Feather arrays are patterned by interacting signalling and cell density waves.

Feathers are arranged in a precise pattern in avian skin. They first arise during development in a row along the dorsal midline, with rows of new feather buds added sequentially in a spreading wave. We show that the patterning of feathers relies on coupled fibroblast growth factor (FGF) and bone morphogenetic protein (BMP) signalling together with mesenchymal cell movement, acting in a coordinated reaction-diffusion-taxis system. This periodic patterning system is partly mechanochemical, with mechanical-chemical integration occurring through a positive feedback loop centred on FGF20, which induces cell aggregation, mechanically compressing the epidermis to rapidly intensify FGF20 expression. The travelling wave of feather formation is imposed by expanding expression of Ectodysplasin A (EDA), which initiates the expression of FGF20. The EDA wave spreads across a mesenchymal cell density gradient, triggering pattern formation by lowering the threshold of mesenchymal cells required to begin to form a feather bud. These waves, and the precise arrangement of feather primordia, are lost in the flightless emu and ostrich, though via different developmental routes. The ostrich retains the tract arrangement characteristic of birds in general but lays down feather primordia without a wave, akin to the process of hair follicle formation in mammalian embryos. The embryonic emu skin lacks sufficient cells to enact feather formation, causing failure of tract formation, and instead the entire skin gains feather primordia through a later process. This work shows that a reaction-diffusion-taxis system, integrated with mechanical processes, generates the feather array. In flighted birds, the key role of the EDA/Ectodysplasin A receptor (EDAR) pathway in vertebrate skin patterning has been recast to activate this process in a quasi-1-dimensional manner, imposing highly ordered pattern formation.

Loss of cilia causes embryonic lung hypoplasia, liver fibrosis, and cholestasis in the talpid3 ciliopathy mutant.

Sonic hedgehog plays an essential role in maintaining hepatoblasts in a proliferative non-differentiating state during embryogenesis. Transduction of the Hedgehog signaling pathway is dependent on the presence of functional primary cilia and hepatoblasts, therefore, must require primary cilia for normal function. In congenital syndromes in which cilia are absent or non-functional (ciliopathies) hepatorenal fibrocystic disease is common and primarily characterized by ductal plate malformations which underlie the formation of liver cysts, as well as less commonly, by hepatic fibrosis, although a role for abnormal Hedgehog signal transduction has not been implicated in these phenotypes. We have examined liver, lung and rib development in the talpid(3) chicken mutant, a ciliopathy model in which abnormal Hedgehog signaling is well characterized. We find that the talpid(3) phenotype closely models that of human short-rib polydactyly syndromes which are caused by the loss of cilia, and exhibit hypoplastic lungs and liver failure. Through an analysis of liver and lung development in the talpid(3) chicken, we propose that cilia in the liver are essential for the transduction of Hedgehog signaling during hepatic development. The talpid(3) chicken represents a useful resource in furthering our understanding of the pathology of ciliopathies beyond the treatment of thoracic insufficiency as well as generating insights into the role Hedgehog signaling in hepatic development.

The intrinsic innervation of the lung is derived from neural crest cells as shown by optical projection tomography in Wnt1-Cre;YFP reporter mice.

Within the embryonic lung, intrinsic nerve ganglia, which innervate airway smooth muscle, are required for normal lung development and function. We studied the development of neural crest-derived intrinsic neurons within the embryonic mouse lung by crossing Wnt1-Cre mice with R26R-EYFP reporter mice to generate double transgenic mice that express yellow fluorescent protein (YFP) in all neural crest cells (NCCs) and their derivatives. In addition to utilizing conventional immunohistochemistry on frozen lung sections, the complex organization of lung innervation was visualized in three dimensions by combining the genetic labelling of NCCs with optical projection tomography, a novel imaging technique that is particularly useful for the 3D examination of developing organs within embryos. YFP-positive NCCs migrated into the mouse lung from the oesophagus region at embryonic day 10.5. These cells subsequently accumulated around the bronchi and epithelial tubules of the lung and, as shown by 3D lung reconstructions with optical projection tomography imaging, formed an extensive, branching network in association with the developing airways. YFP-positive cells also colonized lung maintained in organotypic culture, and responded in a chemoattractive manner to the proto-oncogene, rearranged during transfection (RET) ligand, glial-cell-line-derived neurotrophic factor (GDNF), suggesting that the RET signalling pathway is involved in neuronal development within the lung. However, when the lungs of Ret(-/-) and Gfrα1(-/-) embryos, deficient in the RET receptor and GDNF family receptor α 1 (GFRα1) co-receptor respectively, were examined, no major differences in the extent of lung innervation were observed. Our findings demonstrate that intrinsic neurons of the mouse lung are derived from NCCs and that, although implicated in the development of these cells, the role of the RET signalling pathway requires further investigation.