Editing Invagination (section)

=== ''Drosophila'' ventral furrow ===
[[File:Ventral_furrow_formation_in_drosophila_embryo.png|thumb|426x426px|Formation of the ventral furrow in a ''Drosophila'' embryo. Cell nuclei (blue), membranes (green), and myosin (red) are stained.]]
One of the most well studied models of invagination is the ventral furrow in ''Drosophila melanogaster''. The formation of this structure is one of the first major cell movements in ''Drosophila'' gastrulation. In this process, the prospective [[mesoderm]]––the region of cells along the [[Anatomical terms of location|ventral]] midline of the embryo––folds inwards to form the ventral furrow. This furrow eventually pinches off and becomes a tube inside the embryo and ultimately flattens to form a layer of tissue underneath the ventral surface.<ref name=":6">{{Cite book |last=Gilbert |first=Scott F. |url=https://www.ncbi.nlm.nih.gov/books/NBK9983/ |title=Developmental Biology |last2=Gilbert |first2=Scott F. |date=2000 |publisher=Sinauer Associates |isbn=978-0-87893-243-6 |edition=6th}}</ref>

Ventral furrow formation is driven by apical constriction of the future mesoderm cells, which first flatten along the apical surface and then contract their apical membranes. The classical models for how apical constriction worked in this context were based on the “purse-string” mechanism where an actin-myosin band around the circumference of the apical cell surface contracts.<ref name=":7">Gheisari, Elham, Mostafa Aakhte, and H. -Arno J. Müller. 2020. “Gastrulation in ''Drosophila Melanogaster'': Genetic Control, Cellular Basis and Biomechanics.” ''Mechanisms of Development'' 163 (September):103629. <nowiki>https://doi.org/10.1016/j.mod.2020.103629</nowiki>.</ref>  However, more recent investigations have revealed that, while there is a circumferential band of actin associated with cell junctions on the side of cells, it is actually an actin-myosin network arranged radially across the apical surface that powers apical constriction.<ref name=":8">Martin, Adam C., Matthias Kaschube, and Eric F. Wieschaus. 2009. “Pulsed Contractions of an Actin–Myosin Network Drive Apical Constriction.” ''Nature'' 457 (7228): 495–99. <nowiki>https://doi.org/10.1038/nature07522</nowiki>.</ref> This structure acts like a radial version of a muscle sarcomere.<ref name=":3" /> Force generated by myosin results in contraction towards the center of the cell. The cells do not contract continuously but rather have pulsed contractions. In between contractions, the actin network around the circumference of the cell helps stabilize the reduced size of the cell, allowing for a progressive decrease in size of the apical surface.<ref name=":8" /> In addition to apical constriction, adhesion between cells through adherens junctions is critical for transforming these individual cell-level contractions into a deformation of a whole tissue.

Genetically, formation of the ventral furrow relies on the activity of the [[Transcription factor|transcription factors]] ''[[Twist-related protein 1|twist]]'' and ''[[SNAI1|snail]]'', which are expressed in the prospective ventral mesoderm before furrow formation.<ref name=":7" /> Downstream of ''twist'' is the Fog signaling pathway, which controls the changes that occur in the apical domain of cells.<ref>Manning, Alyssa J., and Stephen L. Rogers. 2014. “The Fog Signaling Pathway: Insights into Signaling in Morphogenesis.” ''Developmental Biology'' 394 (1): 6–14. <nowiki>https://doi.org/10.1016/j.ydbio.2014.08.003</nowiki>.</ref>