Functional analysis of trophoblast giant cells in parthenogenetic mouse embryos

Diploid mouse embryos containing only maternal DNA (parthenotes) fail, in part, because the inner cell mass does not induce the trophoblast to grow. In this study, we asked whether any of the defects in parthenotes may arise from alterations in trophoblast function. We examined the expression of genes important for normal trophoblast function and found several trophoblast genes that were expressed at normal levels in the primary trophoblast cells of parthenotes: E-cadherin, a cell adhesion molecule, was expressed normally in both the ICM and trophectoderm of parthenogenetic blastocysts and blastocyst outgrowths; the gene for Hxt, a basic helix-loop-helix factor that regulates trophoblast development, was expressed in both zygotic and parthenogenetic giant cells; placental lactogen-1, a hormone that is normally secreted by trophoblast giant cells, was expressed in most of both parthenogenetic and normal trophoblast cells; and the 92 kDa matrix metalloproteinase, gelatinase B, also known as MMP-9, was secreted at equivalent levels by both zygotic and parthenogenetic blastocyst outgrowths. However, once the outgrowths had developed, a subpopulation of trophoblast cells in parthenogenetic embryos had decreased DNA replication and significantly fewer nucleoli per nucleus than did zygotic embryos. Moreover, the parthenogenetic trophoblast cells growing out from blastocysts had a decreased viability in culture. These data suggest that, although parthenogenetic embryos are able to initiate primary trophoblast differentiation, the stability and continued differentiation of trophoblast giant cells may be abnormal. Our data support the hypothesis that the deficiency of secondary trophoblast giant cells may contribute to the decline of parthenogenetic embryos and suggest that the factors controlling this subset of trophoblast are distinct from those for primary trophoblast.