THE PEM HOMEOBOX GENE IS X-LINKED AND EXCLUSIVELY EXPRESSED IN EXTRAEMBRYONIC TISSUES DURING EARLY MURINE DEVELOPMENT

We previously reported the isolation of a cDNA clone for a homeobox-co ntaining gene designated Pem, shown by Northern analysis of Day 7 thro ugh Day 16 mouse embryos to be expressed in extraembryonic tissues. In this study, Pem gene expression was further examined using in situ hy bridization and immunocytochemistry to determine the spatial distribut ion of Pem transcripts and protein in peri-implantation embryos and in embryoid bodies (EBs). Low amounts of Pem mRNA were detected in undif ferentiated EBs. When EBs were induced to differentiate, the outer cel l layer of visceral or parietal endoderm expressed both Pem mRNA and p rotein. In developing embryos, no Pem protein was detectable in the un compacted morula; 12% of the nuclei in compacted morulae were Pem posi tive, while 25% of the blastocyst trophectoderm and 15% of inner cell mass cells expressed Pem protein. Shortly after implantation, in 5.5 a nd 6.5 d.p.c. embryos, Pem expression was limited to extraembryonic ti ssues and was present in distal and proximal visceral endoderm, pariet al endoderm, and ectoplacental cone. By 7.5-8.5 d.p.c. neither Pem RNA nor protein was found in the distal squamous visceral endoderm, which surrounds the embryonic region of the egg cylinder, nor in the pariet al endoderm. Expression was retained in the proximal columnar epitheli um of the visceral endoderm. Prominent Pem expression was observed in the chorion, in trophoblast-derived cells of the ectoplacental cone, a nd in secondary giant cells, localized in the nuclear compartment. Pem was localized to the X chromosome and found to be expressed in cell l ineages where only the maternal X chromosome is active. The data indic ate a possible role for Pem in regulating genes involved in the differ entiation of extraembryonic tissues. (C) 1994 Academic Press, Inc.