Organisms are composed of a variety of structures including muscles, internal organs, and brains, all of which are created through a process known as self-organization. In a study published in the online journal, Cell Reports, the research team examined how the spindle apparatus self-organizes. Composed of fibrous molecules called microtubules, this structure is responsible for the segregation of chromosomes between daughter cells.
Researchers around the world are interested in the mechanisms of spindle formation because if chromosome segregation does not take place correctly in human cells, the process can cause cancer or birth defects. Previous studies have identified molecular motors and a range of other molecules involved in spindle formation. But certain fundamental data remain missing, particularly concerning the relationship between the amount of microtubules and the size and shape of spindles.
Using fluorescence microscopy, Jun Takagi and his colleagues at Waseda University observed self-organizing spindles from the eggs of aquatic frogs. Based on their observations, the team derived a simple mathematical model describing the relationship between the size and shape of the spindle apparatus and the density and amount of microtubules. This successful characterization of the key parameters that determine spindle structures during self-organization is particularly useful in understanding the physical mechanisms of ‘self-organization’ in orderly structures.
We also established a technique for cutting a spindle into two halves using glass microneedles (
■ Results and conclusions drawn from the study
Our 3D observation of metaphase spindles that self-organized in Xenopus egg extracts revealed that spindle shape and microtubule density were constant irrespective of spindle size, whereas spindle size was correlated with the microtubule amount. We quantitatively defined the spindle shape and the microtubule density, based on which we successfully derived a simple equation describing the relationship among all the parameters. In this equation, spindle size is explained by microtubule amount in addition to the other parameters that are independent of spindle size (i.e., the spindle shape and microtubule density).
When a spindle was cut into two fragments using glass microneedles, each fragment regained its original spindle shape and microtubule density within five minutes of cutting (Figure 3). This indicates that the independent associations between spindle size and spindle shape or microtubule density was maintained even in the cut fragments. Regarding the microtubule amount in each fragment, it was reduced by half or more due to cutting and, at the same time, each fragment became smaller than the original spindle. These findings again indicate preservation of the correlation between spindle size and microtubule amount in the cut fragments. Furthermore, when two cut fragments were allowed to contact each other, they fused together and eventually became a single spindle resembling the one before cutting (Figure 3).
These results demonstrate that spindle size is correlated with microtubule amount, and that spindle shape and microtubule density are dynamically maintained and unaffected by the physical intervention of ‘cutting.’■ Potentially universal effects and social significance of the study
Journal informationThe study reported here was conducted with the financial support of a Grant-in-Aid for Scientific Research. The results have been published in an article entitled ‘Using micromanipulation to analyze control of vertebrate meiotic spindle size’ in Cell Reports, an online journal from Cell Press.
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