Dating groups yolk
To identify its underlying mechanisms, biologists at the Institute of Science and Technology Austria (IST Austria) teamed up with their colleagues from theoretical physics.
Within these two hours, the cytoplasm, which will later form the animal body, must separate completely from the yolk, which the larva is going to feed on.
Previously, cell biologists had proposed that local expansion of the cell surface at one pole of the egg mediates this segregation.
Instead, they are actively pushed, or rather squeezed, towards the opposite vegetal pole of the egg by comet-like actin structures -- particular actin structures whose function had not been reported in developmental processes before.
The combination of these pulling and pushing forces ensures a robust segregation of the cytoplasm and yolk granules within the developing embryo.
In the bulk of the cell, far away from the cell surface, filaments of actin and myosin -- proteins also involved in muscle cell contraction -- form a dense mesh.