Luminal signalling links cell communication to tissue architecture during organogenesis

Abstract

Morphogenesis is the process whereby cell collectives are shaped into differentiated tissues and organs. The self-organizing nature of morphogenesis has been recently demonstrated by studies showing that stem cells in three-dimensional culture can generate complex organoids, such as mini-guts, optic-cups and even mini-brains. To achieve this, cell collectives must regulate the activity of secreted signalling molecules that control cell differentiation, presumably through the self-assembly of microenvironments or niches. However, mechanisms that allow changes in tissue architecture to feedback directly on the activity of extracellular signals have not been described. Here we investigate how the process of tissue assembly controls signalling activity during organogenesis in vivo, using the migrating zebrafish lateral line primordium. We show that fibroblast growth factor (FGF) activity within the tissue controls the frequency at which it deposits rosette-like mechanosensory organs. Live imaging reveals that FGF becomes specifically concentrated in microluminal structures that assemble at the centre of these organs and spatially constrain its signalling activity. Genetic inhibition of microlumen assembly and laser micropuncture experiments demonstrate that microlumina increase signalling responses in participating cells, thus allowing FGF to coordinate the migratory behaviour of cell groups at the tissue rear. As the formation of a central lumen is a self-organizing property of many cell types, such as epithelia and embryonic stem cells, luminal signalling provides a potentially general mechanism to locally restrict, coordinate and enhance cell communication within tissues.

Epithelial cell behaviours during neurosensory organ formation

“…Inhibition of Fgf signalling or lumen disassembly results in delayed deposition of the organs, whereas Fgf3 overexpression accelerates organ deposition (Durdu et al, 2014). Altogether, these findings show that Fgf signalling accounts for the directed motility of follower cells, acts as an attractive signal for proneuromasts, and regulates the deposition and thus patterning of centripetal organs (Chitnis et al, 2012;Dalle Nogare et al, 2014;Durdu et al, 2014;Lecaudey et al, 2008). Importantly, this dissection of the molecular networks that signal during pLL primordium migration has revealed the subtle changes in cell behaviours that are involved during this process.…”

“…During pLL primordium migration, Fgf signalling activity is required for the migration of follower cells adjacent to the leading domain, and for the formation and deposition of proneuromast rosettes (Chitnis et al, 2012;Dalle Nogare et al, 2014;Donà et al, 2013;Durdu et al, 2014;Lecaudey et al, 2008). Studies have shown that Fgf8-expressing/derived cells also contribute to molar placode formation; these cells likely form rosettes temporarily and show only minor displacement when Fgf8 activity is compromised (Prochazka et al, 2015).…”

Seeing is believing: multi-scale spatio-temporal imaging towards in vivo cell biology

“…by combining intravital imaging of entire organisms with EM, which we here refer to as intravital CLEM Karreman et al, 2014Karreman et al, , 2016Maco et al, 2013). Intravital CLEM enabled important discoveries in several areas of life science, such as cell biology (Avinoam et al, 2015), neuroscience (Allegra Mascaro et al, 2013;Maco et al, 2013), cancer research (Karreman et al, 2016), virology (Hellström et al, 2015 and developmental biology (Durdu et al, 2014;Goetz et al, 2014), and is highly likely to contribute to future breakthroughs in biological research.…”

“…In resin-and flat-embedded small organisms, it facilitates the retrieval of a precise ROI located within the intravital image and, for instance, afforded the characterization of how fibroblast growth factor (FGF) activity controls the frequency at which rosette-like mechanosensory organs, e.g. the zebrafish lateral line primordium, are deposited through the assembly of microluminal structures that constrain FGF signaling (Durdu et al, 2014). Studies performed in C. elegans provide additional examples of laser-assisted targeted ultramicrotomy; these have helped to further clarify at high resolution the formation of excretory canals, as well as the contribution of exosomes to alae formation (Hyenne et al, 2015) (Fig.…”

“…The second, and biggest, challenge resides in the difficulty to retrieve the ROI in voluminous resin-embedded 3D samples (up to a few mm 3 ). In small organisms and embryos, anatomical landmarks, such as fluorescent or visible structures that have been imaged in vivo can be utilized to provide key anchor points for pinpointing the event of interest in the resin-embedded tissue (Durdu et al, 2014;Goetz et al, 2014Goetz et al, , 2015Kolotuev et al, 2010Kolotuev et al, , 2013Müller-Reichert et al, 2007). This has proven to be very useful in zebrafish embryos for correlating the blood flow that is sensed by endothelial cilia to their inner ultrastructure (Fig.…”