Covers early mouse embryonic development - formation of the blastocyst, gene regulatory network and cellular mechanisms producing the trophoblast, endoderm and ICM. Extra reading with sources cited.
Mouse has a life cycle of 9 weeks from fertilisation
to mature adult which is relatively short for a
mammal
Mammalian eggs are much smaller and internal
development leads to difficulties in observation and
experimentation
Genetic analysis relatively easy - main mammalian
model organism and first whole genome sequenced
Mice with particular genetic constitution can also be
routinely produced using transgenic techniques/ produce knockout mice (where some
genes rendered inactive)
Vertebrate Techniques
Earliest studies involved microscopy – direct observation, superseded by digital-
imaging technology
Detecting the expression of genes in situ developed in latter half of 20 th century –
both riboprobes to detect gene transcripts and immunohisochemisty – using
antibodies to detect expression of proteins the transcripts encode = mapping
expression patterns of developmentally important genes
DNA microarray analysis – detect and identify expression of large number of genes
simultaneously
Transplantation, gene silencing, overexpression or misexpression
,Early stages
1. Fertilisation occurs in the ampulla of the oviduct – meiosis is completed, first
cleavage occurs 1 day later, cilia in the oviduct push embryo toward the uterus
(cleavages occur throughout the journey)
Unique to mammalian embryos:
- Very slow divisions – taking place 12-24 hours apart
- Unique orientation of mammalian blastomeres in relation to each other: first
cleavage is meridional division, second cleavage – one divides meridionally, one
equatorially = rotational cleavage
- Marked asynchrony of early cell division – blastomeres do not divide
simultaneously, embryos do not increase exponentially from 2-8 cell stages
(frequently contain odd number of cells)
- Newly formed nuclei (rather than the oocyte cytoplasm) produce proteins
necessary for cleavage and development
In mouse and goat – switch from maternal to zygotic control occurs at the 2-cell
stage; in humans between 4- and 8- cell stages (Piko and Clegg, 1982)
2. At E4.5 the blastocyst hatches from zona pellucida to implant in the uterus
3. Cleavage forms a ball of cells (=morula) which
until the 8-cell stage are loosely associated
4. Blastocyst = asymmetric hollow ball of cells
surround a fluid cavity (=blastocoel)
Compaction
Following 3rd cleavage, blastomeres suddenly
huddle together to form a compact ball off cells
Cell adhesion proteins (eg. E-Cadherin) become
expressed
Tightly packed arrangement stabilised by tight
junctions that form between outside cells, sealing
off inside of sphere (which no longer have access to external fluid)
Cells within sphere form gap junctions – thereby enabling small molecules and ions to
pass between them
After compaction, cells become polarised – exterior surfaces carry microvilli,
interior surfaces smooth
Microvilli extend and contract to pull cells together
, Formation of the blastocyst
Cells of 8-cell embryo divide to produce a 16-
cell morula = internal cells (Inner cell mass
ICM) and the external/trophoblast cells
(trophectoderm)
Trophectoderm gives rise to the chorion (outer
embryonic membrane of the placenta
ICM gives rise to amnion (inner embryonic membrane), allantois, and the embryo
proper - ICM (by the 64-cell stage ICM and trophoblast are completely separate
cell layers) also actively supports the trophoblast, secreting proteins such as Fgf4
to stimulate the trophoblast cells to divide (Tanaka et al., 1998)
Cells which give rise to trophectoderm or ICM appear to be random (Dard et al.,
2009)
Cavitation: Vectorial fluid is pumped by the trophectoderm into the interior of the
blastocyst, causing trophectoderm to expand and form a fluid-filled cavity
(blastocoel) containing ICM (inner cells are connected by junctions) at one end
Cavitation achieved by Na+/K+-ATPase and Na+/H+ exchanger that pump Na+ into
central cavity – accumulation of sodium ions draws n water osmotically, enlarging the
blastocoel (Ekkert et al,, 2004); seems to be stimulated by oviduct cells while
embryo is travelling to uterus (Xu et al., 2004)
From E.3.5-4.5…
ICM divided into 2 regions: surface layer in contact with blastocoel becomes the
primitive endoderm (extra-embryonic membranes - amnion) + primitive
ectoderm/epiblast (embryo proper as well as some extra-embryonic components)
Trophectoderm (outer shell) differentiates to form trophoblast stem cells, giant
trophoblast cells and placenta
The signals
Trophoblast cells express Eomesodermin and Cdx2
All ICM cells express Oct4 and downregulate Eomes and
Cdx2
Primitive endoderm cells express GATA6, GNCF, Sox17
Epiblast cells express nanog, Sox2 and Stat3
Gene Regulatory Network (GRN) for ICM development
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