title: Sequential activation of apical and basolateral contractility drives ascidian endoderm invagination.
abstract: BACKGROUND
  Epithelial invagination is a fundamental morphogenetic behavior that transforms a flat cell sheet into a pit or groove. Previous studies of invagination have focused on the role of actomyosin-dependent apical contraction; other mechanisms remain largely unexplored.
  
  
  RESULTS
  We combined experimental and computational approaches to identify a two-step mechanism for endoderm invagination during ascidian gastrulation. During Step 1, which immediately precedes invagination, endoderm cells constrict their apices because of Rho/Rho-kinase-dependent apical enrichment of 1P-myosin. Our data suggest that endoderm invagination itself occurs during Step 2, without further apical shrinkage, via a novel mechanism we call collared rounding: Rho/Rho-kinase-independent basolateral enrichment of 1P-myosin drives apico-basal shortening, whereas Rho/Rho-kinase-dependent enrichment of 1P and 2P myosin in circumapical collars is required to prevent apical expansion and for deep invagination. Simulations show that boundary-specific tension values consistent with these distributions of active myosin can explain the cell shape changes observed during invagination both in normal embryos and in embryos treated with pharmacological inhibitors of either Rho-kinase or Myosin II ATPase. Indeed, we find that the balance of strong circumapical and basolateral tension is the only mechanism based on differential cortical tension that can explain ascidian endoderm invagination. Finally, simulations suggest that mesectoderm cells resist endoderm shape changes during both steps, and we confirm this prediction experimentally.
  
  
  CONCLUSIONS
  Our findings suggest that early ascidian gastrulation is driven by the coordinated apposition of circumapical and lateral endoderm contraction, working against a resisting mesectoderm. We propose that similar mechanisms may operate during other invaginations.
authors.affiliation: Center for Cell Dynamics, Friday Harbor Laboratories, 620 University Road, Friday Harbor, WA 98250, USA.
authors.forename: Kristin
authors.initials: K
authors.lastname: Sherrard
authors.forename: François
authors.initials: F
authors.lastname: Robin
authors.forename: Patrick
authors.initials: P
authors.lastname: Lemaire
authors.forename: Edwin
authors.initials: E
authors.lastname: Munro
mesh_terms: D000577:Amides / Q000494:pharmacology; D000818:Animals; D004707:Endoderm / Q000196:embryology* / Q000378:metabolism; D006576:Heterocyclic Compounds, 4 or More Rings / Q000494:pharmacology; D008954:Models, Biological; D009218:Myosins / Q000037:antagonists & inhibitors / Q000378:metabolism; D011725:Pyridines / Q000494:pharmacology; D014561:Urochordata / Q000196:embryology*; D020742:rhoA GTP-Binding Protein / Q000378:metabolism
chemical_list: D000577:Amides; D006576:Heterocyclic Compounds, 4 or More Rings; D011725:Pyridines; C108830:Y 27632; C472645:blebbistatin; D009218:Myosins; D020742:rhoA GTP-Binding Protein