Proposed origin of chimeras, mosaics and monochorionic twins

Abstract

OBJECTIVE: The purpose of this presentation is to explore the influence of controlled oocyte maturation on embryonic development and how such manipulation could result in chimeric or mosaic offspring, or in monochorionic twins.DESIGN: An experimental animal model, using cultured mouse oocytes, was used to impede the rotation of the meiotic spindle and to control the volume of the released polar body during the first metaphase division. The fertilizability and developmental potential of the resulting oocyte and associated polar body were then examined.MATERIALS AND METHODS: The first and second meiotic divisions were studied using fluorescent and time-lapse microscopy. Controlled compression of oocytes prevented rotation of the first meiotic spindle, which remained parallel to the surface of the oolemma. This resulted in the division of the maturing oocytes into two approximately equal cells, one of which was equivalent to an enlarged first polar body. Such 2-celled oocytes were cultured with sperm after localized openings were made in their zona pellucida using laser. Fertilization and embryonic development were photographed at frequent intervals.RESULTS: Each of the two cells in the oocytes established a second meiotic spindle. Both cells were fertilizable, released a second polar body and formed a male and female pronucleus. Each cell cleaved to establish twin 2-cell embryos within the zona pellucida. After each twin embryo had cleaved to beyond the 4-cell stage, it became difficult to distinguish individual embryos due to mingling of their blastomeres. Following intimate aggregation of blastomeres a single compacted morula was established. This progressed to a blastocyst, but expansion failed to occur. In some cases, both cells were fertilized within the zona but only one produced a cleaving embryo. In such cases the developing half produced a blastocyst that contained a large neighbouring cell.CONCLUSIONS: A single oocyte may give rise to twin intra-zonal embryos that subsequently combine to form a chimera, or a mosaic embryo, containing two maternal and two paternal genetic contributions. It can be deduced that the composite embryo could contain either an XX/XX, or XX/XY, or XY/XY genotype. But the developmental potential of such chimeric entities needs to be evaluated using genetic analysis. In addition, if the original twin embryos do not combine, their separate development with fusion and sharing of trophoblasts, could give rise to monochorionic twins. OBJECTIVE: The purpose of this presentation is to explore the influence of controlled oocyte maturation on embryonic development and how such manipulation could result in chimeric or mosaic offspring, or in monochorionic twins. DESIGN: An experimental animal model, using cultured mouse oocytes, was used to impede the rotation of the meiotic spindle and to control the volume of the released polar body during the first metaphase division. The fertilizability and developmental potential of the resulting oocyte and associated polar body were then examined. MATERIALS AND METHODS: The first and second meiotic divisions were studied using fluorescent and time-lapse microscopy. Controlled compression of oocytes prevented rotation of the first meiotic spindle, which remained parallel to the surface of the oolemma. This resulted in the division of the maturing oocytes into two approximately equal cells, one of which was equivalent to an enlarged first polar body. Such 2-celled oocytes were cultured with sperm after localized openings were made in their zona pellucida using laser. Fertilization and embryonic development were photographed at frequent intervals. RESULTS: Each of the two cells in the oocytes established a second meiotic spindle. Both cells were fertilizable, released a second polar body and formed a male and female pronucleus. Each cell cleaved to establish twin 2-cell embryos within the zona pellucida. After each twin embryo had cleaved to beyond the 4-cell stage, it became difficult to distinguish individual embryos due to mingling of their blastomeres. Following intimate aggregation of blastomeres a single compacted morula was established. This progressed to a blastocyst, but expansion failed to occur. In some cases, both cells were fertilized within the zona but only one produced a cleaving embryo. In such cases the developing half produced a blastocyst that contained a large neighbouring cell. CONCLUSIONS: A single oocyte may give rise to twin intra-zonal embryos that subsequently combine to form a chimera, or a mosaic embryo, containing two maternal and two paternal genetic contributions. It can be deduced that the composite embryo could contain either an XX/XX, or XX/XY, or XY/XY genotype. But the developmental potential of such chimeric entities needs to be evaluated using genetic analysis. In addition, if the original twin embryos do not combine, their separate development with fusion and sharing of trophoblasts, could give rise to monochorionic twins.