Editing Invagination (section)

=== Apical constriction ===
{{main|Apical constriction}}
[[File:Invagination_by_apical_constriction.jpg|thumb|459x459px|Apical constriction leading to invagination of a monolayer of cells]]
Apical constriction is an active process that results in the shrinkage of the apical side of the cell. This causes the cell shape to change from a column or cube-shaped cell to become wedge-shaped. Apical constriction is powered by the activity of the proteins [[actin]] and [[myosin]] interacting in a complex network known as the actin-myosin cytoskeleton. Myosin, a motor protein, generates force by pulling filaments of actin together. Myosin activity is regulated by the [[phosphorylation]] of one of its [[Protein subunit|subunits]], [[Myosin light chain|myosin regulatory light chain]]. Thus, kinases such as [[Rho-associated coiled-coil kinase]] (ROCK), which phosphorylate myosin, as well as [[Phosphatase|phosphatases]], which [[Dephosphorylation|dephosphorylate]] myosin, are regulators of actomyosin contraction in cells.<ref name=":1">Martin, Adam C., and Bob Goldstein. 2014. “Apical Constriction: Themes and Variations on a Cellular Mechanism Driving Morphogenesis.” ''Development'' 141 (10): 1987–98. <nowiki>https://doi.org/10.1242/dev.102228</nowiki>.</ref>

The arrangement of actin and myosin in the [[cell cortex]] and the way they generate force can vary across contexts. Classical models of apical constriction in embryos and epithelia in [[cell culture]] showed that actin-myosin bundles are assembled around the circumference of the cell in association with [[Adherens junction|adherens junctions]] between cells. Contraction of the actin-myosin bundles thus results in a constriction of the apical surface in a process that has been likened to the tightening of a purse string.<ref name=":1" /> More recently, in the context of a cultured epithelium derived from the mouse [[organ of Corti]], it has also been shown that the arrangement of the actin and myosin around the cell circumerence is similar to a muscle [[sarcomere]], where there are a repeating units of myosin connected to antiparallel actin bundles.<ref name=":2">Ebrahim, Seham, Tomoki Fujita, Bryan A. Millis, Elliott Kozin, Xuefei Ma, Sachiyo Kawamoto, Michelle A. Baird, et al. 2013. “NMII Forms a Contractile Transcellular Sarcomeric Network to Regulate Apical Cell Junctions and Tissue Geometry.” ''Current Biology'' 23 (8): 731–36. <nowiki>https://doi.org/10.1016/j.cub.2013.03.039</nowiki>.</ref> In other cells, a network of myosin and actin in the middle of the apical surface can also generate apical constriction. For example, in cells of the ''Drosophila'' ventral furrow, the organization of actin and myosin is analogous to a muscle sarcomere arranged radially.<ref>Heer, Natalie C., and Adam C. Martin. 2017. “Tension, Contraction and Tissue Morphogenesis.” ''Development'' 144 (23): 4249–60. <nowiki>https://doi.org/10.1242/dev.151282</nowiki>.</ref><ref name=":3">Coravos, Jonathan S., and Adam C. Martin. 2016. “Apical Sarcomere-like Actomyosin Contracts Nonmuscle ''Drosophila'' Epithelial Cells.” ''Developmental Cell'' 39 (3): 346–58. <nowiki>https://doi.org/10.1016/j.devcel.2016.09.023</nowiki>.</ref> In some contexts, a less clearly organized “cortical flow” of actin and myosin can also generate contraction of the apical surface.<ref name=":2" />