Geometric constraints affect three-dimensional cell morphologies and packing within epithelial tissues. Cells in confining geometries skew or deform and change their neighbors frequently in order to arrange within limited available space.
These findings were published in the Molecular Biology of the Cell in 2017.
Rupprecht JF et al. Geometric constraints alter cell arrangements within curved epithelial tissues. Molecular Biology of the Cell. 2017. 28(25). 3582-3594. doi: 10.1091/mbc.E17-01-0060.
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Figure: The illustration depicts the arrangement of cells during cellularization in Drosophila embryo. a) Epithelial cell layer formed by the ingression of the plasma membrane around each nucleus of a multinucleated syncytium. Cells along the curved apical pole are shown in blue. b) Three-dimensional morphologies and arrangement of cells at the curved apical pole: they have a larger apical surface and skew towards the trunk region. c) Cells at the apical pole frequently change their neighbors along their apical-basal axes (seen as blue-blue neighboring cells on the apical side and blue-red neighboring cells on the basal side).
The study explores the effects of geometry on epithelial tissue organization in Drosophila embryos, by quantifying the three-dimensional morphologies and arrangement of cells in the highly curved embryo head and the flatter trunk regions. Cells in the head region are shorter and less crowded than cells in the trunk. Additionally, cells in the embryo head skew or deform and frequently change their neighbors in order to arrange within the highly curved head region of the Drosophila embryo. The effects of confining geometry on cell shape and arrangement were tested and validated using a two-dimensional vertex model.
Understanding the basics
In the earliest stages of development, when the tissues are still taking shape, the physical properties of the microenvironment can direct cell differentiation, and initiate the coordinated movement of groups of cells to establish the patterns that will define how the body is arranged. Later, as tissue integrity is impacted by years of use, damage through injury or disease, and subsequent repair mechanisms, the physical properties of the cellular microenvironment will have drastically changed. These changes can lead altered cell behavior, that at times leads to the onset of disease. The importance of the physical microenvironment of cells in development is most prominent in the earliest stages of development, which are collectively referred to as embryogenesis.
Cell-cell signaling refers to inter-cellular communication through the transduction of chemical, mechanical or electrical signals, facilitated by the formation of specialized cell-cell adhesion junctions. There are three main types of cell-cell adhesive junctions in mammals that detect and transduce signals from neighboring cells: tight junctions, adherens junctions, and desmosomes.
Cell polarity refers to the intrinsic asymmetry observed in cells, either in their shape, structure, or organization of cellular components. Most epithelial cells, migrating cells and developing cells require some form of cell polarity for their function. These cells receive information about their surroundings via extracellular biochemical and mechanical cues and translate those information into polarity of the plasma membrane, its associated proteins and cytoskeletal organization. Once established, cell polarity is maintained by transcytosis, in which vesicles carry incorrectly-localized membrane proteins to the correct regions in the plasma membrane. In addition, tight junctions, which act as ‘fences’ against transmembrane diffusion, lock the asymmetry in place. Therefore, mechanobiology plays an essential regulatory role in both the establishment and maintenance of cell polarity.
The Study in Detail
- The cell packing density was lesser at the embryo head than in the trunk region. This was quantified by measuring the nearest neighbor distance, which was slightly reduced towards the trunk region.
- Cells in the embryo head were shorter due to reduced basal extension than cells in the trunk. This was confirmed by measuring cell invagination depths, which were shorter 10 +/- 6 % shorter in the head region than in the embryo trunk.
- At the embryo head, cells frequently exchanged their neighbors, which were driven by T1 transitions occurring along their apical-basal axes. The frequency of T1 transitions and cell rearrangements was lower in cells in the trunk region.
- The cellular rearrangements in the embryo head driven by T1 transitions were found to be independent of actomyosin contractility since no changes in actin or myosin localization were observed corresponding to cellular rearrangements.
- Some differences in cell morphologies were observed between the embryo head and the trunk region. While the apical surface area was larger in cells at the embryo head than in the trunk, the differences in basal surface area between the two regions was considerably small.
- The cells at the embryo head skewed or deformed towards the trunk region after their basal surfaces had extended 15 micrometers. This was shown to occur due to effects of geometric constraints, wherein cells in the head region deformed towards the less-constrained trunk in an attempt to rearrange in limited available space.
- In smaller and rounder mutant embryos, the cell density and cell surface area at the embryo head were reduced than in the wild type embryos. Cell skew was also reduced in the head region of the mutant embryos.
Methods and Controls used in the study
- Confocal and light sheet microscopy were used to image three-dimensional cell morphologies and arrangements in the head and trunk regions of Drosophila embryos. A microfluidic device for used for mounting the embryos for imaging.
- A two-dimensional vertex model was used to test and validate the effects of confining geometries on cell morphologies and arrangements observed using imaging techniques.
Applications and Future Directions
- The effects of curved geometry on cell shape and arrangement provides insights into the organization of similar biological systems such as the midgut wherein epithelial tissues are found within curved environments.
- The tissue dynamics described in this study could offer clues for understanding the basis of topological defects that arise during tissue and organ formation.
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