When HeLa cells were grown in the presence of nitrous oxide (N2O) under pressure (80 lb/in2) mitosis was inhibited and the chromosomes displayed a typical colchicine metaphase (c-metaphase) configuration when examined by light microscopy. When the cells were returned to a 37°C incubator, mitosis was resumed and the cells entered G1 synchronously. Ultrastructural studies of N2O-blocked cells revealed a bipolar spindle with centriole pairs at each pole. Both chromosomal and interpolar (pole-to-pole) microtubules were also present. Thus, N2O, unlike most c-mitotic agents, appeared to have little or no effect upon spindle microtubule assembly. However, the failure of chromo somes to become properly aligned onto the metaphase plate indicated an impairment in normal prometaphase movement. The alignment of spindle microtubules was frequently atypical with some chromosomal microtubules extending from kinetochores to the poles, while others extended out at acute angles from the spindle axis. These ultrastructural studies indicated that N2O blocked cells at a stage in mitosis more advanced than that produced by Colcemid or other c-mitotic agents. Like Colcemid, however, prolonged arrest in mitosis with N2O led to an increased incidence of multipolar spindles.

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