Expression of glucose-phosphate isomerase in relation to growth of the mouse oocyte in vivo and in vitro

Mouse zygotes as recipients in embryo cloning

Zygotes have not been recognized as nuclear recipients since enucleated zygotes receiving nuclei from beyond two-cell stage embryos are not able to form blastocysts. In the present study, a new technique of zygote enucleation is presented, which consists in selectively removing the nuclear membrane with genetic material of pronuclei, but leaving other pronuclear components in the cytoplasm. With selective enucleation it is possible - after transfer of eight-cell stage nuclei - to obtain 70.5 and 7.8% of preimplantation and full-term development respectively. Origin of cloned mice from introduced nuclei was confirmed by the coat colour and glucose phosphate isomerase (GPI) isozyme of the donor. We suggest that some pronuclear factors - taken away from the zygotes in the karyoplasts upon classical enucleation - are needed to reprogram the introduced nuclei.

Foetal fibroblasts introduced to cleaving mouse embryos contribute to full-term development

Foetal fibroblasts (FFs) labelled with vital fluorescent dye were microsurgically introduced into eight-cell mouse embryos, three cells to each embryo. FFs were first identified in the inner cell mass (ICM) in about one-third of embryos, whereas in three quarters of embryos FFs were located among trophoblast cells. Some elimination of FFs from trophoblast occurred later on. Eventually, in blastocysts’ outgrowths, an equally high contribution from FFs progeny (60%) was found in both ICM and trophoblast. Three days after manipulation, FFs resumed proliferationin vitro. More than three FFs were found in 46.2% of embryos on day 4. On the 7th dayin vitroin 70% of embryos more than 12 FFs were found, proving at least three cell divisions.

To study postimplantation development, the embryos with FFs were transferred to pseudopregnant recipients a day after manipulation. After implantation, FFs were identified by electrophoresis for isozymes of glucose phosphate isomerase (GPI). A single 11-day embryo delayed to day 8 proved chimeric by expressing both donor isozyme GPI-1B and recipient GPI-1A. Similar chimerism was found in the extraembryonic lineage of 11% of embryos by day 12. Starting from day 11 onwards, in 32% of normal embryos and in 57% of foetal membranes, hybrid GPI-1AB isozyme, as well as recipient isozyme, was present. Hybrid GPI-1AB can only be produced in hybrid cells derived by cell fusion, therefore, we suggest that during postimplantation development, FFs are rescued by fusion with recipient cells. In the mice born, hybrid isozyme was found in several tissues, including brain, lung, gut and kidney.

We conclude that somatic cells (FFs) can proliferate in earlyembryonic environment until early postimplantation stages. Foetuses and the mice born are chimeras between recipient cells and hybrid cells with contributions from the donor FFs. Transdifferentiation as opposed to reprogramming by cell fusion can be considered as underlying cellular processes in these chimeras.

Experimentally produced diploid〈-〉triploid mouse chimaeras develop up to adulthood

Spontaneous diploid-triploid chimaeras occur sporadically in various mammalian species including man, but so far have never been produced experimentally. In order to get a deeper insight into the developmental consequences of this anomaly, we have developed two procedures that enabled for the first time to produce routinely diploid-triploid embryos, foetuses, and animals in the mouse. These procedures are: (1) aggregation of cleaving diploid embryos with triploid embryos produced by suppression of the second polar body in zygotes, and (2) fusion of a haploid karyoplast with one blastomere of the two-cell diploid embryos. The first procedure yielded 23 living and 6 dead postimplantation embryos and foetuses (age: 8th-19th day) out of which 22 were chimaeric. In addition, three chimaeric neonates reached adulthood. Two animals were fertile, and one--an overt chimaera--was an infertile male. The rate of postimplantation development of aggregation chimaeras was normal or only slightly retarded, and with one exception the foetuses were morphologically normal. Generally, the highest contribution of the 3n component in extra-embryonic structures was noted in the yolk sac, and usually it was higher than its contribution to the organs of the body. Chimaerism was most often noted in the liver, the heart, the intestine, and the lungs. Participation of triploid cells to all tissues studied, both in the body and in extra-embryonic structures, appeared to decrease slightly as development progressed. The second procedure yielded 10 foetuses and 6 adults. Three foetuses were chimaeric. Six fertile adults were probably non-chimaeras: the triploid component was absent in the coat and in the blood.

Gene silencing: maturation of mouse fetal germ cells in vitro

Nuclear reprogramming is essential during gametogenesis for the production of totipotent zygotes. Here we show that premeiotic female germ cells derived from mouse fetuses as early as 12.5 days post coitum are able to complete meiosis and genomic imprinting in vitro and that these matured oocytes are highly competent in supporting development to full term after nuclear transfer and in vitro fertilization. To our knowledge, this is the first time that complete oogenesis has been successfully accomplished in vitro.

Does monozygotic twinning occur in mice?

Published reports suggest that the incidence of monozygotic twinning in women is increased after hormonally induced ovulation. Since some statistical evidence exists to indicate that monozygotic twinning may also occur in mice, we attempted to devise a mouse system in which the incidence of such twinning could be compared after spontaneous versus hormonally induced ovulation, in order to analyse the developmental basis of such an effect. We used phenotypic identity in litters segregating for ten genetic loci (not all independent) to indicate possible twin pairs. DNA finger-printing using three human minisatellite probes was then performed blind on these pairs and on sibling controls. From a total of over 2000 mice born, 40 apparently identical pairs were identified, on which DNA finger-printing was successfully carried out on 35 pairs. All proved to be derived from different zygotes. We conclude that monozygotic twin pairs are either extremely rare in the stock of mice that we studied, or have such reduced viability that their chance of surviving to weaning is low.

Development of mouse germ cells in cultures of fetal gonads

Mouse gonadal tissue was studied under various conditions of in vitro culture, with the aim of clarifying some of the somatic-cell influences that regulate the development of germ cells in the mammalian gonad. Gonadal ridges, with or without the adjacent mesonephric region, were removed from mouse embryos 10.5-12.5 days post coitum (dpc). In an organ culture system, the female ridges showed good development, with no masculinization. All germ cells entered meiosis at the expected time. Although some oocytes entered the growth phase, many primordial follicles were observed. 11.5- and 12.5-day male ridges formed testis cords, and the germ cells developed as T-prospermatogonia. In 10.5-day ridges, cells resembling Sertoli cells differentiated but did not form testis cords, and the germ cells entered meiosis. We conclude that full differentiation of the supporting cell lineage was not achieved when culture was begun at 10.5 dpc; our findings suggest that immature Sertoli cells neither form testis cords nor inhibit the entry of germ cells into meiosis. When the ridges were fragmented and cultured in gas-permeable dishes, the somatic cells grew out as a monolayer on which the germ cells rested. Under these conditions male germ cells did not enter meiosis and did not survive for more than a few days. Female germ cells entered meiosis. In contrast to the organ culture system, many of the surviving oocytes entered the growth phase during the second week of culture, reaching diameters of up to 60 microns. This suggests that normal follicular cell investment may play a crucial role in maintaining the oocyte in a state of developmental arrest. The growing oocytes showed the oocyte-specific expression of the enzyme glucose phosphate isomerase. It seems that the initiation and maintenance of both oocyte growth and oocyte-specific gene expression can take place in the absence of normal follicular cell investment.