After thawing a set of frozen embryos for line recovery, what is the next action?

• A. Implant in a pseudopregnant female
• B. Backcross to the donor strain
• C. Cross to wild-type
• D. Cryopreserve again

Answer: (A)

Transferring the thawed embryos into a pseudopregnant recipient female is the next step. After thawing for line recovery, those embryos must be carried to term, and a receptive uterus is provided by a pseudopregnant dam. This hormonal and uterine state supports embryo implantation and gestation, which is essential to reestablish the desired line. Re-cryopreservation wouldn’t yield live animals, and using the embryos to backcross to the donor strain or to wild-type would alter the genetic background rather than recover the intended line.