How does a millimeter-sized cell find its center?

Abstract

Microtubules play a central role in centering the nucleus or mitotic spindle in eukaryotic cells. However, despite common use of microtubules for centering, physical mechanisms can vary greatly, and depend on cell size and cell type. In the small fission yeast cells, the nucleus can be centered by pushing forces that are generated when growing microtubules hit the cell boundary. This mechanism may not be possible in larger cells, because the compressive force that microtubules can sustain are limited by buckling, so maximal force decreases with microtubule length. In a well-studied intermediate sized cell, the C. elegans fertilized egg, centrosomes are centered by cortex-attached motors that pull on microtubules. This mechanism is widely assumed to be general for larger cells. However, re-evaluation of classic experiments in a very large cell, the fertilized amphibian egg, argues against such generality. In these large eggs, movement of asters away from a part of the cell boundary that they are touching cannot be mediated by cortical pulling, because the astral microtubules are too short to reach the opposite cell boundary. Additionally, Herlant and Brachet discovered a century ago that multiple asters within a single egg center relative to the cell boundary, but also relative to each other. Here, we summarize current understanding of microtubule organization during the first cell cycle in a fertilized Xenopus egg, discuss how microtubule asters move towards the center of this very large cell, and how multiple asters shape and position themselves relative to each other.

Figure 1

Cartoon of a frog egg shortly after fertilization. The vegetal part (bottom) is heavily filled with yolk. The sperm enters randomly at the animal (top) part of the egg. The radial grow of the sperm aster leads to the movement of the centrosome towards the cell’s center. All microgrographs in this paper were taken in the plane shown.

Figure 2

Overview of microtubule organization during the first cell cycle in X. laevis. Top: Immunostaining against tubulin, bar = 500 μm. Arrows indicate positions of centrosomes. Time (t) is normalized to first cleavage. Bottom: cartoon of corresponding time with the path of the centrosome in red and microtubules in black. A) and B) Growth of sperm aster moves centrosome towards the center of the cell. C) After sperm aster breaks down first mitotic spindle forms D) At telophase the astral microtubules grow out and the centrosomes are moved to the centers of the future daughter cells.

Figure 3

Models on how asters could find the center of a very large cell. A) In the simple pushing model the force that can be transmitted via microtubules is inversely proportional to the square of their length. B) In the pushing with stiffened microtubules meshwork model the aster is stiffened via bundling, crosslinking or embedding in an elastic gel . Microtubules can transfer relevant pushing forces over long distance. C) In the pulling on the cortex with limited attachment sites model minus end directed motors are saturated with microtubules. The sum of forces on the centrosome points towards the cell’s center. D) In the pulling on the cytoplasm model minus-end directed motors are attached to a component of the cytoplasm (e.g. yolk or cytoskeleton). The longer a microtubule, the more motors are pulling which leads to centering.,

Figure 4

How do asters notice each other? A) Sperm asters in polyspermic embryo space each other apart creating microtubule-sparse regions between them. Reprint by Herlant and Brachet (1910), kind permission of Springer Link. B) Immunostaining of telophase in a di-spermic embryo with magnification of region between asters. Bars are 500 μm and 50 μm. C to E: Models that could explain how asters could notice each other: C) Multivalent plus-end directed motors push asters apart. D) A physical barrier is created between asters. E) Orientation-dependent microtubule depolymerizer chews up microtubules that enter with opposing polarity.