- Open Access
No strings attached: new insights into epithelial morphogenesis
© Davidson; licensee BioMed Central Ltd. 2012
- Received: 13 December 2012
- Accepted: 19 December 2012
- Published: 20 December 2012
The dramatic ingression of tissue sheets that accompanies many morphogenetic processes, most notably gastrulation, has been largely attributed to contractile circum-apical actomyosin 'purse-strings' in the infolding cells. Recent studies, however, including one in BMC Biology, expose mechanisms that rely less on actomyosin contractility of purse-string bundles and more on dynamics in the global cortical actomyosin network of the cells. These studies illustrate how punctuated actomyosin contractions and flow of these networks can remodel both epithelial and planarly organized mesenchymal sheets.
- Retinal Pigment Epithelium
- Cell Sheet
- Retinal Pigment Epithelium
- Epithelial Sheet
- Epithelial Morphogenesis
Several recent papers highlight ways in which adhesion and the geometry of cells in tissues can convert the highly localized contractions of actomyosin arrays into cell- and tissue-shaping processes. Goldstein and co-workers  have shown that it is not the frequency of actomyosin contractions but how these contractions couple neighboring cells through cell junctions that dictate their efficiency in transmitting tension and directing cell shape changes. The nature of these couplings somehow dictates transmission of force from deeply positioned septate junctions between adjacent cells to the plane of the apical cortex. Since epithelial cells in Drosophila and C. elegans are under constant tension, the transmission or relaxation of tension plays a critical role in directing bulk movements such as germ band elongation and dorsal closure in Drosophila and during gastrulation and ventral enclosure in C. elegans.
Supracellular actomyosin bundles at the margins of tissues can play a comparable role in organizing cortical actin dynamics. Supracellular bundles of contractile actin bundles are commonly found at the margins of wounded epithelial sheets and at the boundaries between germ layers. One prominent bundle is found at the leading edge of the enveloping layer as the layer engulfs the yolk cell during zebrafish gastrulation. Recent studies of epiboly , a movement that stretches an epithelial sheet to completely enclose the embryo early during gastrulation, showed that significant tension exists within the cortex of the yolk cell perpendicular to the supracellular actin bundles at the leading edge of the enveloping cell layer. Tension in the yolk cell cortex correlates directly with the direction of actomyosin network flow in the cortex in a manner similar to the cortical flow seen in the apical cortex of actively contracting epithelial cells. The persistence of these forces in driving epiboly even when embryos are 'cylindrical' suggests these forces are frictional in nature rather than simply tensional. Frictional forces, from the flow of underlying actomyosin cortex in the yolk, are thought to pull on the bundled actin in the enveloping layer, drawing the enveloping layer toward the posterior end of the yolk ball. Interestingly, such a mechanism, termed the cortical tractor model, was first hypothesized in the 1980s by amphibian embryologist Antone Jacobson  to account for epithelial folding during neurulation (Figure 1c). Folding of the brain in amphibians involves progressive apical constriction coordinated with changes in cell height. Since many cells contribute to the folding, each cell need only adopt a small change in shape. Collectively, small forces could be summed across a broad field of cortically tractoring cells to produce a fold. Further investigations combining predictive models and biophysical experiments might be able to test the role of cortical flows in driving ingression or exclusion of individual cells from tightly integrated epithelial sheets .
In most of the cases discussed above the forces of contractile actin bundles in circum-apical junctions, in the apical cortex, or from frictional flows of actomyosin, are directed against strong attachment points - for example, as circum-apical belts of actin draw in neighboring cells at sites of cell-cell adhesion or as supercellular actomyosin structures at the margin of a wound or germ layer pull the layer over adjacent tissues. Attachment of the cortex to these fixed points is critical if forces generated by one cell are to be summed with those generated by others and transmitted across a tissue. The transmission of such forces coordinates large scale tissue movements that are necessary either to re-shape the tissue in a plane (as in convergent extension) or to direct out-of-plane bending moments that drive folding (for example, in neurulation). However, not all tissues are connected by these types of structures - important examples are mesenchymal tissues and less strongly polarized epithelial sheets - and these tissues also need to transmit force and coordinate large-scale movements.
The single-cell movements of ingression, intercalation, and extrusion share many features with coordinated epithelial movements that shape large cell sheets. Study of these events in controlled mechanical environments , as well as in complex mechanical environments such as the C. elegans embryo, hold great promise for understanding how similar molecular dynamics in the cytoskeleton control a diverse set of cell and tissue movements and bring us closer to understanding how normal development works and how dysregulation might lead to birth defects.
We apologize in advance to the many authors whose work we were not able to not cite here due to space constraints. LD is supported by grants from the National Science Foundation (IOS-0845775) and the National Institutes of Health (HD044750 and ES019259).
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