How Does A Millimeter-Sized Cell Find Its Center?
When mitosis occurs, the nucleus is at the center of the cell, and the two daughter cells split symmetrically. But how does the nuclues find the center of the cell? While there seems to be reasonable explanations for small and intermediate sized cells, the mechanism for large (millimeter) sized cells is unclear. This paper proposes a few possibilities.
Below are immunostaining pictures of the egg of a clawed frog Xenopus laevis during its first cell cycle after fertilization. At time t=0 the sperm enters the egg, and at time t=1 the first cleavage occurs.
The sperm upon entering the cell carries with it a centrosome from which a radial network of microtubules grow. This network, called the sperm aster, somehow moves the centrosome towards the center of the cell. When a microtubule touches the female nucleus, it is somehow pulled to the center of the aster. Thus by the end of the sperm aster growth the genetic material of both the sperm and the egg are at the center of the cell. The sperm aster breaks down and the mitotic spindles are formed. Then two astral microtubules begin to grow and move towards the centers of the new cells, bringing the genetic material of the daughter cells with them. This paper studies how the asters are able to locate the center of the cell.
Finding the Center: Proposed Models
The paper discusses four possible methods by which the asters can find the center of the cell.
A. Simple Microtubule Pushing According to this model the microtubules grow out radially from the aster center until they reach the cell boundary. At the boundary, they keep growing, thereby pushing the aster center away. Therefore the side of the cell where the aster starts out will have many microtubules touching that edge, and they will push the center away. A microtubule is only able to sustain a certain amount of force due to buckling, and this force is proportional to <math>1/L^2</math> where L is the length of the microtubule. Another way to explain how the aster centers might be that the center might have microtubules attached to different parts of the cell boundaries, but the microtubules that are longer aren't able to push as hard as the shorter microtubules, so the center moves away from the cell boundary it is closest to and zeroes in on the center.
While this model is believed to be the mechanism by which aster centering occurs in small cells such as fission yeast cells, the model doesn't work as great for larger cells like the frog egg. The length the microtubules must grow for these cells is larger, and so the pushing force is very weak. The cell would need a very large amount of microtubules to use this mechanism to push the aster to the center, and this is not observed.
Pushing with a stiffened microtubule meshwork