Editing Invagination (section)

=== Neural tube formation ===
{{main|Neurulation}}
[[File:Neural_tube_formation_in_mouse.jpg|thumb|462x462px|Cartoon of neural tube formation in a mouse embryo, showing the median hinge point and points of tissue buckling along the sides]]
Scientists have studied the process of neural tube formation in vertebrate embryos since the late 1800s.<ref name=":0" /> Across vertebrate groups including [[Amphibian|amphibians]], [[Reptile|reptiles]], [[Bird|birds]], and [[Mammal|mammals]], the neural tube (the embryonic precursor of the [[spinal cord]]) forms through the invagination of the neural plate into a tube, known as primary neurulation. In [[fish]] (and in some contexts in other vertebrates), the neural tube can also be formed by a non-invagination-mediated process known as secondary neurulation.<ref name=":6" /> While some differences exist in the mechanism of primary neurulation between vertebrate species, the general process is similar. Neurulation involves the formation of a medial hinge point at the middle of the neural plate, which is where tissue bending is initiated. The cells at the medial hinge point become wedge shaped. In some contexts, such as in ''[[Xenopus]]'' frog embryos, this cell shape change appears to be due to apical constriction.<ref>Nikolopoulou, Evanthia, Gabriel L. Galea, Ana Rolo, Nicholas D. E. Greene, and Andrew J. Copp. 2017. “Neural Tube Closure: Cellular, Molecular and Biomechanical Mechanisms.” ''Development'' 144 (4): 552–66. <nowiki>https://doi.org/10.1242/dev.145904</nowiki>.</ref><ref>Christodoulou, Neophytos, and Paris A. Skourides. 2015. “Cell-Autonomous Ca2+ Flashes Elicit Pulsed Contractions of an Apical Actin Network to Drive Apical Constriction during Neural Tube Closure.” ''Cell Reports'' 13 (10): 2189–2202. <nowiki>https://doi.org/10.1016/j.celrep.2015.11.017</nowiki>.</ref> However, in chickens and mice, bending at this hinge point is mediated by a process called basal wedging, rather than apical constriction.<ref name=":4" /><ref>Ybot-Gonzalez, Patricia, and Andrew J. Copp. 1999. “Bending of the Neural Plate during Mouse Spinal Neurulation Is Independent of Actin Microfilaments.” ''Developmental Dynamics'' 215 (3): 273–83. <nowiki>https://doi.org/10.1002/(SICI)1097-0177(199907)215:3</nowiki><273::AID-AJA9>3.0.CO;2-H.</ref><ref>Schoenwolf, Gary C., David Folsom, and Ardis Moe. 1988. “A Reexamination of the Role of Microfilaments in Neurulation in the Chick Embryo.” ''The Anatomical Record'' 220 (1): 87–102. <nowiki>https://doi.org/10.1002/ar.1092200111</nowiki>.</ref> In this case, the cells are so thin that the movement of the [[Cell nucleus|nucleus]] to the basal side of the cell causes a bulge in the basal part of the cell. This process may be regulated by how the cell divisions take place. Contractions of actin-myosin cables are also important for the invagination of the neural plate. Supracellular actin cables stretching across the neural plate help pull the tissue together (see {{Section link|2=Supracellular cables|nopage=yes}}). Furthermore, forces pushing into the neural plate from the adjacent tissue also may play a role in the folding of the neural plate.<ref>Suzuki, Makoto, Hitoshi Morita, and Naoto Ueno. 2012. “Molecular Mechanisms of Cell Shape Changes That Contribute to Vertebrate Neural Tube Closure.” ''DGD'' 54 (3): 266–76. <nowiki>https://doi.org/10.1111/j.1440-169X.2012.01346.x</nowiki>.</ref><ref>Morita, Hitoshi, Hiroko Kajiura-Kobayashi, Chiyo Takagi, Takamasa S. Yamamoto, Shigenori Nonaka, and Naoto Ueno. 2012. “Cell Movements of the Deep Layer of Non-Neural Ectoderm Underlie Complete Neural Tube Closure in Xenopus.” ''Development'' 139 (8): 1417–26. <nowiki>https://doi.org/10.1242/dev.073239</nowiki>.</ref><ref>Hackett, Deborah A., Jodi L. Smith, and Gary C. Schoenwolf. 1997. “Epidermal Ectoderm Is Required for Full Elevation and for Convergence during Bending of the Avian Neural Plate.” ''Developmental Dynamics'' 210 (4): 397–406. <nowiki>https://doi.org/10.1002/(SICI)1097-0177(199712)210:4</nowiki><397::AID-AJA4>3.0.CO;2-B.</ref>