At the time of cell division, chromatin becomes condensed into the chromosomes. Accordingly, for chromosome analysis, an inhibitor of spindle fiber such as colchicine is commonly added to the cell culture to restrain the cell division at metaphase. Because the prophase chromosomes are longer and less condensed than metaphase chromosomes, the prophase chromosomes are much more useful than metaphase ones for high-resolution karyotyping. The present study was conducted to investigate the effect of adding ethidium bromide (EtBr), an inhibitor of chromatin condensation, into chick embryo culture on the induction of prophase chromosomes. Injection of 1 ㎍ EtBr into the air sac of chick embryo at the end of the culture resulted in an increase in the ratio of prophase chromosomes, indicating that chromatin condensation was effectively inhibited by this reagent. Additions) treatment of the culture with 100 ㎍ colchicine following the EtBr improved the gross morphology of the chromosomes, with no adverse effect on the number of prophase chromosomes. Moreover, the relative length and number of G-bands of macrochromosomes at prophase were 1.7-2.0 times as long as and about 2 times as great as those at metaphase, respectively.
As the EtBr was highly effective in inhibiting the condensation of chick chromosomes, we propose the usage of EtBr as an improved method for preparing high-resolution chromosomes for the chick embryos.
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