How you measure spindle size depends entirely on which type of spindle you’re working with. In cell biology, a mitotic spindle is the structure that pulls chromosomes apart during cell division, and its size is measured in micrometers using fluorescence microscopy. In sleep science, a sleep spindle is a burst of brain wave activity captured on an EEG, measured by its duration in seconds and its amplitude in microvolts. This guide covers the practical methods for both.
Measuring Mitotic Spindle Size
The mitotic spindle is a football-shaped structure made of protein filaments called microtubules. It forms inside a dividing cell and is responsible for separating copies of chromosomes to opposite ends of the cell. Spindle “size” can refer to its length (pole to pole), its width at the widest point, or its total volume. In human cell lines, spindle lengths typically range from about 7 to 27 micrometers. Common human cell lines like HEK293 and HeLa produce spindles averaging around 11 to 14 micrometers long.
The simplest measurement is pole-to-pole length: the straight-line distance between the two pointed ends of the spindle. You can capture this from a single fluorescence microscopy image if the spindle lies flat in the focal plane. Width is measured across the spindle’s midsection, perpendicular to the pole-to-pole axis. Both measurements can be taken manually using the line tool in ImageJ or FIJI.
3D Volumetric Measurement With Spindle3D
Flat, 2D measurements miss a lot of information. A spindle that looks the same length as another might have very different total volume, and volume turns out to be a better predictor of how spindle size scales with cell size. To capture this, researchers acquire Z-stacks of fluorescent images (a series of slices through the cell at different depths) and reconstruct the spindle in three dimensions.
A free, open-source plugin called Spindle3D, available for FIJI, automates this process. It takes 3D fluorescent image data of spindles and chromatin, segments the spindle shape, and calculates volume by adding up all the tiny 3D units (voxels) inside the segmented region. It also computes average spindle width by measuring the width of the binary mask at multiple angles in 10-degree steps, then averaging those values. This gives a more complete morphometric profile than length alone. The plugin pairs with the segmentation software Ilastik, which helps separate the spindle signal from background noise.
Before Spindle3D, there was no standardized open-source tool for 3D spindle analysis, so labs used custom scripts or manual annotation. If you need reproducible, comparable data across experiments, an automated pipeline like this eliminates a major source of variability.
What Controls Mitotic Spindle Size
Spindle size is not fixed. It scales with cell size, following two distinct patterns depending on how big the cell is. In small cells, spindle size increases roughly in proportion to cell volume. In large cells, the spindle hits an upper limit and stops growing even as the cell gets bigger.
The underlying mechanism involves how fast the microtubules that make up the spindle can grow. Microtubule growth rate scales with cell volume, which explains why spindles get shorter as cells divide and become smaller. Specific proteins that regulate microtubule growth act as limiting factors. In the well-studied roundworm embryo, for example, reducing levels of a protein called CLS-2 slows microtubule growth and shrinks the spindle proportionally. The total supply of tubulin (the building block of microtubules) is sufficient to build two or three full spindles in a single cell, so tubulin alone isn’t the bottleneck. Instead, the regulatory proteins that control how tubulin assembles appear to be the key constraint.
Cell state matters too. When stem cells begin to differentiate, their spindles become about 24% smaller compared to undifferentiated cells of the same size. This shift is linked to changes in how the cell’s interior density and protein distribution affect the balance between different populations of microtubules.
Measuring Sleep Spindles on EEG
Sleep spindles are brief bursts of rhythmic brain activity in the 11 to 16 Hz frequency range, visible on an EEG during non-REM sleep. Their “size” is characterized by two measurements: duration and peak-to-peak amplitude.
The American Academy of Sleep Medicine sets a minimum duration threshold of 0.5 seconds. Most spindles last between 0.5 and 1 second. However, expert scorers frequently identify spindles as short as 0.3 seconds: about 14% of expert-identified spindles fall in that 0.3 to 0.5 second range, suggesting the official cutoff may be slightly conservative.
Amplitude is measured as the maximum peak-to-peak voltage within the spindle event after the EEG signal has been filtered to isolate the 11 to 16 Hz band. In one large study of older adults, the mean maximum peak-to-peak amplitude was 27 ± 11 microvolts, which represents moderate-to-low amplitude spindles. Younger populations tend to produce higher amplitudes.
How Amplitude and Frequency Are Calculated
To get precise measurements, the raw EEG signal containing the spindle is band-pass filtered between 11 and 16 Hz. After filtering, the oscillation frequency is determined by dividing the sampling frequency by the average distance between consecutive peaks (measuring both the tops and bottoms of the wave, then averaging). The maximum peak-to-peak amplitude is simply the largest voltage difference between any adjacent peak and trough within the filtered spindle event.
Spindle detection can be done manually by trained scorers or through automated algorithms. Both approaches have trade-offs. A crowdsourced comparison of experts, non-experts, and automated methods found meaningful disagreement even among experts, particularly for low-amplitude or borderline-duration events. Automated detectors offer consistency but need careful validation against expert-scored gold standard datasets. Spindles shorter than 0.3 seconds are generally discarded as too brief to score reliably.
Choosing the Right Measurement for Your Purpose
For mitotic spindles in a cell biology experiment, start with pole-to-pole length if you need a quick comparison between conditions. If your question involves scaling relationships or you’re comparing across cell types of different sizes, invest the time in 3D volumetric analysis with Spindle3D, since width and volume capture shape changes that length alone misses.
For sleep spindles, duration and amplitude are the standard metrics, but also consider spindle density (number of spindles per minute of non-REM sleep) and oscillation frequency, since slow spindles (around 11 to 13 Hz, predominant over frontal brain regions) and fast spindles (13 to 16 Hz, predominant over central and parietal regions) appear to reflect different neural processes. Reporting all four parameters gives the most complete picture of spindle activity.