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Effects of Sex Chromosome Dosage on Placental Size in Mice¹

National Institute for Medical Research,³ London NW7 1AA, United Kingdom School of Medicine,⁴ Mie University, Tsu 514-8507, Japan Max-Planck-Institut für Molekulare Genetik,⁵ 14195 Berlin, Germany

Mice of the XO genotype with a paternally derived X chromosome (X^pO) have placental hyperplasia in late pregnancy, although in early pregnancy the ectoplacental cone, a placental precursor, is smaller in X^pO mice than in their XX sibs. This early size deficiency of the ectoplacental cone is apparently a consequence of X^p imprinting, because X^mO embryos (with a maternally derived X chromosome) are unaffected. In the present study we sought to establish whether X^pO placental hyperplasia in late pregnancy is also a consequence of X^p imprinting. Placental weight data were first collected from litters that included X^pO or X^mO fetuses and XX controls. Comparison of XO placentae with XX placentae showed that X^pO and X^mO placentae are hyperplastic. This finding suggested that the hyperplasia might be an X dosage effect, and this hypothesis was supported by the finding that XY male fetuses from the same crosses also had larger placentae than their XX sibs. Further analysis of a range of sex-chromosome variant genotypes, including X^mYSry-negative females and XXSry transgenic males, showed that mouse fetuses with one X chromosome consistently had larger placentae than littermates with two X chromosomes, independent of their gonadal/androgen status.

¹ H.I. was the recipient of a Yamada Science Foundation Travelling Fellowship.

² Correspondence: Paul S. Burgoyne, Division of Developmental Genetics, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, U.K. FAX: 44 208 906 4477; pburgoy{at}nimr.mrc.ac.uk

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