Mechanical coordination is sufficient to promote tissue replacement during metamorphosis in Drosophila

Abstract

During development, cells coordinate to organize in coherent structures. Although it is now well established that physical forces are essential for implementing this coordination, the instructive roles of mechanical inputs are not clear. Here, we show that the replacement of the larval epithelia by the adult one in Drosophila demands the coordinated exchange of mechanical signals between two cell types, the histoblasts (adult precursors) organized in nests and the surrounding larval epidermal cells (LECs). An increasing stress gradient develops from the center of the nests toward the LECs as a result of the forces generated by histoblasts as they proliferate and by the LECs as they delaminate (push/pull coordination). This asymmetric radial coordination of expansive and contractile activities contributes to epithelial replacement. Our analyses support a model in which cell-cell mechanical communication is sufficient for the rearrangements that implement epithelial morphogenesis.

ECM remodeling and spatial cell cycle coordination determine tissue growth kinetics

“…Furthermore, cell areas in the anterior nest are also spatially variable, with cells around the periphery of the nest having a larger apical area than their counterparts in the nest center ( Figure 1B, middle row). This may be due to their location at the interface with LECs, which have been shown to exert forces on the boundary histoblast when undergoing apoptosis (Prat-Rojo et al, 2020;Teng et al, 2017). As cell division is the dominant factor in histoblast nest growth, we focused subsequent analysis on their proliferation dynamics.…”

“…Following a period of three cleavage divisions, the histoblasts enter an expansion phase (around 14-16 hAPF) when they proliferate and grow, displacing the surrounding Larval Epidermal Cells (LECs, large polyploid cells that formed the larval epidermis), which are extruded and engulfed by macrophages (hemocytes) patrolling the hemolymph underneath the epidermal layer (Bischoff and Cseresnyes, 2009;Michel and Dahmann, 2020;Nakajima et al, 2011;Ninov et al, 2007;Prat-Rojo et al, 2020;Teng et al, 2017) ( Figure 1B). LEC death requires both ecdysone signaling and displacement by the expanding histoblast nests (Michel and Dahmann, 2020;Nakajima et al, 2011;Ninov et al, 2007;Prat-Rojo et al, 2020;Teng et al, 2017). Once the histoblasts cover the entire abdominal surface, proliferation ceases and differentiation proceeds to give rise to the adult abdomen.…”

“…A limitation of studies in the Drosophila wing disc is its inaccessibility for these live imaging tools to follow individual cell behavior and mechanical properties over time in the growing tissue. Thus, we chose to analyze the growth of the Drosophila abdominal epidermis, where individual cells can be imaged and tracked in vivo during the pupal stages when the animal is sessile (Bischoff and Cseresnyes, 2009; Mangione and Martin-Blanco, 2018; Prat-Rojo et al, 2020).…”

“…During the larval period, the histoblasts are quiescent (G2 arrest) (Bryant and Schneiderman, 1969; Mandaravally Madhavan and Schneiderman, 1977), and are induced to enter the cell cycle by the pulse of the steroid hormone ecdysone that triggers the larval/pupal transition (0 h After Puparium Formation – APF) (Ninov et al, 2007). Following a period of three cleavage divisions, the histoblasts enter an expansion phase (around 14-16 hAPF) when they proliferate and grow, displacing the surrounding Larval Epidermal Cells (LECs, large polyploid cells that formed the larval epidermis), which are extruded and engulfed by macrophages (hemocytes) patrolling the hemolymph underneath the epidermal layer (Bischoff and Cseresnyes, 2009; Michel and Dahmann, 2020; Nakajima et al, 2011; Ninov et al, 2007; Prat-Rojo et al, 2020; Teng et al, 2017) (Figure 1B). LEC death requires both ecdysone signaling and displacement by the expanding histoblast nests (Michel and Dahmann, 2020; Nakajima et al, 2011; Ninov et al, 2007; Prat-Rojo et al, 2020; Teng et al, 2017).…”

Multi-view confocal microscopy enables multiple organ and whole organism live-imaging

“…As exemplified by numerous past and recent studies, the imaging Drosophila pupa without immersion has been key in the exploration of developmental and repair processes and to characterize core and conserved genetic and biophysical mechanisms driving cell differentiation as well as tissue proliferation, shaping, architecture and repair (Aigouy et al, 2010;Arata et al, 2017;Corson et al, 2017;Curran et al, 2017;Diaz-de-la-Loza et al, 2018;Etournay et al, 2015;Founounou et al, 2013;Franz et al, 2018;Gho et al, 1999;Guirao et al, 2015;Lemke and Schnorrer, 2018;Levayer et al, 2015;Mauri et al, 2014;Michel and Dahmann, 2020;Prat-Rojo et al, 2020;Ray et al, 2015;Sarov et al, 2016). In particular, using spinning disc confocal microscopy, many studies have focused on the dynamics of a single tissue including the pupal wing and dorsal thorax (notum) tissues as well as histoblast nests.…”