Microtubule organization within mitotic spindles revealed by serial block face scanning EM and image analysis

Serial block face scanning electron microscopy (SBF-SEM) is a powerful method to analyze cells in 3D. Here, working at the resolution limit of the method, we describe a correlative light-SBF-SEM workflow to resolve microtubules of the mitotic spindle in human cells. We present four examples of uses for this workflow which are not practical by light microscopy and/or transmission electron microscopy. First, distinguishing closely associated microtubules within K-fibers; second, resolving bridging fibers in the mitotic spindle; third, visualizing membranes in mitotic cells, relative to the spindle apparatus; fourth, volumetric analysis of kinetochores. Our workflow also includes new computational tools for exploring the spatial arrangement of MTs within the mitotic spindle. We use these tools to show that microtubule order in mitotic spindles is sensitive to the level of TACC3 on the spindle. Jo ur na l o f C el l S ci en ce • A dv an ce a rt ic le Introduction In readiness for cell division, eukaryotic cells build a mitotic spindle. This miniature machine, made of microtubules (MTs) and associated proteins, maneuvers the duplicated chromosomes and segregates them so that the two daughter cells each receive a complete copy of the genome. Understanding how this MT-based structure is organized and how it works is a major theme in cell biology (Petry, 2016). In human cells, the mitotic spindle is a fusiform structure, which at metaphase is virtually spherical. MTs radiate from the two spindle poles, in one of three classes. 1) Astral MTs extend from the poles back toward the plasma membrane; 2) Interpolar MTs run from one pole toward the other pole; 3) Kinetochore MTs which are attached to the kinetochore on the chromosome (Mastronarde et al., 1993; McDonald et al., 1992; Helmke et al., 2013). Kinetochore MTs are bundled together to form kinetochore fibers (K-fibers) and at metaphase, 20–40 MTs constitute a single K-fiber bundle (Booth et al., 2011, 2013; Sikirzhytski et al., 2014; McEwen et al., 1997). A network of crosslinking material, termed the “mesh”, is thought to hold kinetochore MTs together throughout their length (Hepler et al., 1970; Nixon et al., 2015). So far one protein complex containing TACC3, clathrin and ch-TOG has been identified as being a component of the mesh (Booth et al., 2011; Nixon et al., 2015). The MTs of the mitotic spindle are densely packed and so resolving their organization in 3D is challenging. Advances in super-resolution imaging and expansion microscopy have improved the view of mitotic MTs by light microscopy (Mikhaylova et al., 2015; Chozinski et al., 2016), yet the gold standard method is still electron microscopy (EM). Reconstructions of entire spindles in 3D using serial sectioning and transmission electron microscopy (TEM) have told us much about mitotic spindle structure (Mastronarde et al., 1993; McDonald et al., 1992; Redemann et al., 2016). However these methods are extremely laborious, technically challenging, and prone to error. The advent Jo ur na l o f C el l S ci en ce • A dv an ce a rt ic le of automated EM methods, such as serial block face scanning electron microscopy (SBF-SEM) has promised to speed up the analysis of complex cellular structures in 3D (Hughes et al., 2014). The spatial resolution of SBFSEM is reported to be around 10 nm, making it ideal for the study of MTs (25 nm diam.) in the mitotic spindle. Whether this resolution can be reached routinely for such analysis is unclear. The first aim of the present study was to apply SBF-SEM to the problem of resolving MTs in the mitotic spindle. The resulting datasets are large and require robust statistical methods for analysis and our second aim was therefore to provide computational tools to aid understanding of such datasets. The resulting workflow was then used to further understand how TACC3 levels influence MT organization in mitotic spindles. Results Visualization and 3D reconstruction of mitotic spindle MTs We began by finding the optimal conditions for imaging MTs in mitotic cells by SBF-SEM. These conditions, described in the Methods section, allowed us to image a volume containing almost the entire spindle and to visualize bundles of MTs in the spindle running from the pole to the chromosomes (Figure 1). The resulting datasets could be segmented, rendered in 3D, and analyzed computationally (Figure 1A and Methods). This workflow resulted in the visualization of a 3D rendering of the mitotic spindle (Figure 1B,C). Even under these optimized conditions, we were working at the resolution limit of the instrument and this hampered our ability to resolve every single MT in the mitotic spindle (see below). Where single MTs could be discerned we were unable to resolve the ends of MTs (Figure 1B). Such resolution is possible with TEM and tomography, however those methods are incredibly labor-intensive. While SBF-SEM has limitations, our simple and rapid workflow allows the accurate mapping of mitotic spindle architecture sampled beyond the resolution limit of light microscopy; and to do this with much higher throughput. Jo ur na l o f C el l S ci en ce • A dv an ce a rt ic le Limitations of MT mapping by SBF-SEM Since the images were captured at the resolution limit of the instrument, a key question is whether or not we have single MT resolution. To look at this, we focused on astral MTs which are known from serial section TEM to exist as single, isolated MTs. Figure 2A shows that clear density, attributable to individual astral MTs, can be identified in our images and easily segmented. This observation indicates that the method has single MT resolution. We expected the detection of single MTs in regions of low MT density to be easier than discerning single MTs in the main body of the spindle. To examine MT detection in more dense regions, we counted MTs which terminate at kinetochores and compared these numbers with previous observations by TEM. On average we resolved 6 MTs terminating at the kinetochore (median, IQR = 5, 4 – 9, cell = 4 N ) (Figure 2B-D and Supplementary Movie 1). This varied somewhat between datasets, from 4 (IQR = 3.5, 1.5 – 5) to 8 (IQR = 4, 6 – 10), but is fewer than the 20-23 MTs reported at metaphase by serial section TEM (McEwen et al., 1998, 1997). Comparing the number of MTs at a given kinetochore with the number terminating at its sister, showed that they were broadly comparable (Figure 2D). If the number of MTs at kinetochores is quite invariant, this suggests that local differences in MT detection underlie the variability within a dataset. These data indicate that in higher density regions of the spindle, we do not have single MT resolution. We conclude that detection of single MTs is possible, but that MT detection is not uniform throughout the volume. The sample of MTs that can be resolved is nonetheless substantial and sufficient for further analysis. Jo ur na l o f C el l S ci en ce • A dv an ce a rt ic le Spatial statistics of MTs visualized by SBF-SEM The large-scale high-resolution view of MTs resulting from our workflow was well-suited for the development of geometric models to describe mitotic spindle organization. These methods and associated code are detailed in the Methods section, and an overview is presented here. First, the volumetric density of MTs in the spindle could be determined. As an input, we take the segmented MTs from SBF-SEM data and this is used to calculate the total volume of MTs. This value is then normalized to the volume of spindle which is visible in the dataset (Figure 3A). This normalization step is essential because at the magnification necessary to resolve MTs, the spindle volume is often incomplete. To test the sensitivity of this volume density calculation, we compared cells treated with cold ( 4 C ) versus warm (37 C ) media. When cells are treated with cold media, any MTs which are not stably attached at both ends become depolymerized leaving only the Kfibers intact (Rieder, 1981). In HeLa cells and in two different cell lines derived from Glioblastoma patients, we measured a decrease in MT density to approximately one-third of the volume recorded in control cells, in warm media (Figure 3B). Second, the spatial organization of MTs in the spindle is analyzed. The aim is to determine the order – or degree of alignment – of MTs in the mitotic spindle. To do this, an idealized spindle is generated computationally using the coordinates of the segmented MTs, and simple geometric principles (Figure 4 and Methods section ). The measured angle between each MT segment and its idealized counterpart is calculated ( ( ) i  in Figure 4A). For an ideal mitotic spindle, in which all microtubules are aligned, all angles will equal 0 . For real mitotic spindles, the angles deviate considerably from the ideal. Therefore, the magnitude of the angle deviations from the idealized spindle indicate a departure from perfect alignment. For example, the microtubules in the control Jo ur na l o f C el l S ci en ce • A dv an ce a rt ic le GFP-expressing cell shown throughout this paper, have a median angle of 11.2 (IQR = 14.8, 5.2 – 20.0 ) (Figure 5A). Previously, we found that overexpression of TACC3 caused changes in the degree of alignment of MTs (Nixon et al., 2015). These conclusions were based on tomography of a single thin section cut orthogonally to the K-fiber axis. Our new statistical method meant that this comparison could now be done on a near-global, whole-spindle scale. To look deeper into this phenomenon, we wanted to increase or decrease the TACC3 levels on the spindle and examine changes in MT order. Therefore, in addition to GFP-expressing control cells, we processed cells conditionally overexpressing GFP-TACC3 and also cells where the endogenous TACC3 was depleted by RNAi and a mutated form (S558A) of GFP-TACC3 that was refractory to RNAi was expressed. This mutant cannot bind clathrin and therefore cannot localize to the mitotic spindle (Hood et al., 2013). These three genotypes were examined in both warm and cold conditions (Figure 5C and Supplementary Figure 1). Deviations from the ideal were compared using 1 2 ( , )    for all combinations of data (see Methods section for further details). This comparison of angle deviations from these datasets showed that mitotic spindles in cells with more TACC3 had less order than controls, under warm conditions (Figure 5A and 5C). Similar deviations were seen in the cells expressing TACC3(S558A) in place of endogenous TACC3. This analysis extends, and is in agreement with, our previous conclusion that MT order in mitotic spindles is influenced by TACC3 levels on the mitotic spindle (Nixon et al., 2015). The comparison between warmand cold-treated samples revealed that MTs in cells expressing TACC3(S558A) in place of endogenous TACC3 were more disorganized after cold treatment. Interestingly, the cold-treated cells overexpressing TACC3 did not show the same phenotype. The order of MTs in this condition was similar to cold-treated GFP-expressing cells, which may suggest that elevated TACC3 levels protect MTs against cold-induced changes in spatial organization. Jo ur na l o f C el l S ci en ce • A dv an ce a rt ic le Volumetric analysis of kinetochores SBF-SEM datasets also allowed the segmentation of kinetochores, and therefore the calculation of their volume (Figure 6). The average volume of a kinetochore was 6 3 8.17 0.3010 nm  (mean  s.e.m.). The surface area of the outer face of the kinetochore, as measured by serial section TEM is 2 0.16 0.05μm  (McEwen et al., 1998), which would mean a thickness of 50 nm. This is very similar to the thickness in our segmented images, indicating agreement and accurate quantification. Since the pairing of kinetochores can be easily determined in the dataset, we tested the idea that sister kinetochores have similar volumes and that kinetochore volumes vary across the dataset. A plot of sister kinetochore volumes is shown in Figure 6C. One-way ANOVA indicated that there were significant differences between pairs of kinetochores, F(33,34) = 1.918, 12 M SE 4.6 10    , p = 0.032. Randomization of the data and retesting by one-way ANOVA gave further confidence to this result (Figure 6D). These findings suggest that large kinetochores are paired with large kinetochores and small with small. Two further observations from SBF-SEM of mitotic cells: spindle encapsulation and bridging fibers In addition to the mitotic spindle and associated chromosomes, the membranes of mitotic cells could also be rendered in 3D. This allowed us to explore the relationship between intracellular membranes and the mitotic spindle. We found that the mitotic spindle was situated within an “exclusion zone” which was largely free of membranes and organelles (Figure 7, Supplementary Figure 2 and Supplementary Movie 2). Outside of this zone, the endoplasmic reticulum (ER) was densely packed and mitochondria were distributed within the folds of ER. These observations agree with previous work which described the organization of ER membranes in mitotic cells (Puhka et al., 2007, 2012) and with more recent work which proposed that although mitosis in human cells is open, the spindle is encapsulated by membrane Jo ur na l o f C el l S ci en ce • A dv an ce a rt ic le (Schweizer et al., 2015). The purpose of this encapsulation is to exclude organelles and concentrate the proteins necessary to build a mitotic spindle rapidly (Schweizer et al., 2015). Whilst studying the organization of MTs in the mitotic spindle, we also had the opportunity to look for the presence of “bridging fibers”. These small bundles of MTs have been proposed to span from one K-fiber emanating from one pole, across the sister kinetochores and reach the K-fiber which emanates from the opposing pole (Kajtez et al., 2016; Tolic and Pavin, 2016). Close inspection of our segmented datasets showed the presence of structures which may correspond to bridging fibers. Examples are shown in Supplementary Movie 3 and a single example is shown in Figure 8. We could not find evidence for bridging fibers associated with every K-fiber pair, although this is probably because our coverage is incomplete rather than an indication that there is not a one-to-one association of bridging fibers with each K-fiber pair. Discussion SBF-SEM is commonly used to examine cellular and multicellular structures on the scale of tens to hundreds of micrometers. Whether or not this mode of imaging could discern small subcellular structures such as MTs (25 nm diam.) in complex networks such as the mammalian mitotic spindle was untested. Our results indicate that it is possible to visualize a large sample of MTs in the mitotic spindle of a human cell. The resulting datasets were complex and required robust analytical tools which were developed and presented as part of this paper. We applied this workflow to test the influence of TACC3 levels at the mitotic spindle on the organization of MTs on a whole-spindle scale. Jo ur na l o f C el l S ci en ce • A dv an ce a rt ic le The influence of TACC3 on mitotic spindle structure has previously been examined using two extreme methods. In the first, individual high magnification views of subsections of the spindle have been viewed by electron tomography (Nixon et al., 2015). Second, low spatial resolution views of whole spindles were captured by light microscopy (Hood et al., 2013; Gergely et al., 2000). What was missing was a technique which combines the advantages of both methods to visualize MT order on the scale of the whole mitotic spindle. Superresolution methods cannot currently fulfill this gap, and the present study shows that SBF-SEM can; and does so at an appropriate scale to answer questions about MT organization. Moreover, the automation of the image capture and data processing means that it is practical to analyze spindles in this way. While the resolution of this method is lower than TEM and electron tomography, its power lies in the higher throughput and lower requirement for technical expertise. SBF-SEM is not suitable for answering questions that absolutely require single MT resolution and particularly to resolve MT ends (McIntosh et al., 2013), however it can compete with TEM and electron tomography. We showed that 35 pairs of kinetochores could be easily segmented from a single SBF-SEM dataset, a number sufficient to test whether or not sister kinetochores were of a comparable size. Capturing a similar dataset by serial section TEM would be extremely challenging and laborious, with no clear resolution advantage in this case. Our analytical workflow allowed us to assess MT density and MT order. While it is possible to approximate the volume of MTs in cells using light microscopy, the higher resolution of SBF-SEM provides more precise quantification of the MT volume in a mitotic cell. Determining the order of MTs in a dense array is not currently possible by light microscopy even in superresolution modes and the advantage of SBF-SEM here is therefore clear. The simple description that we developed was based on comparison with an Jo ur na l o f C el l S ci en ce • A dv an ce a rt ic le idealized spindle. We found that ellipsoids were a good model for angular comparison, out-performing other geometric shapes and comparison regimes based on near-neighbors or on the spindle axis (see Methods). Our data indicate that too much TACC3 (or too little) affects MT order in mitotic spindles. Previous work has shown that mitotic progression is slowed in cells overexpressing TACC3 or in cells depleted of TACC3 using RNAi (Nixon et al., 2015; Gergely et al., 2003; Lin et al., 2010). The current picture is that the major function of TACC3 in mitotic spindles at metaphase is as part of a TACC3–ch-TOG–clathrin complex that is assembled by Aurora-A phosphorylation, rather than as a MT +tip protein. The TACC3–ch-TOG– clathrin complex is an important component of the “mesh” which is thought to maintain the integrity of the K-fiber MTs. The present data indicate that the sensitivity of mitosis to TACC3 levels is because of changes in the composition of the mesh which alter K-fiber structure and organization; it strongly suggests that these changes prevent the chromosome movements required for highfidelity mitosis. However, we cannot exclude the possibility that other functions of TACC3 contribute to MT order or to mitotic progression. These 3D views of mitotic cells also allowed us to visualize the spindle exclusion zone and putative “bridging fibers” of the mitotic spindle. Clearly distinguishing bona fide bridging fibers from other microtubules is challenging, even at this resolution. Some examples presented here clearly look like bridging fibers which run in parallel to the respective sister K-fibers (Kajtez et al., 2016; Tolic and Pavin, 2016). Others highlighted in Supplementary Video 3 were less clear. If these structures are not bridging fibers, they could be interpolar MTs, or K-fiber MTs which are laterally or merotelically attached to kinetochores (Cimini et al., 2001). A formal test to determine if these MT bundles are associated with the sister K-fibers will require TEM analysis. Jo ur na l o f C el l S ci en ce • A dv an ce a rt ic le Analysis of MT order by SBF-SEM requires operating at the resolution limit of the instrument. We optimized the staining of the sample, section depth, the magnification and accelerating voltage/beam energy to be able to visualize MTs. This protocol allowed us to sample a large fraction of MTs in the spindle, but fell short of a complete view of the mitotic spindle for three reasons. First, the spindle volume was incomplete since the magnification required to see MTs resulted in an XY area of 9.2 x 9.2 μm . Although this limitation can be circumvented by stitching fields-of-view for each slice. Second, we could resolve single MTs in parts of the dataset, however in more dense regions, we were unable to resolve all MTs. For example, we found fewer than expected numbers of MTs terminating at each kinetochore. Third, the section depth of 60 nm resulted in missing MT information. The step size matched the beam penetration depth, because heavily stained objects such as membranes could be accurately followed with no underor over-sampling. However, thin objects such as MTs that are weakly stained appear discontinuous, presumably because they are only visible at the upper surface of the block. Note that the spindle exclusion zone actually improves MT visualization because the organelle-free zone does not interfere with MT detection and segmentation. Thinner sectioning, possibly using focused ion beam-SEM, would improve visualization further. For these reasons, one caveat associated with the method presented here is that the analysis is performed on only a subset of the MTs in the mitotic spindle. We believe that this subset is a good representation of the mitotic spindle: it was sufficient to show differences in MT density and changes in MT order in the mitotic spindle. Future directions for better detection of MTs in mitotic cells include improved SEM detector technology and high pressure freezing as a fixation method. It remains to be seen if any of these innovations would allow full reconstruction of the mitotic spindle. While improvements in detection may allow greater coverage, the protocol presented here is sufficient Jo ur na l o f C el l S ci en ce • A dv an ce a rt ic le for large-scale analysis of MT order in human cells; and in higher throughput than traditional EM methods. Materials and Methods Cell biology HeLa cells which overexpress TACC3 upon doxycycline induction were described previously (Nixon et al., 2015). Cells were maintained in Dulbeccoâ€TMs Modified Eagleâ€TMs Medium (DMEM) plus 10% fetal bovine serum (FBS) and 1% penicillin/streptomycin, in a humidified incubator at 37 C and 5% CO2. The cell culture medium was further supplemented with G418 ( 1 300μg ml ) for the parental HeLa TetOn cells, and with G418 and Hygromycin B ( 1 200μg ml ) for the HeLa TetOn cells stably transfected with the TACC3 plasmid. Glioblastoma cell lines were cultured using Neurobasal medium, supplemented with B27, N2, human fibroblast growth factor ( 1 40ng ml ) and human epidermal growth factor ( 1 40ng ml ). For experiments to express GFP-TACC3(S558A), we used a plasmid (pBrain-GFP-TACC3(S558A)KDP-shTACC3) which knocks down the expression of endogenous TACC3 and simultaneously expresses GFPTACC3(S558A) which is refractory to the shRNA (Gutiérrez-Caballero et al., 2015). Cells were transfected using GeneJuice, according to manufacturerâ€TMs instructions. Three HeLa conditions were studied: first, parental HeLa TetOn cells transiently transfected with pEGFP-C1 only as a control; second, the inducible GFP-TACC3 HeLa TetOn stable cell line; third, parental HeLa TetOn cells transiently transfected with pBrain-GFP-TACC3(S558A)KDP-shTACC3 construct. Cells were synchronized by thymidine-Ro3306 treatment (Nixon et al., 2015) and grown on gridded dishes (MatTek) to allow correlation between light and electron microscopy. Synchronization using thymidine-Ro3306 is preferred to nocodazole or similar drugs which perturb microtubules. For cold Jo ur na l o f C el l S ci en ce • A dv an ce a rt ic le treatment of mitotic cells, cells were imaged to ensure the cells were at metaphase, the growth media removed and replaced with ice-cold growth media and incubated on ice for 10 min. Media was then replaced with cold fixative. Electron Microscopy Fluorescence imaging was done using a Nikon Eclipse Ti-U microscope with standard filter sets for visualization of GFP. Light and epifluorescence micrographs were acquired using a Photometrics Myo camera, 20x air and 60x oil objectives, and NIS Elements acquisition software. Cells were imaged in a 37 C temperature-controlled chamber (OKOlab). Chemical fixation, processing and sectioning for TEM, was done as described previously (Booth et al., 2013). To prepare samples for SBF-SEM, the cells were also processed in a correlative manner to ensure only metaphase cells expressing the protein of interest were chosen. Processing was as described with the following extra steps to better visualize ultrastructural components of the cell by SEM. Formaldehyde (0.5%) and glutaraldehyde (3%) fixative solution was prepared in phosphate buffer ( 1 0.1mol l ), with 0.1% tannic acid and 3% sucrose. Two osmication steps: first, a 1 h reduced osmium step where 2% OsO4 was prepared in 1.5% potassium ferrocyanide solution, in phosphate buffer. Five 3 min ddH2O washes, before 0.1% thiocarbohydrazide (TCH) was added for 30 min at 60 C as a mordant. Five 3 min ddH2O washes. cond, OsO4 step (2% in ddH2O) was applied for 30 min. The cells were again washed in water as before, then stained overnight using 1% uranyl acetate in ddH2O at 4 C . The following day, water washes were repeated before incubating with lead aspartate at 60 C for 30 min. The cells were then dehydrated using graded ethanol. Cells were infiltrated using Epon 812 Hard premixed resin kit (TAAB). Once fully polymerized (48 h at 60 c ), dish and coverslip were removed and the cell located. This area was then excised using a junior hacksaw and razor blades to leave an approximately 2 mm piece of resin containing the cell. This cube of resin was mounted using Silver Dag onto a cryo pin which fits into the chuck of the 3View platform Jo ur na l o f C el l S ci en ce • A dv an ce a rt ic le (Agar Scientific). The area around the cell was fine-trimmed using fresh glass knives, this produced an approximately 2 100μm block face. Once trimmed, the block was coated with a silver paint using an eyelash, before a 15 nm gold/palladium coating was evaporated onto the block surface, again to reduce surface charging (Quorum Technologies, East Sussex, UK). The cell was imaged using the Gatan 3View System and Digital Micrograph software (Gatan, Abingdon,UK) and FEI Quanta 250 (FEI, Eindhoven, Netherlands). When aligned, the 3View was allowed to cut-and-image the block surface, with 100 nm z-slices, initially optimizing the contrast and magnification for that particular block, before leaving the platform to run until the cell was fully sectioned and imaged. A 768 x 768 pixel image, 60 nm sections, 60μs pixel dwell time, 2.1 keV, and 21000x magnification (12 nm pixel size, 9.216 x 9.216 s  ) was optimal for imaging the spindle zone. These conditions needed to be fine-tuned for the individual sample, and were a compromise between reduced levels of sample charging, magnification and resolution. Image analysis All 3D segmentation and surface rendering was performed using Amira software (Visualization Sciences Group, FEI). The voxel size in nm was entered upon loading of the file into Amira. MTs in SBF-SEM volume stacks were segmented manually in Amira by tracing all the MTs visible in each slice throughout the entire volume, this segmentation was rendered to generate a 3D surface. In addition, where membranes, chromosomes, kinetochores and centrioles were visible, these were segmented in a semi-automated manner, using pixel density to segment objects in 3D. The signal-to-noise ratio of segmented MTs was 3.5 0 .4  (mean  s.e.m., = 1373 im ages N ), i.e. lower than electron tomographic data, which hampered automatic tracing (Weber et al., 2012). The segmentation files (*.am files) were processed in ImageJ/FIJI to create binary images which could be fed into Igor Pro 7.0. Again code for Jo ur na l o f C el l S ci en ce • A dv an ce a rt ic le analysis was custom-written in Igor Pro 7.0 and can be found at https://github.com/quantixed/VolumeFinder. For volume analysis, the image stacks are interpreted and pixels above threshold are counted for each slice of the stack. A 3D convex hull is drawn around all points to measure the volume for normalization. For ellipsoid comparison, described in detail below, the images are skeletonized in ImageJ/FIJI. This process converts each MT segment into a 1 pixel thick object. Each object is read by Igor and converted to a unique 2D vector by fitting a line y = a + bx to the xy coordinates of the object. All objects lie in the plane of the section/image and so a 2D vector is sufficient. The resulting straight lines are used for mathematical modeling. The modeling and comparison was done in Igor Pro7.0 for code, see https://github.com/quantixed/VolumeFinder. To ensure reproducibility and for error-checking the modeling and analysis was independently rewritten in R using the xyz coordinates of straight lines as an input. This code is also available at https://github.com/quantixed/VolumeFinder, where a detailed walkthrough document is also provided. Mathematical modeling A more detailed description follows; in summary, straight lines (representing MT segments from the SBF-SEM dataset) were compared to computer-generated counterparts that were placed in the ideal location. All straight lines were translated and rotated such that the spindle was vertical with the poles aligned on the z-axis and the center of the spindle at the origin. The midpoint of each straight line was used to make an ellipsoid that passes through this point and through both spindle poles. The tangent of a section through this ellipsoid at the midpoint was used for comparison with the original straight line. This was done after putting the straight lines and the idealized counterparts back in their original location, and projecting both segments onto a single xy-plane. Jo ur na l o f C el l S ci en ce • A dv an ce a rt ic le We developed this analysis method because the spindle is a fusiform structure, and therefore comparison of each MT segment to the spindle axis gave a broad range of deviations that made comparisons between conditions impossible (Supplementary Figure 3). Angle comparisons between each MT segment and its neighboring segments were not representative since MT density was not uniform throughout the volume (Supplementary Figure 3). The calculations for ellipse comparison rely on simple geometric principles and make use of specified locations of the two spindle poles. This is possible because the force balance in the metaphase spindle ensures a symmetrical arrangement: the chromosomes are situated approximately at the equator, and the spindle poles are equidistant from the metaphase plate. A more simplified model, straight-line or diamond-shaped spindle, was less realistic than ellipses (Supplementary Figure 3). More complex curves are likely to yield a better fit, and are possibly justified in light of data showing that MTs do not necessarily extend spindle pole to kinetochore (Sikirzhytski et al., 2014; Elting et al., 2014); however, the inclusion of more parameters is not desirable. Observed lines We observe a collection of straight lines, {1, 2, , } i n  , denoted ( ) ( ) ( ) ( ) := ( , , ) i i i i x y z L L L L . Each line may be summarized by its two endpoints; ( ) (0) i L and ( ) (1) i L , switching the labels of endpoints ( ) (0) i L and ( ) (1) i L results in the same line object. Section 0 justifies a particular choice of ( ) (0) i L and ( ) (1) i L . Line locations and orientations are determined from a stack of images collected at increasing depth through the cell with each line segment being extracted from a single image within this stack as described in Section 0. The co-ordinate axes are aligned such that the image planes are perpendicular to the z-direction. Jo ur na l o f C el l S ci en ce • A dv an ce a rt ic le Elliptical model We wish to compare observed lines to paths lying on the surface of an ellipsoid model. In fact, we restrict ourselves to spheroids (i.e. ellipsoids with rotational symmetry about the z axis) because we do not expect there to be any promotion/restriction to MT growth in either the xor y-directions. The surface of such a spheroid centered at the origin contains all vectors of the form = ( , , ) x y z v v v v such that