Table 1.

Milestones: fluidization and unjamming transitions in development.

Year	Ref.	Animal model	Key point
2010	(Bénazéraf et al., 2010)	Chicken	Raised the hypothesis that anteroposterior (AP) body axis elongation is an emergent tissue property that arises due to collective cell motion in the presomitic mesoderm (PSM).
2013	(Lawton et al., 2013)	Zebrafish	Identified transitions in tissue fluidity during tailbud elongation. Suggested that hindered collective cellular motion may cause a ‘traffic jam’ in the posterior tailbud and lead to a contorted trunk.
2014	(David et al., 2014)	Xenopus	In a variety of different regions in the gastrula, cell-cell adhesion and tissue viscosity are positively proportional and, together, limit the rate of multicellular rearrangements and motion.
2018	(Barriga et al., 2018)	Xenopus	Underlying the cephalic neural crest, the head mesoderm goes through a tissue level stiffening transition which promotes EMT-mediated collective migration of crest cells.
	(Atia et al., 2018)	Drosophila	Unjamming transition imposes a specific statistical distribution that governs cell shape in both inert systems, and during collective cellular migration in the formation of the ventral furrow.
	(Mongera et al., 2018)	Zebrafish	Rapid fluctuation of cells in the mesodermal progenitor zone (MPZ), coupled with a cell-cell adhesion gradient, promotes an unjamming transition which decreases a tissue-level yield stress and underlies body axis elongation.
2019	(Petridou et al., 2019)	Zebrafish	A drop in the tissue-level viscosity in the blastoderm suggests that a fluidization process permits the spreading of the epithelial enveloping layer (i.e. doming) and the initiation of cellular intercalations.
	(Iyer et al., 2019)	Drosophila	Increased E-cadherin turnover, in response to mechanical stress in the wing epithelium, induces cell rearrangements and promotes a viscous behavior.
	(Spurlin et al., 2019)	Chicken	Adjacent to the tip of the branching epithelium in the avian lung is an unjammed mesenchymal tissue, through which the branch tip can expand and eventually bifurcate.
2020	(Saadaoui et al., 2020)	Quail	Due to cell division events, the embryonic epithelium transition s to a fluid-like state thus enabling large-scale tensile forces to induce multicellular flows governing the formation of the primitive streak.
	(Wang et al., 2020)	Drosophila	Cell shape, together with the degree of alignment between cells, predict the onset of cell rearrangements in the converging and extending germband epithelium.
	(Jain et al., 2020)	Tribolium	During epiboly, tensile forces from actomyosin cable contribute to the fluidization of the leading edge in the spreading epithelium.
2021	(Petridou et al., 2021)	Zebrafish	Random division events in the blastoderm lead to a small but critical change in cell-cell connectivity, which in turn causes tissue-level rigidity to transition abruptly.
	(Kim et al., 2021)	Zebrafish	Aided by a generalized vertex model that accounts for both confluent and sub-confluent regimes, it is shown that cell-cell contact length fluctuations actively drive tissue fluidization.