During gastrulation, an embryo is dramatically restructured by cell and tissue movements [1
] to position the three germ layers (endoderm, ectoderm, and mesoderm), and to assemble the organ primordia. The paramount morphogenetic task during this process is to internalize surface cells. Four major mechanisms of internalization have been described: invagination (the inward folding of a group of cells), involution (ingrowth and curling inward of a group of cells), ingression (the migration of individual cells from the surface to the interior) and epiboly (growth of a group of cells around another group) [1
]. How different organisms bring about this multiplicity of morphogenetic mechanisms that deploy common molecular machineries is poorly understood.
Contractile actomyosin networks are probably the best-studied common molecular assemblies driving morphogenesis [2
]. A prominent morphogenetic mechanism that uses pulsatile actomyosin networks during gastrulation is apical constriction [3
]. Oscillatory apical contractions of an apical actomyosin network exert pulling forces on discrete cell-cell junctions, which leads to changes in the shape of cells in the tissue [3
]. It is thought that these oscillatory contractions cooperatively lead to tissue bending [3
]. Another example for the deployment of contractile actomyosin networks is epithelial resealing [5
]; for example, embryonic wound closure in Drosophila, Xenopus
, zebrafish, and mouse. In this process,, cells at the epithelial margin form dynamic lamellipodial and filopodial protrusions, and assemble a supracellular actomyosin cable that draws the hole closed, in a similar way to a purse-string.
Both oscillatory contractility and purse-string closures in their canonical form invoke mechanical coherence of individual dynamic components to result in supracellular force-generating systems [6
]. Interestingly, it has become clear that supracellular structures are very likely not basic structures, but are in fact emergent features of higher organisms [5
]. Moreover, contractile behaviors that are apparently similar on the cellular level (for example, oscillatory contractility) can result in markedly different outcomes, depending on the respective subcellular organization and behavior of actomyosin [8
In the roundworm Caenorhabditis elegans
, gastrulation begins at the 26-cell stage, when the 2 endodermal cells Ea and Ep internalize on the ventral side of the embryo to form the gut primordium [9
]. This is followed by the multipolar internalization of mesoderm, primordial germ cells, and ectodermal cells to form the pharynx, body musculature, and neuronal tissue, respectively, all from the ventral side [9
]. Cell internalization has been mainly studied for the endodermal precursors Ea/Ep, and depends on proper fate specification through Wnt signaling [12
], on regulators of apicobasal polarity [15
], on apical accumulation and activation of the protein non-muscle myosin (NMY)-2 [15
], and on cell-cell adhesion [20
]. Although apical constriction has been considered a morphogenetic mechanism in light of the requirement of apical NMY-2 for endoderm internalization [10
], it has also been shown that mesodermal cells extend over the endoderm, indicating that neighboring cells might actively contribute to internalization [10
]. How the surrounding tissues and the internalizing cells are coordinated to achieve internalization is not well understood. Notably, cell internalization occurs with a cellular architecture that lacks several aspects of cell-cell coordination compared with, for example, Drosophila, Xenopus
, or zebrafish; cells have not yet formed coherent tissues, and assembly of polarized apical junctions and deposition of extracellular matrix occur after completion of gastrulation [22
]. This suggests that clear differences in the cellular mechanisms of gastrulation must exist.
In this study, we investigated the primary morphogenetic module for cell internalization during C. elegans gastrulation. During cell internalization, surrounding cells form centripetal extensions that converge into multicellular rosettes to seal over internalizing cells. Extension formation of the surrounding cells correlates with and seems to depend on apical contractile flows in the internalized cells. We showed that this morphogenetic module could adapt to severe topological alterations, providing a mechanistic explanation for plasticity and scalability of gastrulation, which is needed for recurrent deployment of rosette formation throughout gastrulation to internalize different numbers of cells with different sizes and spatial configurations. Besides the recurrent use of this morphogenetic module, we found that globally, coplanar cell divisions thin out and spread the surface cell layer over time. We suggest that the combination of coplanar division and recurrent rosette formation for piecemeal internalization constitutes a system-level solution of volume rearrangement under spatial constraint. Finally, we provide evidence that recurrent rosette formation mediates both local and global rearrangement of cell positions and tissue patterns in the surface layer.