“…Similar to the situation in Xenopus, in these other cases, the outer cells follow different developmental pathways from the inner cells (enveloping layer and embryo proper in zebrafish; trophectoderm versus inner cells mass in the mouse) (Johnson and Ziomek, 1981;Pedersen et al, 1986;Fleming, 1987;Sutherland et al, 1990;Kimmel et al, 1995). However, neither the existence of determinants nor the spatial pattern of these divisions has been established for the mouse or zebrafish early embryo.…”
Section: Oriented Divisions Occur During Early Development Of Other Vmentioning
A key feature of early vertebrate development is the formation of superficial, epithelial cells that overlie nonepithelial deep cells. In Xenopus, deep and superficial cells show a range of differences, including a different competence for primary neurogenesis. We show that the two cell populations are generated during the blastula stages by perpendicularly oriented divisions. These take place during several cell divisions, in a variable pattern, but at a percentage that varies little between embryos and from one division to the next. The orientation of division correlates with cell shape suggesting that simple geometric rules may control the orientation of division in this system. We show that dividing cells are molecularly polarised such that aPKC is localised to the external, apical, membrane. Membrane localised aPKC can be seen as early as the one-cell stage and during the blastula divisions, it is preferentially inherited by superficial cells. Finally, we show that when 64-cell stage isolated blastomeres divide perpendicularly and the daughters are cultured separately, only the progeny of the cells that inherit the apical membrane turn on the bHLH gene, ESR6e. We conclude that oriented cell divisions generate the superficial and deep cells and establish cell fate diversity between them.Movie available online
“…Similar to the situation in Xenopus, in these other cases, the outer cells follow different developmental pathways from the inner cells (enveloping layer and embryo proper in zebrafish; trophectoderm versus inner cells mass in the mouse) (Johnson and Ziomek, 1981;Pedersen et al, 1986;Fleming, 1987;Sutherland et al, 1990;Kimmel et al, 1995). However, neither the existence of determinants nor the spatial pattern of these divisions has been established for the mouse or zebrafish early embryo.…”
Section: Oriented Divisions Occur During Early Development Of Other Vmentioning
A key feature of early vertebrate development is the formation of superficial, epithelial cells that overlie nonepithelial deep cells. In Xenopus, deep and superficial cells show a range of differences, including a different competence for primary neurogenesis. We show that the two cell populations are generated during the blastula stages by perpendicularly oriented divisions. These take place during several cell divisions, in a variable pattern, but at a percentage that varies little between embryos and from one division to the next. The orientation of division correlates with cell shape suggesting that simple geometric rules may control the orientation of division in this system. We show that dividing cells are molecularly polarised such that aPKC is localised to the external, apical, membrane. Membrane localised aPKC can be seen as early as the one-cell stage and during the blastula divisions, it is preferentially inherited by superficial cells. Finally, we show that when 64-cell stage isolated blastomeres divide perpendicularly and the daughters are cultured separately, only the progeny of the cells that inherit the apical membrane turn on the bHLH gene, ESR6e. We conclude that oriented cell divisions generate the superficial and deep cells and establish cell fate diversity between them.Movie available online
“…Starting at the 32-cell stage, as the outside cells of the embryo are becoming fully committed to the TE lineage (48,49), a fluid-filled cavity known as the blastocoel begins to form ( Figure 1). The presence of a blastocoel is essential for proper development of the ICM (49).…”
Section: Symmetric Versus Asymmetric Cell Divisions Up To the 32-cellmentioning
confidence: 99%
“…Starting at the 32-cell stage, as the outside cells of the embryo are becoming fully committed to the TE lineage (48,49), a fluid-filled cavity known as the blastocoel begins to form ( Figure 1). The presence of a blastocoel is essential for proper development of the ICM (49). During blastocoel formation water may enter the embryo via an osmotic gradient, as a result of Na + /K + ATPases that produce an accumulation of Na + on the basolateral side of the TE (50).…”
Section: Symmetric Versus Asymmetric Cell Divisions Up To the 32-cellmentioning
Mammalian preimplantation development, which is the period extending from fertilization to implantation, results in the formation of a blastocyst with three distinct cell lineages. Only one of these lineages, the epiblast, contributes to the embryo itself, while the other two lineages, the trophectoderm and the primitive endoderm, become extraembryonic tissues. Significant gains have been made in our understanding of the major events of mouse preimplantation development, and recent discoveries have shed new light on the establishment of the three blastocyst lineages. What is less clear, however, is how closely human preimplantation development mimics that in the mouse. A greater understanding of the similarities and differences between mouse and human preimplantation development has implications for improving assisted reproductive technologies and for deriving human embryonic stem cells.
IntroductionThe period of preimplantation development in mammals, extending from egg fertilization to implantation of the blastocyst in the uterus, is a key stage during which the first three major cell lineages of the embryo and its extraembryonic membranes are set aside. These three lineages contribute to distinct tissues in later development: the epiblast (EPI) gives rise to the fetus itself; the trophectoderm (TE) goes on to form the majority of the fetal contribution to the placenta; and the primitive endoderm (PE) becomes the parietal and visceral endoderm, which later contributes to the yolk sac. Knowledge about how these lineages develop during the preimplantation period has major clinical implications for increasing the success of assisted reproductive strategies (ARTs) such as in vitro fertilization (IVF) and preimplantation genetic diagnosis (PGD), preventing the high rate of early pregnancy loss in humans, and improving the derivation of stem cell lines from human embryos.Much of what we know about preimplantation development has come from studies in the mouse, which has been used as a model for the early human embryo for over 40 years. Here, we review what has been learned from the mouse about the major events of mammalian preimplantation development and discuss recent work that has shed new insight on how the three blastocyst lineages come to be established. Despite the significant progress that has been made, we still know little about how closely the events of preimplantation development in the mouse reflect the human situation. We compare between mouse and human development where possible and point out where more investigation of early human development could be especially worthwhile.
“…The first cell fate decision is initiated after transition from the four-cell stage to the eight-cell stage and during the progression into the 32-cell stage (Pedersen et al, 1986). During this time, localization of the individual blastomeres permits the distinction of inner and outer blastomeres and this spatial difference is likely to contribute to the observed divergent gene expression (Jedrusik et al, 2008).…”
Section: Reprogramming and Preimplantation Development (Pid)mentioning
Pluripotent cells of the blastocyst inner cell mass (ICM) and their in vitro derivatives, embryonic stem (ES) cells, contain genomes in an epigenetic state that are poised for subsequent differentiation. Their chromatin is hyperdynamic in nature and relatively uncondensed. In addition, a large number of genes are expressed at low levels in both ICM and ES cells. Also, the chromatin of naturally pluripotent cells contains specialized histone modification patterns such as bivalent domains, which mark genes destined for later developmentally-regulated expression states. Female pluripotent cells contain X chromosomes that have yet to undergo the process of X chromosome inactivation. Collectively, these features of very early embyronic chromatin are required for the successful specification and production of differentiated cell lineages. Artificial reprogramming methods such as somatic nuclear transfer (SCNT), ES cell fusion-mediated reprogramming (FMR), and induced pluripotency (iPS) yield pluripotent cells that recapitulate many features of naturally pluripotent cells, including many of their epigenetic features. However, the route to pluripotent epigenomic states in artificial pluripotent cells differs drastically from that of their natural counterparts. Here, we compare and contrast the differing routes to pluripotency under natural and artificial conditions. In addition, we discuss the intrinsically metastable nature of the pluripotent epigenome and consider epigenetic aspects of reprogramming that may lead to incomplete or inaccurate reprogrammed states. Artificial methods of reprogramming hold immense promise for the development of autologous cell graft sources and for the development of cell culture models for human genetic disorders. However, the utility of artificially reprogrammed cells is highly dependent on the fidelity of the reprogramming process and it is therefore critically important to assess the epigenetic similarities between embryonic and induced pluripotent stem cells.
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