branching morphogenesis. However, it is not clear if altered tissue mechanics influence patterns of proliferation along the embryonic airway epithelium nor if individual branching modes are affected differently by changes in luminal pressure. Here, we focused on mechanisms of lateral branching and used as a model system the embryonic avian lung, which forms exclusively via this branching mode during early development. We used microinjected fluid droplets or pharmacological modulators of fluid secretion to alter luminal fluid pressure either locally or globally within cultured embryonic lungs. Somewhat surprisingly, we found both local and global increases in luminal pressure to suppress the formation of new lateral branches while also promoting increased epithelial proliferation. In a consistent manner, decreased luminal pressure led to an increase in lateral branching morphogenesis. Morphometric analysis of airway branching patterns revealed that altered luminal pressure shifts the overall branching program, rather than simply changing rates of morphogenesis. Taken together, these results highlight the importance of mechanical forces during airway branching and suggest that different branching modes may be affected differently by luminal fluid pressure.