The Spindle

The existence of the spindle was for long debated, and it was considered by some to be a fixation artifact. However, its presence has now been confirmed. The evidence is based on the facts (1) that the spindle is visible under contrast and polarizing microscopes, (2) that it can be displaced by ultra-centrifugation, and (3) that it has been possible is essential for mitosis. If cells are treated are with colchicine, which inhibits spindle formation, anaphasic movement of the two groups of chromosomes to the poles does not take place.

The spindle is largely a protein gel. ATPase activity has been associated with the spindle, suggesting a contractile mechanism similar to that of muscle fibres, although the load on the spindle is much less.

Types of spindles. There are two types of spindles, anastral and astral. The anastral spindle is without centrioles, blepharoplasts or asters. It is typically found in higher plants, but has also been reported in several animal ova and Protista. Usually the spindle symmetrical, and consists of two equal half-spindles.

Spindle fibres. Three types of spindle fibres have been distinguished, chromosomal fibres, continuous fibres and interzonal fibres. Chromosomal fibres extend from the kinetochore (centromere) to the pole of the spindle. They shorten during cell division and pull the daughter chromosomes apart. They are formed by the centromeres of the chromosomes.

Continuous fibres extend from pole to pole without attaching to the chromosomes. They elongate during cell division and push the spindle poles apart, thus separating the chromosomes. The fibres from between the poles, regardless of whether chromosomes are present or not. They are formed even after the chromosomes are removed by microsurgery.

Microtubules. Electron microscope studies show that the spindle fibre consists of a bundle a microtubules embedded in a proteins matrix Microtubules are long cylinders, about 250a in diameter.



A section through the cylinder shows that it is hollow. The wall of the cylinder is electron-dense and is approximately 50-70A thick. The wall appears to be made up of 13 longitudinally arranged filamentous subunits with a centre-to-centre spacing of 50A. The filaments appear to be made up of small monomers of globular proteins (tubulin) about 40-50A in diameter. The dimmers polymerize to form filaments, which then aggregate to form microtubules.

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