During embryonic development, cells use the information contained in their genome to build the organism. This is achieved in successive morphogenetic events during which cells divide, die, deform and move. It is the specific order and combination of morphogenetic events that build distinct species. To understand morphogenesis, one needs to take into account evolutionary biology, genetics, cell biology and biophysics.
Research in the lab focuses on the mechanical aspects of mammalian morphogenesis. We use genetics, microscopy and biophysical tools to study the morphogenesis of mammalian embryos.
Morphogenesis of the pre-implantation embryo
Pre-implantation development shapes the blastocyst, which consists of a squamous epithelium enveloping a fluid-filled cavity and an inner cell mass. Three key morphogenetic events shape the blastocyst: compaction, internalization and cavitation.
PRE-IMPLANTATION DEVELOPMENT
https://www.youtube.com/watch?v=VnMJGEuQHxs
Timelapse of pre-implantation development of the mouse embryo (snapshots taken every 15 min)
At the 8-cell stage, compaction is a developmentally-controlled cell-cell adhesion process that rounds up the embryo. As for the rounding up of a liquid droplet, compaction is driven by the surface tension of the cells. The surface tension of the embryo increases during the 8-cell stage as the actomyosin cortex strengthens. At the same time, this increase in contractility initiates periodic contractions that travel around the cell surface. See Pulsatile cell-autonomous contractility drives compaction in the mouse embryo.
At the 16-cell stage, cells can adopt either an inner or outer position within the embryo. The 8- to 16-cell stage division is instrumental in determining the position of individual cells. This division can be oriented, daughter cells can be pushed inside or outside as result of the mitotic spindle orientation, and asymmetric, daughter cells can acquire distinct components giving them different contractile properties. See Asymmetric division of contractile domains couples cell positioning and fate specification. These different contractile properties result in the self-organised sorting of the cells in either inner or outer position. See Adhesion functions in cell sorting by mechanically coupling the cortices of adhering cells. The positioning of the cells controls their genetic program and their differentiation into embryonic or extra-embryonic tissues. Interestingly, cell mechanics seem to influence this differentiation. See Asymmetric division of contractile domains couples cell positioning and fate specification. It remains unclear how this could happen.
Internalization
https://www.youtube.com/watch?v=Cqp5AeatbgE
Timelapse of the internalisation of a chimeric doublet of cells expressing an actin marker (LifeAct) in different colours (snapshots taken every 15 min)
At the 32-cell stage, a cavity forms between the trophectoderm and inner cells. During this process, multiple lumens grow and eventually fuse into a single cavity. As the cavity grows, cell-cell contacts between trophectoderm and ICM cells are broken.
Pecowaco: Periodic cortical waves of contraction
During morphogenesis, actomyosin contractility is an essential engine of cell and tissue shape changes. Although these shape changes appear continuous, they actually are powered by pulses of contractions, which repeat periodically. Interestingly, these cyclic contractile events are conserved in evolution and can take various forms.
PERIODIC CONTRACTIONS OF THE 8-CELL STAGE MOUSE EMBRYO
https://www.youtube.com/watch?v=EwlSJn4QtNo
Timelapse of periodic contractions of an 8-cell stage mouse embryo in the absence of calcium (snapshot taken every 5 s).
In the mouse embryo, periodic contractions begin at the 8-cell stage, at the onset of compaction. See Pulsatile cell-autonomous contractility drives compaction in the mouse embryo.
PERIODIC CORTICAL WAVES
https://www.youtube.com/watch?v=whiH9SwbJO8
Timelapse of periodic cortical waves of contraction (snapshot taken every 5 s). Left panel shows phase contrast, middle panel shows LifeAct-GFP and right panel shows the plasma membrane with its local curvature overlaid on top in false color.
After asymmetric division, periodic cortical waves of contraction can be used to predict which cell is destined to become internalised and embryonic tissue. Indeed, extra-embryonic blastomeres remained on the surface due to their low contractility. See Asymmetric division of contractile domains couples cell positioning and fate specification.
PERIODIC CORTICAL WAVES AFTER ASYMMETRIC DIVISION
https://www.youtube.com/watch?v=PkdeZ8NC3Rs
Timelapse of periodic cortical waves of contraction (snapshot taken every 5 s). 16-cell stage doublet resulting from the asymmetric division of an isolated 8-cell stage blastomere. Membrane is shown in magenta and LifeAct-GFP in green. The unpolarised blastomere on top shows prominent periodic cortical of contraction while the polarised sister cell on the bottom remains quiet. The polarised blastomere will later envelop its unpolarised sister cell, which will become embryonic tissue.
Measuring cell and tissue mechanics of mammalian embryos
A micropipette aspiration set up can easily be coupled to high-resolution fluorescence confocal microscopes to relate cell mechanics to sub-cellular processes. See Adhesion functions in cell sorting by mechanically coupling the cortices of adhering cells.
Micropipette aspiration can be used to measure surface tension and viscosity of cells and tissues. See Pulsatile cell-autonomous contractility drives compaction in the mouse embryo.
Snapshot of a micropipette-held 8-cell stage embryo (left) during a surface tension mapping experiment. Surface tensions γcm are measured for all blastomeres sequentially by finding the critical pressure Pc giving a deformation of the size of the micropipette Rp and measuring the radius of curvature of the cell Rc. Surface tensions γcm from contacting blastomeres can then be used to calculate the tension along cell-cell contacts γcc by taking into account the contact angles θ.
Using a dual pipette aspiration assay, the strength of adhesion of contacting cells can be measured in vitro. See Dual pipette aspiration: a unique tool for studying intercellular adhesion.
SEPARATION FORCE MEASUREMENT
https://www.youtube.com/watch?v=i5UZ9eLquHA
Zebrafish mesoderm progenitors are brought into contact before being separated by pulling the micropipettes apart while applying increasing pressures in a stepwise fashion with the left micropipette.
Using a dual pipette aspiration assay, the relative contribution of contractility and adhesion to cell-cell contact formation can be measured in vitro. See Adhesion functions in cell sorting by mechanically coupling the cortices of adhering cells
Using a dual pipette aspiration assay, elastic properties of the actomyosin cortex can be estimated in vitro. See Active elastic thin shell theory for cellular deformations.
