Draft:Cortical tension in embryonic development

Introduction

Cortical tension is involved in the cleavage stage of the embryo, and is particularly important for cell division.

Cortical tension is a specific type of tension generated by actomyosin complexes at the cell cortex, a layer below the plasma membrane that allows for morphological changes in the cell. In embryonic development, cortical tension plays a critical role in multiple stages of development, including gastrulation, epiboly, neural tube formation, and cell elongation. It also plays a role in more nuanced mechanisms of axes formation, including cell size, polarity, and migration. Cortical tension is primarily driven by the actomyosin complex, but is also controlled by other mechanisms, including adhesion, cytoskeletal involvement, and mechanotransduction. Cortical tension has been shown to induce various signaling pathways to drive development through various stages. While cortical tension is usually studied in a variety of model organisms, it is highly relevant to human embryonic development, including blastocyst development, cell adhesion, and extraembryonic tissue stiffness.

Table of contents

1. Background
2. C. elegans (nematode)
3. D. melanogaster (fruit fly)
4. D. rerio (zebrafish)
5. Mouse
6. Conclusion

Background

Cortical tension has been studied extensively in numerous model organisms using various techniques and methodologies. In micropipette aspiration, the most common (and simplest) technique, cells are pulled into micropipettes and a deformation index is measured. Another common technique that is used is atomic force microscopy (AFM), where a probe applies a small force to the cell to measure deformation resistance. Cell blebbing is also observed, as inflation and deflation of blebs usually correlate to changes in cortical tension. These techniques as a whole can help inform how cortical tension and flow are differentially affected in embryonic development, and in particular, be examined in various types of model organisms.

C. elegans (nematode)

In C. elegans, the gradient of cortical tension across the cell cortex (also known as cortical flow) can drive cell polarity and cell fate determinant localization. Specifically, this gradient can pattern the anterior-posterior axis and the dorsal-ventral axis, and can also promote cell adhesion to drive or halt proliferation and downstream signaling pathways. Cortical tension and cortical flow localizes aPAR domain proteinss, including PAR-3, PAR-6, and PKC-3, which all determine apical polarity. Furthermore, the maternal determinant GSK-3 has been shown to drive this cortical flow, which is then necessary for polarity determination. This cortical flow can also determine Wnt signaling pathways, which drive cell & germ layer differentiation. Finally, this flow determines planar asymmetry, which ultimately drives cell intercalation later in embryonic development.

Example of anisotropic movement, which is random in direction & magnitude.

Embryonic development in C. elegans is also driven by asymmetrical cell divisions, where one portion of the embryo will undergo faster and a higher number of mitotic divisions to facilitate axis polarization in a certain manner. The manner in which these asymmetric divisions polarize to one side of the embryo is also driven by cortical flow, but is driven anisotropically – that is to say, cortical tension randomly varies in magnitude when examined in different directions. This contradicts previous suggestions that cortical flow is a simple tension gradient, and when cortical flow was visualized both magnitudinally and directionally using laser microscopy, it was determined that cortical flow does not generate mechanical tension gradients, but anisotropic levels of cortical tension that are also driven by friction/viscosity, and serve for long range axes determination.

D. melanogaster (fruit fly)

Embryonic development has been studied extensively in Drosophila melanogaster; specifically, many important discoveries in developmental patterning that are relevant to human embryonic growth have been made in Drosophila, making it an important model organism in which to study cortical tension.

Similar to C. elegans, cortical tension in Drosophila is well known to play a large role in cell shape & polarity, axes formation, and cell fate determination. In Drosophila, the mechanism of cell intercalation, or cell type mixing, is what drives patterning of the dorsal-ventral axis, and the following patterning of the anterior-posterior axis. The forces that drive cell intercalation are typically located at cell junctions, and it has been shown that cortical elasticity is directly correlated with cortical tension. In particular, myosin driven forces in the cell control this elasticity, which in turn alters cortical tension and drives cell junction remodeling, allowing for cells to intercalate successfully.

Drosophila has also been shown to drive cortical tension through localization of cell determinants for downstream cell fate determination. During dorsoventral patterning, a layer of cells called the amnioserosa undergoes severe constriction, and these contractions are regulated by the PAR complex discussed earlier, and the Rho signaling pathway, which also involves the RhoGEF domain protein RhoGEF2. RhoGEF2 is particularly localized cortically, and as a result, is expected to affect cortical tension during this constriction in some way. Therefore, RhoGEF2 mutants were introduced to see if cortical tension was affected, and without RhoGEF2, the contractility in the amnioserosa was lost. Further along in development, Drosophila retinal development seems to involve cell-cell adhesion facilitated by cell cortex contraction through cortical tension. As a result, cortical tension plays a role in Drosophila embryonic development through a variety of mechanisms.

D. rerio (zebrafish)

The zebrafish is a particularly interesting and versatile organism to study development because its embryo is transparent, allowing for easy visualization and imaging of embryonic development. Epiboly is a developmental process in zebrafish development that utilizes the three tissue layers – the enveloping layer, the yolk syncytial layer, and the yolk cytoplasmic layer – and reorganizes the embryo in such a way that the blastoderm envelops the yolk at the vegetal pole. In this process, the layers thin and spread, involving numerous mechanical forces. By measuring force in vivo zebrafish embryos, it was determined that cortical tension gradients are the primary driving force behind tissue movement.

During epiboly, there is also anisotropic cortical tension present in the enveloping layer. Because the enveloping layer undergoes cell elongation to spread across the yolk layers, it requires varying levels of tension to regulate elongation across the tissue layer. When this tension is ablated, cells spontaneously fuse, suggesting that this cortical tension is necessary to prevent cell fusion.

Furthermore, germ layer organization is also driven by cortical tension in zebrafish. The three progenitor germ layers – ectoderm, mesoderm, and endoderm – are typically organized early in development, and this organization is driven by cell migration and adhesion. Cortical tension, here, is particularly driven by actomyosin forces, and these forces drive cell-cell adhesion differentially depending on the progenitor germ layer. This adhesion is also partially controlled by Nodal/TGF-b signaling, particularly in ectoderm progenitors.

Mouse

The mouse is one of the most important model organisms to examine development, because the mammalian embryonic development of mouse zygotes is starkly similar to that of humans, allowing for us to understand genetic disease progression more closely.

During mouse blastocyst development, one particular feature of early organ formation is lumenal pressure, which determines preliminary vasculature and endocrine organs. This lumenal pressure is dependent on cell-cell adhesion, which in turn is dependent on cortical tension. Both tissue stiffness and cortical tension are interrelated, and when cortical tension reaches a certain threshold, the blastocyst collapses and the adhesive properties are lost. As a result, the maintenance of cortical tension in regards to cell lumen pressure is critical to ensuring embryo shape and cell organization.

While most studies regarding cortical tension are done in embryos (post-fertilization), work has also been done to clarify that oocyte meiosis also requires cortical tension regulation pre-fertilization. In particular, oocytes that are cryopreserved through vitrification (cooling process) seem to show deficiencies in cortical tension. Indeed, vitrification results in decreased oocyte cortical tension, and artificially increasing cortical tension improves oocyte viability by reducing meiotic errors.

In a similar vein, decreasing cortical tension in normal oocytes (not cryopreserved or vitrified) also impairs their overall function; particularly, decreasing cortical tension affects meiosis by impairing chromosome segregation, leading to chromosomal instability and aneuploidy, which is a key issue in embryonic development.

Cortex remodeling is an additional primary influencing factor of cortical tension, and both theoretical in silico modeling and in vitro artificial remodeling have shown that altering cortex remodeling affects cortical tension, and in turn affects spindle formation & migration. This also affects oocyte meiosis, leading to further defects post-fertilization.

Conclusion

While cortical tension in embryonic development has been studied widely in various model organisms, as enumerated above, the difficulty lies in elucidating which mechanisms are applicable to human embryonic development. Furthermore, visualization of human embryos in vivo is difficult both ethically and experimentally, resulting in limited information on how these mechanisms affect human development. Understanding cortical tension in development can elucidate how various genetic diseases form, and also inform further cancer research on how cells can remodel their fate in similar mannerisms to embryonic development.

See also section

Cell cortex

Cell adhesion

Germ layer

Cleavage (embryo)