In most animal cells, the mitotic apparatus – the microtubule-based machine that sorts chromosomes – includes two more or less distinct populations of microtubules.  The spindle is a bundle of bundles focused at the centrosomes at each pole of the mitotic apparatus, and hooked up to the chromosomes in the middle.  These are the apparent agents of, or at least the scaffold for, chromosome alignment and segregation.  The asters, on the other hand, are the radial arrays of microtubules that extend from each pole. 

Before chromosomes separate, the asters are relatively dense, as in the first image in the series above, and are quite dynamic in real life.  Coincident with chromosome separation, as most of the spindle fibers disassemble, the astral microtubule array grows steadily. 

In these sand dollar zygotes, few if any astral microtubules reach the cell surface before chromosome separation, but a few minutes later, when chromosomes are about half-way to the spindle poles, astral microtubules have reached the polar cell surface.  In many living embryos, the presence of the astral microtubules can be easily detected at this stage because cytoplasmic organelles align along them, causing the appearance of “astral rays” that are visible in even the simplest of light microscopes. 

By the time chromosomes reach the spindle poles a few minutes later, astral microtubules have reached the cell surface everywhere.  At this time or very soon after, the cell begins to cleave itself in two; most of the spindle microtubules have disassembled, and the asters pervade the cytoplasm as the nuclei reassemble and the cleavage furrow progresses. 

Because they so obviously connect the cell surface to the mitotic apparatus, it is among the oldest of past-times among cell biologists to speculate how it might be that astral microtubules – or astral rays, before microtubules were discovered in the electron microscope – convey information about the position and state of the spindle to the cell surface, telling it where it needs to contract to divide the cell.