Summary. The possibility that the embryonic diapause associated with delayed implantation in mice is maintained by limitation of an essential amino acid, energy substrate or concentration of ions was examined by comparing the rates of DNA synthesis in delayed implanting embryos that were 'reactivated' by incubation in 'complete' medium or in one of several specially formulated 'deficient' media. It was found, in agreement with earlier observations, that an increase in the rate of DNA synthesis could be detected within 12 h and continued through 72 h in complete medium. An identical pattern was found when embryos were incubated in medium deficient in amino acids and vitamins. Similar patterns of activation were observed in the absence of all metabolizable substrates, a drastically reduced concentration of Na+, and even in a medium consisting only of 25 mm-bicarbonate buffer, NaCl and KCl. The embryos incubated in the more drastically deficient media appeared to be damaged after 18–24 h. Nevertheless, the observation that the rate of DNA synthesis did not remain depressed suggests that such deficiencies are not the means by which embryonic dormancy is maintained in utero.
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