Kinetochores and microtubules in multipolar mitosis and chromosome orientation.

The ultrastructure of the kinetochore and the orientation of kinetochore microtubules were studied in multipolar divisions of cultured rat-kangaroo cells (Pt-K1). The metaphase kinetochore exhibited a lamellar structure and most of the chromosomes expressed a bipolar microtubule orientation. A multipolar kinetochore orientation was observed in some chromosomes. Equal or unequal portions of a chromatid's kinetochore and a corresponding number of kinetochore microtubules may be oriented to two different poles. Curved or bent continuous microtubules were observed in the vicinity of chromosomes showing multipolar orientation. The findings are in accordance with Östergren's theory of ‘auto-orientation’ [8]. It is speculated that orientation of a chromatid kinetochore to more than one pole might be a possible regular event during the process of chromosome orientation prior to full metaphase in bipolar mitosis.