A cutaway of the dorsal sector of a midgastrula, showing post-involution IMZ tissues ( B). In the top panel, a vegetal view at the onset of gastrulation ( A Video 1, 285 min) shows expression of CT (white symbols, implying circumferential convergence and thickening perpendicular to the surface of the embryo) in the Involuting Marginal Zone (IMZ), and the effect of its convergence in producing involution (black arrows) and blastopore closure (green arrows, ( A). Blastopore closure is a systems-level process involving mechanical interplay of a number of regional morphogenic movements in amphibians ( Schechtman, 1942 Gerhart and Keller, 1986 Keller and Shook, 2004 Davidson, 2008) and other species as well ( Davidson et al., 1995). In amphibians, blastopore closure occurs as the ring of presumptive mesoderm lying at the margin of the blastopore, called the ‘Involuting Marginal Zone’ (IMZ), rolls, or ‘involutes’, over the blastoporal lip and simultaneously converges, or decreases in circumference, thereby ‘squeezing’ the blastopore shut and internalizing the presumptive mesoderm in one motion ( Figure 1, top panel Video 1, left embryo). Semi-quantitative biomechanical properties of embryonic cells and tissues have been inferred from responses to micro-dissection ( Beloussov, 1990 Fernandez-Gonzalez et al., 2009 Ma et al., 2009 Solon et al., 2009 Martin et al., 2010 Fouchard et al., 2011), and in other cases, direct quantitative measurements have been made ( Adams et al., 1990 Moore, 1994 Davidson, 1995 Davidson et al., 1995 Moore et al., 1995b Davidson et al., 1999 Zhou et al., 2009 Zhou et al., 2010 Luu et al., 2011 David et al., 2014 Feroze et al., 2015).Ī major component of gastrulation in amphibian embryos, such as those of Xenopus laevis, in many species of invertebrates and anamniotes, as well as in a few amniotes (see Stern, 2004), is ‘blastopore closure’. Understanding the physical aspects of tissue movements is essential for understanding how cells and gene products function in morphogenesis ( Keller et al., 2003 Keller et al., 2008), and thus biomechanical measurements, mathematical modeling, and rigorous engineering standards play increasing roles in experimental analyses (see Jacobson and Gordon, 1976 Hardin and Cheng, 1986 Priess and Hirsh, 1986 Hardin, 1988 Hardin and Keller, 1988 Koehl, 1990 Hutson et al., 2003 Keller et al., 2008 Rodriguez‐Diaz et al., 2008 Toyama et al., 2008 Layton et al., 2009 Varner et al., 2010). Major morphogenic (shape-generating) movements in the development of multicellular organisms occur by integration of local, force-generating activities and force-transmitting properties of individual cells into ‘morphogenic machines’ that act across the tissue-level length scale. These results illuminate the mechanobiology of early vertebrate morphogenic mechanisms, aid interpretation of phenotypes, and give insight into the evolution of blastopore closure mechanisms. Uniaxial tensile stress relaxation assays show stiffening of mesodermal and ectodermal tissues around the onset of neurulation, potentially enhancing long-range transmission of convergence forces. Explants from ventralized embryos, which lack tissues expressing CE but close their blastopores, produce up to 2 μN of tensile force, showing that CT alone generates forces sufficient to close the blastopore. These forces are generated by convergent thickening (CT) until the midgastrula and increasingly by convergent extension (CE) thereafter. We show that explanted MZs generate tensile convergence forces up to 1.5 μN during gastrulation and over 4 μN thereafter. Indirect evidence suggests that blastopore closure during gastrulation of anamniotes, including amphibians such as Xenopus laevis, depends on circumblastoporal convergence forces generated by the marginal zone (MZ), but direct evidence is lacking.
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