Murine embryos express TGF-betal as early as 2-4 cell stage. To investigate the functions of TGF-betal in vivo, our laboratory disrupted the TGF-betal gene (Shull et al, Nature 1992: 359, 693-99). Genotypes of pups born from intercrosses of animals heterozygous for the disrupted TGF-betal allele in the 129×CF-1 strain revealed a significant deviation from the mendelian ratio of 1:2:1. Of 806 live offsprings born, 280 were homozygous wild type(+/+), 413 were heterozygous (+/-) and 113 were homozygous mutant (-/-), giving a ratio of 1:1.5:0.4. The +/+ data has been normalized to 1. To determine the gestational stage at which embryonic lethality occurred, genotyping of embryos derived from heterozygous crosses were undertaken at various developmental stages. In the 129×CF-1 strain, genotypic analysis of 398 blastocysts showed the ratio to be 1:1.3:0.5 (+/+: +/-: -/-). Next, in vitro development of preimplantation embryos derived from TGF-betal heterozygous crosses in the same strain was assessed. 134 four cell stage embryos out of 140 could be successfully cultured to healthy compacted morula/blastocyst stage embryos, the culturing efficiency being no different from controls. To demonstrate that the embryonic loss cannot be accounted for by differential fertilization rates, crosses were undertaken between +/+ and+/- animals in both sexes. Of 170 pups born to TGF-betal +/- male and +/+ female the ratio of +/+: +/- pups was 1:1.5. When TGF-betal +/+ males were mated with TGF-betal +/- females, the ratio of +/+: +/- pups was 1: 0.82(N=180) (p>0.05). Thus there was no under-representation of +/- embryos at fertilization. These results suggest that in the 129×CF-1 strain, approximately 50% TGF-betal -/- embryos fail to develop past the first two cell divisions in preimplantation embryogenesis.

Data is also presented showing that there is strain dependence of the TGF-betal requirement for embryogenesis. In six different mouse strains, the percentage of -/- offprings from TGF-betal +/- parents ranged from 1% to 23%. These results demonstrate that the embryonic lethality associated with TGF-betal mutation is highly dependent upon the background strain.