(772g) The Role of Actomyosin Generated Tension in Coordinating Cell Movements During Zebrafish Development

Introduction

Cells engage in collective
migration in circumstances ranging from wound healing to embryonic development.
While much is known about how single cells migrate, it is unclear how groups of
cells coordinate movements across large distances. In this study we use
zebrafish embryos as a model system to investigate the role of cell-generated
line tensions in generating long-range, coordinated motion. Early zebrafish
development exhibits intricately orchestrated cell migration and patterning.
Within the first 24 hours a mass of cells termed the blastoderm spreads
uniformly over a spherical yolk, starting at one end and moving toward the
opposite pole in a process termed epiboly. During epiboly, the cells at the blastoderm
margin (edge) migrate at a uniform rate and thus reach the opposite pole of the
yolk at the same time (Figure 1A-C). This long-range coordination is
essential in establishing the axes that define the front, back, top and bottom
of the resulting organism. A band of actin and myosin, termed the actin band,
forms near the blastoderm margin. It has been previously shown that the actin
band produces a contractile tension; however its functional role is not known.

Results

We used
mechanical shape perturbations to probe the function of the actin band during
epiboly. In shape-perturbed embryos, we find that the rate of blastoderm
migration is no longer uniform: lagging areas of the margin speed up, while
leading areas slow down. This equalizes progress along the blastoderm margin
perimeter such that epiboly reorients to coincide with the long axis of the
embryo. When the tension within the actin band is disrupted using chemical
treatments such as blebbistatin or reduction of calcium this reorientation no
longer occurs. This observation suggests that the tension within the actin band
is a necessary component in coordinating blastoderm migration along the length
of the margin. We additionally observed that cells along the blastoderm margin
align parallel to the actin band. Treatment with either blebbistatin or calcium
depletion resulted in a markedly rougher margin, suggesting that the actin band
additionally coordinates motion between neighboring cells.

Discussion

Based upon these
results, we developed an analytical model in which the actin band acts like a
pre-tensioned rubber band, introducing a line tension that coordinates blastoderm
migration over millimeter length scales (Figure 1D). This line tension also
provides a resistive force against instabilities at the margin of the migrating
blastoderm, ensuring that neighboring cells progress at similar rates (Figure
1E). Our model is in quantitative agreement with margin migration rates observed
in a wide variety of embryo shapes, suggesting that a simple physical parameter
(actin band tension) is sufficient to account for the long-range cell coordination
that we observe. The physical properties of the zebrafish blastoderm margin are
similar to those of spreading cell sheets observed in a wide variety of
circumstances. We suggest that similar quantitative, physical models may account
for collective cell movements observed in a wide variety of circumstances including
other developmental processes, wound healing, and a variety of tissue
engineering applications.

Figure 1. Mechanical tension leads to long-range coordination
between blastoderm cells in the zebrafish embryo. A-C) Progression of
epiboly (green ? blastoderm, yellow ? yolk, orange ? actin band). A)
Epiboly begins with the blastoderm sitting on one pole of the yolk cell. B)
The blastoderm migrates over the yolk cell until it finishes epiboly by
converging at the pole opposite the initial blastoderm pole (C). D-F)
Description of the roles of actomyosin generated line tension. D) Line
tension (black line) along the boundary of a migrating cell mass (green) generates
local forces (blue) that reorient the cell boundary. E) Zoomed in view
of an instability at the boundary (red square in D). Tension at the margin
(black dotted arrows) generates a net restorative force (black solid arrow). F)
As a result, the cell mass reorients and the boundary maintains a smooth
profile that is parallel to the actin band.