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Mechanisms responsible for establishment of epiblast and hypoblast in preimplantation
mouse embryo”
Scope of research:
Before implanting into the uterine wall, mammalian embryo (blastocyst) establishes
the primary cell lineages that are founders of the foetus and the structures supporting its
growth in the uterine environment i.e. foetal membranes and placenta. The blastocyst is a
vesicle containing a fluid-filled cavity. Two distinct cell lineages are present at this
developmental stage. The trophectoderm (TE) is a layer of cells that forms the wall of the
blastocyst and during further development contributes exclusively to the embryonic part of
the placenta. The inner cell mass (ICM) is composed of pluripotent cells and occupies one
pole of the blastocyst interior. Shortly before implantation, the ICM differentiates into two
distinct subpopulations of cells. Primitive endoderm (PE) (hypoblast) emerges as a monolayer
of cells on the surface of ICM directly facing the blastocyst cavity and after implantation
contributes to endoderm layer of extraembryonic tissue, the yolk sac. Deeper cells of ICM
comprise pluripotent epiblast (EPI) that is a source of cells of the future definite embryo and
some of the extraembryonic membranes, such as allantois and amnion.
There are two alternative models for the establishment of PE and EPI. According to
the first model, position of cells in ICM specifies their fate by differential cues to which cells
inside ICM and in the layer facing the cavity are subjected. This hypothesis assumes that ICM
cells of the early blastocyst are homogeneous population of bipotential cells, each having the
ability to become either EPI or PE. Superficial cells would differentiate as PE, while cells
occupying deeper layers of ICM would become EPI. An alternative model assumes that ICM
of early blastocyst is a heterogeneous mosaic of EPI and PE progenitors having different
developmental potential that will later sort out into the appropriate layers. The heterogeneity
of cells relies on diversified expression of markers specific for EPI and PE, such as Nanog
and Gata6, respectively. Before PE and EPI layers become clearly morphologically distinct,
the cells expressing these genes are intermingled and localised in both deeper and surface
compartments of the early ICM. Gene profiling of ICM or ES cells also revealed the
heterogeneity of the ICM cell population. These results suggest two possible mechanisms for
PE formation. Cells with specific molecular identity (i.e. expressing either PE or EPI markers)
sort out and move to the location corresponding to their destiny. Alternative mechanism
excludes cell movements and suggests that initial gene expression patterns become “adjusted”
in situ in such a way that PE marker genes become down-regulated in deeper cells and upregulated in surface cells, and vice versa for markers of the epiblast. Other studies suggest that
multiple factors – actin-dependent cell movement, adhesion, selective apoptosis and
positional signals - participate in PE and EPI lineage segregation.
The aim of this project is to elucidate the mechanisms that regulate initial phases of
cell differentiation in the mouse embryo, i.e. the specification of PE and EPI. The intended
experiments will address the question whether and when cells allocated to these two ICM
lineages become determined (restricted) in their developmental fate and what factors
(developmental origin, cell position, molecular identity or adhesive properties) are responsible
for this specification.
The candidate will use the combination of experimental approaches including:
micromanipulation of embryos, cell culture, cell and molecular biology techniques.
Requirements:
- knowledge of issues and research methods regarding embryology and cell culture
- prior experience in molecular biology techniques
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