Editing Invagination (section)

=== Sea urchin gastrulation ===
[[File:Sea_urchin_gastrulation.png|left|thumb|175x175px|Invagination of the archenteron during sea urchin gastrulation]]
[[Sea urchin]] gastrulation is another classic model for invagination in embryology. One of the early gastrulation movements in sea urchins is the invagination of a region of cells at the [[Polarity in embryogenesis|vegetal]] side of the embryo (vegetal plate) to become the [[archenteron]], or future gut tube. There are multiple stages of archenteron invagination: a first stage where the initial folding in of tissue occurs, a second stage where the archenteron elongates, and in some species a third stage where the archenteron contacts the other side of the cell cavity and finishes its elongation.<ref name=":6" />

Apical constriction occurs in archenteron invagination, with a ring of cells called “bottle cells” in the center of the vegetal plate becoming wedge-shaped.<ref name=":9">Kimberly, Elizabeth Laxson, and Jeff Hardin. 1998. “Bottle Cells Are Required for the Initiation of Primary Invagination in the Sea Urchin Embryo.” ''Developmental Biology'' 204 (1): 235–50. <nowiki>https://doi.org/10.1006/dbio.1998.9075</nowiki>.</ref> However, invagination does not seem to be solely driven by the apical constriction of bottle cells, as inhibiting actin polymerization<ref name=":10">Lane, Mary Constance, M.A.R. Koehl, Fred Wilt, and Ray Keller. 1993. “A Role for Regulated Secretion of Apical Extracellular Matrix during Epithelial Invagination in the Sea Urchin.” ''Development'' 117 (3): 1049–60. <nowiki>https://doi.org/10.1242/dev.117.3.1049</nowiki>.</ref> or removing bottle cells does not fully block invagination.<ref name=":9" /> Several other mechanisms have been proposed to be involved in the process, including a role for extraembryonic [[extracellular matrix]].<ref>McClay, David R., Jacob Warner, Megan Martik, Esther Miranda, and Leslie Slota. 2020. “Chapter Seven - Gastrulation in the Sea Urchin.” In ''Current Topics in Developmental Biology'', edited by Lilianna Solnica-Krezel, 136:195–218. Gastrulation: From Embryonic Pattern to Form. Academic Press. <nowiki>https://doi.org/10.1016/bs.ctdb.2019.08.004</nowiki>.</ref> In this model, there are two layers of extracellular matrix at the apical surface of cells made of different proteins. When cells from the vegetal plate secrete a molecule ([[chondroitin sulfate proteoglycan]]) that is highly water absorbent into the inner layer, this causes the layer to swell, making the tissue buckle inwards.<ref name=":10" /> Several genetic pathways have been implicated in this process. [[Wnt signaling pathway|Wnt signaling]] through the non-canonical [[planar cell polarity]] pathway has been shown to be important, with one of its downstream targets being the small [[GTPase]] [[Transforming protein RhoA|RhoA]]. [[Fibroblast growth factor|FGF signaling]] also plays a role in invagination.<ref>Lyons, Deirdre C., Stacy L. Kaltenbach, and David R. McClay. 2012. “Morphogenesis in Sea Urchin Embryos: Linking Cellular Events to Gene Regulatory Network States.” ''WIREs Developmental Biology'' 1 (2): 231–52. <nowiki>https://doi.org/10.1002/wdev.18</nowiki>.</ref>