Invagination

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.

[1] 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.

Regardless of the force-generating mechanism that causes the bending of the epithelium, most instances of invagination result in a stereotypical cell shape change.

Apical constriction is powered by the activity of the proteins actin and myosin interacting in a complex network known as the actin-myosin cytoskeleton.

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.

[7] 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.

For example, in cells of the Drosophila ventral furrow, the organization of actin and myosin is analogous to a muscle sarcomere arranged radially.

[9][10] In some contexts, a less clearly organized “cortical flow” of actin and myosin can also generate contraction of the apical surface.

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.

[26] 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.

[25] Downstream of twist is the Fog signaling pathway, which controls the changes that occur in the apical domain of cells.

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.

[24] 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.

In some contexts, such as in Xenopus frog embryos, this cell shape change appears to be due to apical constriction.

[28][29] However, in chickens and mice, bending at this hinge point is mediated by a process called basal wedging, rather than apical constriction.

[37] In this model, there are two layers of extracellular matrix at the apical surface of cells made of different proteins.

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 RhoA.

Invagination consists of the internal movements of a sheet of cells (the endoderm) based on changes in their shape.