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A sensitive RNase mapping technique was used to investigate the expression of individual class I mRNA in the embryo and placenta of the mouse from day 7.5 through day 13 of gestation. Transcripts of the H-2Kd and -Dd genes appeared in both placental and embryonic tissues as early as day 7.5 post coitum and continued to be expressed thereafter. In contrast, H-2Ld mRNA was barely detectable in embryonic RNA until day 18 of pregnancy, although it was present in placental RNA samples on days 10 to 13 of gestation. Qa-region genes demonstrated a different pattern of expression than H-2Dd, -Kd, and -Ld. Transcripts of Q7, one of the genes encoding Qa-2 surface Ag, were detected in the developing embryo on days 9 to 11 post coitum but decreased thereafter. However, Q7d transcripts were not detected in placental tissues at any stage. Q6 mRNA was not detected in any of the tissues, and Q10 mRNA was detected only in day 12 to 13 embryos, where expression is known to be restricted to the fetal liver. Transcripts from T13, a Tla-region gene, and D2d, a D-region gene, were not detected in any of the samples tested. This study has revealed a complex pattern of class I gene regulation both within the genes of the MHC and between the embryo and placental lineages during the midgestational stages of development.
Hedley et al. (Thu,) studied this question.