Polyploidization of extraembryonic tissues during mouse embryogenesis.

It has recently been shown that visceral yolk-sac endoderm is derived from the primitive endoderm of the 4.5-day mouse blastocyst (Gardner & Papaioannou, 1975; Gardner & Rossant, 1979). During development, primitive endodermal cells acquire nuclei with more than four times the haploid amount of DNA. The finding of metaphases with multiple sets of chromosomes suggests that the diploid precursors of such endodermal giant cells become truly polyploid. Amniotic cells also contain giant nuclei but the mechanism by which these arise is uncertain. The giant-cell transformation therefore appears to be a general feature of mouse extraembryonic development rather than a phenomenon restricted solely to trophoblast. The basis and significance of these findings are discussed in relation to the development of other extraembryonic membranes both of plant and animal origin.