Structure of the mammalian kinetochore

The structure of the mammalian trilaminar kinetocnore was investigated using stereo electron microscopy of chromosomes in hypotonie solutions which unraveled the chromosome but maintained microtubules. Mouse and Chinese hamster ovary cells were arrested in Colcemid and allowed to reform microtubules after Colcemid was removed. Recovered cells were then swelled, lysed or spread in hypotonic solutions which contained D₂O to preserve microtubules. The chromosomes were observed in thin and thick sections and as whole mounts using high voltage electron microscopy. Bundles of microtubules were seen directly attached to chromatin, indicating that the kinetochore outer layer represents a differential arrangement of chromatin, continuous with the body of the chromosome. In cells fixed without pretreatment, the outer layer could be seen to be composed of hairpin loops of chromatin stacked together to form a solid layer. The hypotonically-induced unraveling of the outer layer was found to be reversible, and the typical 300 nm thick disk reformed when cells were returned to isotonic solutions. Short microtubules, newly nucleated after Colcemid removal, were found not to be attached to the kinetochore outer layer, but were situated in the fibrous corona on the external surface of the outer layer. This was verified by observations of thick sections in stereo which made it possible to identify microtubule ends within the section. Thus, kinetochore microtubules are nucleated within the fibrous corona, and subsequently become attached to the outer layer. We dedicate this paper to Wolfgang Beermann on the occasion of his 60th birthday in appreciation of many years of friendship and his pioneering contributions in the field of chromosome biology