Karyologically identified homozygous tw18 embryos: extrauterine growth properties and transformation by SV40

A method for identifying individual embryos from crosses segregating homozygous t lethals using the marker chromosome Rb7 is described and applied to the tw18 haplotype. In contrast to wild-type and heterozygous littermates, homozygous tw18/tw18 6 and 8 day embryos have very limited growth potentialities in vitro. When transplanted under the testis capsule, homozygous tw18 embryos (3, 6 and 7 day) produce teratomas at a much lower efficiency than heterozygous and wild-type embryos, and the rare teratomas thus obtained have limited growth potentialities when cultured in vitro. Upon transformation with SV40, however, permanent lines of cells of mesodermal origin, capable of myoblastic or adipocytic differentiation, have been obtained. This shows that the effect of the mutation on cell growth can be overcome by SV40 transformation.