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During mitosis, the connections of microtubules (MTs) to centrosomes and kinetochores are dynamic. From in vitro studies, it is known that the dynamic behavior of MTs is related to the structure of their ends, but we know little about the structure of MT ends in spindles. Here, we use high-voltage electron tomography to study the centrosome- and kinetochore-associated ends of spindle MTs in embryonic cells of the nematode, Caenorhabditis elegans. Centrosome-associated MT ends are either closed or open. Closed MT ends are more numerous and are uniformly distributed around the centrosome, but open ends are found preferentially on kinetochore-attached MTs. These results have structural implications for models of MT interactions with centrosomes.

Gametophyte germlings from unialgal cultures of Membranoptera platyphylla were examined with the electron microscope. The events of mitosis were observed in dividing cells near the thallus apex. In prophase the nucleus is spindle-shaped and surrounded by microtubules and a layer of endoplasmic reticulum. A unique organelle, the polar ring, is present at each pole; its function is not clear. At metaphase the nuclear envelope is intact except for fenestrations at each pole. Spindle microtubules are attached to distinct kinetochores on the chromosomes and continuous pole-to-pole microtubules are present. The nucleolus has dispersed but its granular components are still evident in the nucleoplasm. As the chromosomes separate, the nucleus elongates and finally constricts in the middle to produce 2 daughter nuclei. [ABSTRACT FROM AUTHOR]