Germline and somatic mosaicism in transgenic mice

Analysis of 262 transgenic mouse pedigrees suggests that about 30% of the mice produced by microinjection of plasmids into pronuclei are mosaic in the germline. This implies that in these lines integration of the foreign DNA occurred after the first round of chromosomal DNA replication. In mosaics resulting from delayed integration the transgenic cells are usually distributed to both the trophectoderm and the inner cell mass, but sometimes to only one of these two cell types. Mosaicism of the inner cell mass results in even representation among the somatic tissues, and usually the germline as well; however, the germline is sometimes deficient in or entirely lacks transgenic cells. The germline precursor pool is distinct from the somatic precursor pools; apparently it is either determined prior to the primary germ layers or it is initially composed of fewer cells.