Which of the following processes describes recovering a mouse line?

• A. The chimeras are genotyped and the heterozygous ones are mated together.
• B. The founders resulting from pronuclear injection are mated to wild type to produce offspring that have the transgene.
• C. The frozen embryos are thawed and implanted in a pseudopregnant female.
• D. The frozen embryos are thawed and grown in vitro.

Answer: (C)

Recovering a mouse line involves reviving material that has been stored, and the first essential step is to restart development after thawing. When frozen embryos are thawed, they are typically cultured in vitro to resume development to a stage suitable for transfer to a recipient. This revival in culture allows the embryos to reach a transferable or implantable stage so that they can be carried to term by a pseudopregnant female. In other words, thawing the frozen embryos and allowing them to grow in vitro is the key revival step that enables re-derivation of the line from cryostored material. The other processes described relate to different methods of establishing a line (breeding strategies, transgenic founders) and do not describe reviving a line from cryopreserved embryos.