Evidence for an upper limit to mitotic spindle length
Entry by Andrew Capulli, AP225 Fall 2011
Martin Wuehr, Yao Chen, Sophie Dumont, Aaron Groen, Daniel J. Needleman, Adrian Salic, Timothy J. Mitchison, Evidence for an Upper Limit to Mitotic Spindle Length. Current Biology, 2008, 18, 1256-1261
All living organisms have, at the very least, one thing in common: DNA. Although relative amounts of DNA and chromosomes vary among species (bacteria have a pair of chromosomes while humans have 23 pairs and monkeys have 24 pair for example), DNA contains the coding for future life in any living organism (Note: DNA's role in transcription, translation, protein formation: see wikipedia entry on the "Central Dogma of Molecular Biology": http://en.wikipedia.org/wiki/Central_dogma_of_molecular_biology). It begs the question then, how, if all species are based upon their DNA and we all have DNA, do species scale? Perhaps more clearly: why are certain species large and others small? Does cell size have anything to do with scaling? As discussed in lecture, it is often the goal of physicists to find the length scale; in the world of log-log plots, the slope of those plots is scaling and the rationalization of that slope is the true science. What Needleman et. al. begin to do is address this scaling question in terms of the human body, mitosis spindle fibers, and DNA. The question of scaling the human body as a whole may seem extreme right now, but the authors of this experimental paper have begun to break the ice on the fundamental unit of life: the cell.Needleman et. al. do most of their work in this study investigating the length of mitotic spindle fibers as they scale with cell size. Using Xenopus Laevis cells (a type of frog with large cells easier to study than say, smaller human cells), the authors essentially measure spindle length as it varies with maximum cell length during different stages of mitosis. Further (and what I beleive to be more interesting and maybe conclusive) studies varying DNA amounts in the cells and observing how spindle fiber length scales are done and will be discussed more below.
Summary of Main Experimentation
The purpose of this study was to identify how spindle fibers in mitosis scale with cell size. The Xenopus Laevis cells vary in cell size during mitosis ranging about 10um to about 1300um (cell sized as defined in this study is the maximum distance across an elliptical cell- pole to pole of a mitotic cell). Using Xenopus Laevis cells was therefore advantageous, giving the research team a large variety of cell sizes to examine. Using standard methanol fixation and immunofluorescence, microscopy images like those below (Figure 1) were taken and spindle length calculated as a function of cell size. As can be seen, at different stages in the cell mitosis, spindle fiber length and cell size vary (graphical analysis shown and discussed below). (C) in Figure 1 shows Mitosis 2 in where the image on top shows the cell size and the image below shows the spindle size. Notice that these two measures are drastically different in Xenopus Laevis cells; there appears to be some sort of limit to the spindle length (despite the far greater cell length/size).
More revealing are the data graphically depicted (see second part of Figure 1 below). As the graph shows, each color represents a different stage of cell division and, for the purpose of discussion, each color represents an approximate cell length or size associated with that stage of mitosis. For the purpose of discussion, I will use cell size instead of stage of mitosis for making observations but, as can be seen by the data, the cell size and stage of mitosis can be roughly correlated. As the cell grows (when still relatively small with respect to the whole range of sizes), the spindle lengths grow some what linearly; may quick calculations from the graph say that the spindle is about 60% as long as the cell in this range of the data. However, as cells continue to grow, the spindle length appears to hit an asymptote of about 60um as seen in the graph and claimed by the authors. This is the interesting data: while the cells continue to grow, the spindles do not. Question begin to arise some even by the authors such as if the spindles do not scale with the cell size, how do they 'know' where to align (in the center of the cell)? What is the connection between cell size and spindle size then (ie, which governs which?)