# Evidence for an Upper Limit to Mitotic Spindle Length

## Reference

Evidence for an Upper Limit to Mitotic Spindle Length

Wuhr M, Chen Y, Dumont S, Groen AC, Needleman DJ, Salic A, Mitchison TJ.

Current Biology 18: 1256-1261 (2008)

PMID: 18718761 (Pubget)

## Background

How living systems control the sizes of their various components is, in many instances, an open question. Complex multi-component assemblies within cells are often capable of adjusting their size in the face of changing cell geometries, but the mechanism for this sensing remains a mystery. For example, a finely coordinated process during cell division (meiosis and mitosis) physically separates the condensed chromosomes into two sets. The work is done by a mollecular machine called the mitotic spindle. The challenge faced by the mitotic spindle is as follows: while the size of the cell can change by orders of magnitude, for any given organism the size of the macromolecules being sorted remains constant. So what does the spindle do? Does it change its size, and if it does, what is the change a function of?

In vivo assembly of mitotic and meiotic spindles.

## Experiments

This paper uses the development of fertilized Xenopus laevis eggs into tadpoles to study the relatioship between spindle size and cell size. The choice of this experimental system was justified by both the fact that the sizes of cells changed drastically during development (from 1,200 um to 12 um), and because the size of the eggs makes them easier to manually handle. Embryos at different stages of development were fixed with methanol, dehydrated, disected, and labeled with an anti-$\alpha$ tubulin antibody. The data showed that the spindle size increases with cell size in cells smaller than ~300 um. The spindle size of even larger cells seemed to assymptotically approach ~60 um. This data suggest that the spindle size is determined partially by mechanisms independent of the cell size. An alternative, and potentially complementary, hypothesis is that a limited number of some critical mollecule sets an upper limit on the spindle size - with tubulin coming to mind first.

This paper also contains the first published instance of a mitotic spindle assembly occuring in vitro. In such an experiment the spindle is "naked", allowing its study outside of the confounding impacts of the rest of the cellular machinery. The impressive feat of giving mitotic extract spindles was loosely inspired by prior work on meiotic extract spindles, and involved the use of the anaphase-promoting complex (APC) inhibitor Emil. This technique was used to show that the mitotic spindle lenght decreases by about 10% if the number of chromosomes is halved through using haploid cells. This observation suggest that DNA is a minor factor in the determination of spindle lenght.

## Soft Matter Aspects

A mitotic spindle apparatus consists of microtubules, centrosomes, and some associated proteins. Microtubules are polymers composed of dimers of $\alpha$- and $\beta$- tubulin. The microtubule filaments are polarized, with the end containing the $\alpha$ designated as the minus end. During mitosis, each minus end is attached to a centriole, while the plus end extends towards the chromosomes. The authors have argued in a prior paper that the geometry of the spindle is determined by the balance between the motor transport of tubulin and the the shortening of microtubules due to nucleation.