The effects of lead and X-rays, alone or in combination, on blastocyst formation and cell kinetics of preimplantation mouse embryos in vitro

Regulation of the Embryonic Cell Cycle During Mammalian Preimplantation Development

The preimplantation development stage of mammalian embryogenesis consists of a series of highly conserved, regulated, and predictable cell divisions. This process is essential to allow the rapid expansion and differentiation of a single-cell zygote into a multicellular blastocyst containing cells of multiple developmental lineages. This period of development, also known as the germinal stage, encompasses several important developmental transitions, which are accompanied by dramatic changes in cell cycle profiles and dynamics. These changes are driven primarily by differences in the establishment and enforcement of cell cycle checkpoints, which must be bypassed to facilitate the completion of essential cell cycle events. Much of the current knowledge in this area has been amassed through the study of knockout models in mice. These mouse models are powerful experimental tools, which have allowed us to dissect the relative dependence of the early embryonic cell cycles on various aspects of the cell cycle machinery and highlight the extent of functional redundancy between members of the same gene family. This chapter will explore the ways in which the cell cycle machinery, their accessory proteins, and their stimuli operate during mammalian preimplantation using mouse models as a reference and how this allows for the usually well-defined stages of the cell cycle to be shaped and transformed during this unique and critical stage of development.

Overexpression of CDC14B causes mitotic arrest and inhibits zygotic genome activation in mouse preimplantation embryos

Following fertilization the transition from a highly differentiated oocyte to a totipotent 2-cell embryo requires two unique mitotic cell cycles. The first cell cycle is characterized by a prolonged G(1) phase, DNA replication (S phase) that occurs separately in the female and male pronuclei, and a short G(2) phase that occur in the absence of cell growth. During the second cell cycle, G(1) is short whereas G(2) is prolonged and occurs concurrently with zygotic genome activation, which is essential for progression past the 2-cell stage. CDC14B, a dual specificity phosphatase that counteracts cyclin dependent kinase 1 (CDK1/CDC2A) action, regulates mitosis in somatic cells and prevents premature meiotic resumption in mouse oocytes. It is not known if CDC14B plays a role during the unique mitotic cell cycles of preimplantation development. We report that CDC14B is present in mouse embryos and localizes to mitotic centrosomes and spindles. Overexpressing CDC14B in 1-cell embryos results in 40% and 60% of the embryos arresting at the 1- and 2-cell stages, respectively. Embryos arrested at the 1-cell stage contained reduced CDC2A activity, whereas embryos arrested at the 2-cell stage were in G(2) and failed to activate the zygotic genome. In contrast, overexpressing CDC14B in meiotically-incompetent oocytes, which are arrested in a G(2)-like state and are transcriptionally active, does not repress global transcription. These data suggest that CDC14B is a negative regulator of the 1-to-2-cell transition and of zygotic genome activation in mouse embryogenesis.

Cell cycle regulation during early mouse embryogenesis

Elaboration of a multicellular organism requires highly efficient coordination between proliferation and developmental processes. Accordingly, the embryonic cell cycle exhibits a high degree of plasticity; however, the mechanisms underlying its regulation in vivo remain largely unknown. The purpose of this review is to summarize the data on cell cycle regulation during the early mouse embryonic development, a period characterized by major variations in cell cycle parameters which correlate with important developmental transitions. In particular, we analyse the contribution of mutant mice to the study of in vivo cell cycle regulation during early development and discuss possible contributions of cell cycle regulators to developmental programs.

The effects of 1.5T magnetic resonance imaging on early murine in-vitro embryo development

Although no ionizing radiation is involved, patients undergoing magnetic resonance imaging (MRI) are exposed to powerful static magnetic fields, magnetic field gradients, and radio-frequency fields that may be potentially damaging. Our study aims to document the effect of MRI imaging sequences on early murine embryo development (two-cell to blastocyst stage) in vitro. Two-cell murine embryos were exposed to various lengths of MRI using pulse sequences employed in present day clinical imaging. Early murine embryo development was documented in vitro, and blastocyst development rates were computed for both the control and exposed groups. There were no significant differences detected in the rate of blastocyst formation between the control groups and the embryos exposed to MRI.