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    Home > Biochemistry News > Biotechnology News > The research group of Meng Anming from the School of Life Sciences found that the second polar body of mouse embryos establishes cell fate asymmetry

    The research group of Meng Anming from the School of Life Sciences found that the second polar body of mouse embryos establishes cell fate asymmetry

    • Last Update: 2022-01-23
    • Source: Internet
    • Author: User
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    Humans and mammals ovulate periodically, and the released egg is very large, it inherits most of the cytoplasm of the oocyte and a nucleus that retains two sets of chromosomes; at the same time, it also releases a small volume connected to the egg.
    The first polar body (
    PB1 ), the first polar body contains only a very small amount of cytoplasm, but also has a nucleus containing 2 sets of chromosomes, and the first polar body is rapidly apoptotic
    .
    When the nucleus of a sperm (male pronucleus, which contains
    1 set of chromosomes) enters the egg, the egg expels a very small cell (the second polar body PB2 ) that contains 1 set of chromosomes , so that only the nucleus of the egg (female pronucleus) remains 1 set of chromosomes (identical to the second polar body), the male pronucleus and the female pronucleus fuse to form a new zygotic nucleus containing 2 sets of chromosomes (see video), and the zygote (fertilized egg) enters the embryonic development stage .
    In mice, the second polar body can survive for about 3 days .
    Previous studies in mice found that after the second polar body of the fertilized egg was removed, the shape and number of cells of the blastocyst did not change when the embryo developed to the blastocyst stage.
    Therefore, it is generally believed that the mammalian second polar body is important for embryonic development.
    Useless, its excretion only plays the role of removing
    one set of redundant chromosomes.
    In clinical assisted reproduction, the polar body is removed for preimplantation genetic diagnosis
    .



    The research of Academician Meng Anming's team in mouse models found that the second polar body contains components such as RNA polymerase II and ribosomes involved in gene transcription and mRNA translation
    .
    At the late
    1 -cell stage, the second polar body starts gene transcription like a fertilized egg; from the
    1 -cell stage to the 2 -cell stage, newly synthesized proteins in the second polar body can be detected
    .
    During the early
    1 -cell period, proteins and small RNAs can be transported bidirectionally between the second polar body and the zygote
    .
    By oil drop marking, it can be found that when the first cleavage of the fertilized egg has just begun to appear the cleavage hook, the second polar body is only connected to a blastomere, which is defined as
    pbB , but not to the second polar body Connected blastomeres are defined as npbB
    .
    Through cell fate tracking, it was found that at the blastocyst stage, progeny cells of
    pbB and npbB generally did not intermingle, and the progeny cells of pbB contributed more to the inner cell mass ( ICM , the precursor cells used to form the embryoid body) than npbB
    .
    Removal of the second polar body at
    1 -cell metaphase, the total number of cells and morphology of the resulting blastocysts were not significantly different from controls, but pbB andThe contribution of nbpB to the ICM is roughly equal, the implantation rate of blastocysts is significantly reduced, the embryos that can implant and develop to 9.
    5
    days are significantly smaller and do not survive to birth.
    These abnormalities can be achieved by
    injecting a
    second The polar body extract is overcome .
    When fertilized eggs were treated with microtubule inhibitor nocodazole for a short time to disrupt the connecting channel between the polar body and the fertilized egg, a similar effect of removing the second polar body at the 1 -cell stage was observed .
    They also found that the mRNAs of some genes related to cell pluripotency, such as Wnt7A , were enriched in the second polar body.
    After
    injecting Wnt7A mRNA into pbB of 2 -cell stage embryos after depletion of the second polar body at the 1 -cell stage , blastocyst cells Fate defects can be rescued, but post-implantation development remains abnormal .
    Neither preimplantation cell fate nor postimplantation development appear to be affected if the second polar body is removed at the 2 -cell stage .
    These results suggest that mouse second polar bodies are likely involved in the regulation of preimplantation cell fate decisions and postimplantation embryonic development (see Figure) .
    Although it is not clear whether the second polar body of human fertilized eggs has a similar function, when the second polar body is clinically taken for genetic diagnosis, it should be considered to wait until the 2 -cell stage for sampling






    .


    a.
    The protein translated from the
    mRNA in the second polar body ( PB2 ) of the normal mouse zygote can be transported to the zygote, which is likely to form a concentration gradient, resulting in different amounts of protein obtained by the two daughter cells, resulting in differences in developmental fates , in which the progeny cells of pbB contribute more to the ICM and can develop and be born normally after uterine implantation .

    b.
    After
    PB2 is removed , pbB and npbB at the 2 -cell stage acquire the same developmental fate, and blastocysts may fail to implant, die after implantation, or are stunted and fail to be born .


    This achievement is the first to discover that the second polar body of mammalian fertilized egg has an important biological function, but the molecular mechanism of its function remains to be further studied
    .

    The result was published online on January 10 , 2022 in National under the title " The second polar body contributes to the fate asymmetry in the mouse embryo ".
    Science Review , Ph.
    D.
    students Jin Hongbin and Han Yang of the School of Life Sciences of Tsinghua University are the co-first authors, and Academician Meng Anming is the corresponding author of the paper
    .


    Paper link: https://doi.
    org/10.
    1093/nsr/nwac003



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