The Rare Occurrence of Mitosis Without Spindle Apparatus (“Colchicine Mitosis”) Producing Endopolyploidy in Embryos of the Axolotl

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Abstract
A type of mosaicism was observed in the tailtips of 3 embryos from one spawning. Mitotic figures were abundant but involved chromosome reduplica-tion without nuclear or cell division. Spindles and asters were absent, and the chromosomes were sometimes confined in a nuclear membrane, sometimes scattered in the cytoplasm. This abnormal mitotic cycle is designated as a "c-type mitosis." The tailfin of 1 embryo, which must have been diploid originally, contained large areas of polyploid cells, whereas the tailfins of the other 2 embryos, which must have been haploid originally, had predominantly haploid cells, mixed with diploid, tetraploid, and octoploid cells. Stages in the abnormal mitotic cycles were observed and showed typical c_mitosis characteristics, such as delay in centromere division and parallel arrangement of homolo-gous chromosomes after centromere division. No anaphases with the octoploid no. of single chromosomes were observed, but several large, presumably octoploid nuclei with up to 7 nucleoli were found. All 3 embryos were abnormal and non-viable. Serial sections indicated that polyploid cells were present in all organs and tissues, with "c-mitosis" evident particularly in the epidermis. This type of polyploidizing mitosis was found in 2 other tailtips from 2 different spawnings. The differences between the c-type mitosis observed in axolotl embryos and modifications of mitosis produced by colchicine treatment of amphibian cells are discussed. A process of chromosome doubling, with or without breakdown of the nuclear membrane, but always without any spindle, may be of importance in the production of polyploid cells in normal and tumor tissues of animals.