Apical constriction initiates new bud formation during monopodial branching of the embryonic chicken lung

Abstract

SUMMARYBranching morphogenesis sculpts the airway epithelium of the lung into a tree-like structure to conduct air and promote gas exchange after birth. In the avian lung, a series of buds emerges from the dorsal surface of the primary bronchus via monopodial branching to form the conducting airways; anatomically, these buds are similar to those formed by domain branching in the mammalian lung. Here, we show that monopodial branching is initiated by apical constriction of the airway epithelium, and not by differential cell proliferation, using computational modeling and quantitative imaging of embryonic chicken lung explants. Both filamentous actin and phosphorylated myosin light chain were enriched at the apical surface of the airway epithelium during monopodial branching. Consistently, inhibiting actomyosin contractility prevented apical constriction and blocked branch initiation. Although cell proliferation was enhanced along the dorsal and ventral aspects of the primary bronchus, especially before branch formation, inhibiting proliferation had no effect on the initiation of branches. To test whether the physical forces from apical constriction alone are sufficient to drive the formation of new buds, we constructed a nonlinear, three-dimensional finite element model of the airway epithelium and used it to simulate apical constriction and proliferation in the primary bronchus. Our results suggest that, consistent with the experimental results, apical constriction is sufficient to drive the early stages of monopodial branching whereas cell proliferation is dispensable. We propose that initial folding of the airway epithelium is driven primarily by apical constriction during monopodial branching of the avian lung.

Modulation of apical constriction by Wnt signaling is required for lung epithelial shape transition

“…The apical actomyosin cytoskeleton in the lung epithelium is reorganized during the transition from the pseudoglandular to the canalicular-saccular stages Apical constriction is involved in bud formation during the pseudoglandular stage in vertebrate lung development (Kadzik et al, 2014;Kim et al, 2013). Contractile activity at the apical cell surface was visualized with phospho-threonine-18 and serine-19 myosin II light chain ( ppMLC).…”

“…With growing attention to the concept of mechanical deformation to explain a variety of morphogenetic events (Ciarletta et al, 2014;Drasdo, 2000;Savin et al, 2011;Takigawa-Imamura et al, 2015;Varner et al, 2015), branching formation based on physical buckling (Varner et al, 2015) and local apical constriction (Kim et al, 2013) have been proposed as mechanisms of bud formation. In our model, apical constriction was assumed to occur globally within the cyst, and uniformly sized buds emerge spontaneously without pre-patterning of protruding regions.…”

Mammary epithelial tubes elongate through MAPK-dependent coordination of cell migration

“…Since mammalian organs increase in size and structural complexity during development, researchers have sought to understand the relative contributions of proliferation, cell shape change and migration to lung, salivary gland and kidney tubulogenesis (Andrew and Ewald, 2010;Alescio and Di Michele, 1968;Goldin and Wessells, 1979;Lubarsky and Krasnow, 2003). Recent advances in organotypic culture and imaging have shed light on how these processes drive mammalian tube growth and development (Shamir and Ewald, 2014;Kim et al, 2013;Schnatwinkel and Niswander, 2013;Packard et al, 2013;Larsen et al, 2006;Hsu et al, 2013;Tang et al, 2011).…”

An interplay of geometry and signaling enables robust lung branching morphogenesis

“…The earliest models focused on physical forces (reviewed by Lubkin, 2008), and combinations of experimental and computational studies have since confirmed that mechanical stress and internal pressure influence branching morphogenesis Nelson, 2010, 2012;Kim et al, 2013;Nelson and Gleghorn, 2012;Unbekandt et al, 2008;Varner and Nelson, 2014). WNT signaling affects the epithelial shape of new lung buds, but WNT signaling is not essential for lung branching morphogenesis and is thus not part of the core regulatory network (Kadzik et al, 2014).…”