The effect of oestradiol-17β on fertilized rabbit eggs in vitro was studied following its inclusion in the culture medium.
When oestradiol is present at a concentration of 25 μg/ml and several hours remain between the time of exposure and the expected time of first cleavage, single-celled eggs typically fail to cleave and amphimixis appears to be inhibited. If the concentration of oestradiol is reduced to 10 μg/ml, or if the first cleavage is imminent at the time of exposure, the eggs tend to fragment instead of undergoing normal cleavage, presumably an effect on the cell membrane. Most eggs are apparently unaffected by an oestradiol concentration of 5 μg/ml. A minimal exposure time of about 30 min is necessary to achieve an oestradiol response. Two- and four-celled stages are much more resistant to the oestrogen than are single-celled ones.
The degree of fragmentation is reduced when progesterone is added equimolarly with oestradiol, suggesting that some antagonistic inter-action occurs.
Under incubator conditions the concentration of oestradiol remains constant throughout the time of these experiments but that of oestrone appears to decrease.
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