Invagination is the process of a surface folding in on itself to form a cavity, pouch or tube. In developmental biology, invagination of epithelial sheets occurs in many contexts during embryonic development. Invagination is critical for making the primitive gut during gastrulation in many organisms, forming the neural tube in vertebrates, and in the morphogenesis of countless organs and sensory structures. Models of invagination that have been most thoroughly studied include the ventral furrow in Drosophila melanogaster, neural tube formation, and gastrulation in many marine organisms. The cellular mechanisms of invagination vary from one context to another but at their core they involve changing the mechanics of one side of a sheet of cells such that this pressure induces a bend in the tissue.
The term, originally used in embryology, has been adopted in other disciplines as well.
History The process of tissue invagination has fascinated scientists for over a century and a half. Since the beginning, scientists have tried to understand the process of invagination as a mechanical process resulting from forces acting in the embryo.Ettensohn, Charles A. 1985. "Mechanisms of Epithelial Invagination." The Quarterly Review of Biology 60 (3): 289–307. https://doi.org/10.1086/414426. For example, the Swiss biologist Wilhelm His, observing the invagination of the chick neural tube, experimented with modeling this process using sheets of different materials and suggested that pushing forces from the lateral edges of the neural plate might drive its invagination.D. S., Vijayraghavan, and Davidson L. A. 2017. “Mechanics of Neurulation: From Classical to Current Perspectives on the Physical Mechanics That Shape, Fold, and Form the Neural Tube.” Birth Defects Research 109 (2): 153–68. https://doi.org/10.1002/bdra.23557. Scientists throughout the next century have speculated on the mechanisms of invagination, often making models of this process using either physical analogs,Lewis, Warren H. 1947. “Mechanics of Invagination.” The Anatomical Record 97 (2): 139–56. https://doi.org/10.1002/ar.1090970203. or, especially in recent years, mathematical and computational modeling.
Cellular mechanisms Invagination can be driven by a number of mechanisms at the cellular level. Regardless of the force-generating mechanism that causes the bending of the epithelium, most instances of invagination result in a stereotypical cell shape change. At the side of the epithelium exposed to the environment (the apical side), the surface of cells shrinks, and at the side of the cell in contact with the basement membrane (the basal side), the cell surfaces expand. Thus, cells become wedge-shaped. As these cells change shape, the tissue bends in the direction of the apical surface. In many–– though not all––cases, this process involves active constriction of the apical surface by the actin-myosin cytoskeleton. Furthermore, while most invagination processes involve shrinking of the apical surface, there have been cases observed where the opposite happens - the basal surface constricts and the apical surface expands, such as in optic cup morphogenesis and formation of the midbrain-hindbrain boundary in zebrafish.Tozluoǧlu, Melda, and Yanlan Mao. 2020. “On Folding Morphogenesis, a Mechanical Problem.” Philosophical Transactions of the Royal Society B: Biological Sciences 375 (1809): 20190564. https://doi.org/10.1098/rstb.2019.0564.Sidhaye, Jaydeep, and Caren Norden. 2017. “Concerted Action of Neuroepithelial Basal Shrinkage and Active Epithelial Migration Ensures Efficient Optic Cup Morphogenesis.” Edited by Didier YR Stainier. eLife 6 (April):e22689. https://doi.org/10.7554/eLife.22689.Gutzman, Jennifer H., Ellie G. Graeden, Laura Anne Lowery, Heidi S. Holley, and Hazel Sive. 2008. “Formation of the Zebrafish Midbrain–Hindbrain Boundary Constriction Requires Laminin-Dependent Basal Constriction.” Mechanisms of Development 125 (11): 974–83. https://doi.org/10.1016/j.mod.2008.07.004.
Apical constriction
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 subunits, myosin regulatory light chain. Thus, kinases such as Rho-associated coiled-coil kinase (ROCK), which phosphorylate myosin, as well as phosphatases, which dephosphorylate myosin, are regulators of actomyosin contraction in cells.Martin, Adam C., and Bob Goldstein. 2014. “Apical Constriction: Themes and Variations on a Cellular Mechanism Driving Morphogenesis.” Development 141 (10): 1987–98. https://doi.org/10.1242/dev.102228.
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 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. 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.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. https://doi.org/10.1016/j.cub.2013.03.039. 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.Heer, Natalie C., and Adam C. Martin. 2017. “Tension, Contraction and Tissue Morphogenesis.” Development 144 (23): 4249–60. https://doi.org/10.1242/dev.151282.Coravos, Jonathan S., and Adam C. Martin. 2016. “Apical Sarcomere-like Actomyosin Contracts Nonmuscle Drosophila Epithelial Cells.” Developmental Cell 39 (3): 346–58. https://doi.org/10.1016/j.devcel.2016.09.023. In some contexts, a less clearly organized “cortical flow” of actin and myosin can also generate contraction of the apical surface.
Basal relaxation To maintain a constant cell volume during apical constriction, cells must either change their height or expand the basal surface of their cells. While the process of basal relaxation has been less thoroughly studied, in some cases it has been directly observed that the process of apical constriction occurs alongside an active disassembly of the actin-myosin network at the basal surface of the cell, allowing the basal side of the cell to expand. For example, this has been observed in the Drosophila ventral furrow invaginationPolyakov, Oleg, Bing He, Michael Swan, Joshua W. Shaevitz, Matthias Kaschube, and Eric Wieschaus. 2014. “Passive Mechanical Forces Control Cell-Shape Change during Drosophila Ventral Furrow Formation.” Biophysical Journal 107 (4): 998–1010. https://doi.org/10.1016/j.bpj.2014.07.013.Pearl, Esther J., Jingjing Li, and Jeremy B. A. Green. 2017. “Cellular Systems for Epithelial Invagination.” Philosophical Transactions of the Royal Society B: Biological Sciences 372 (1720): 20150526. https://doi.org/10.1098/rstb.2015.0526. and the formation of the otic placode in the chicken.Sai, XiaoRei, and Raj K. Ladher. 2008. “FGF Signaling Regulates Cytoskeletal Remodeling during Epithelial Morphogenesis.” Current Biology 18 (13): 976–81. https://doi.org/10.1016/j.cub.2008.05.049.Sai, Xiaorei, and Raj K. Ladher. 2015. “Early Steps in Inner Ear Development: Induction and Morphogenesis of the Otic Placode.” Frontiers in Pharmacology 6 (February). https://doi.org/10.3389/fphar.2015.00019.
Changes in cell height Invagination also often involves, and can be driven by, changes in cell height. When apical constriction occurs, this can lead to elongation of cells to maintain constant cell volume, and consequently a thickening of the epithelium. However, shortening of cells along the apical-basal axis can also help deepen the pit formed during invagination.Kondo, Takefumi, and Shigeo Hayashi. 2015. “Mechanisms of Cell Height Changes That Mediate Epithelial Invagination.” DGD 57 (4): 313–23. https://doi.org/10.1111/dgd.12224. Active changes in cell shape to cause cell shortening have been shown to contribute to invagination in a few cases. For example, in the Drosophila leg epithelium, apoptotic cells shrink and pull on the apical surface of the epithelium via an apical-basal cable made up of actin and myosin.Monier, Bruno, Melanie Gettings, Guillaume Gay, Thomas Mangeat, Sonia Schott, Ana Guarner, and Magali Suzanne. 2015. “Apico-Basal Forces Exerted by Apoptotic Cells Drive Epithelium Folding.” Nature 518 (7538): 245–48. https://doi.org/10.1038/nature14152. In the invagination that occurs in ascidian gastrulation, cells first undergo apical constriction and then change their shape to become rounder ––and thus shorter along the apical-basal axis––which is responsible for the completion of the invagination movement.Sherrard, Kristin, François Robin, Patrick Lemaire, and Edwin Munro. 2010. “Sequential Activation of Apical and Basolateral Contractility Drives Ascidian Endoderm Invagination.” Current Biology 20 (17): 1499–1510. https://doi.org/10.1016/j.cub.2010.06.075. During cell division, cells also naturally take on a rounded morphology. The rapid drop in cell height caused by rounding of cells during mitosis has also been implicated in invagination of the Drosophila tracheal placode.Kondo, Takefumi, and Shigeo Hayashi. 2013. “Mitotic Cell Rounding Accelerates Epithelial Invagination.” Nature 494 (7435): 125–29. https://doi.org/10.1038/nature11792.
Supracellular cables