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Embryology 8
In this lecture we will talk about the second week of development of the blastocyst.
The most important event in the second week is forming the bilaminar germ disc which
consists of the primary ectoderm from epiblast and the primary endoderm from hypoblast.
Note: when we talk about the development of the embryo in any day we should mention two
1-the developments in the blastocyst that happens to the embryo.
2-the developments in the endometrium.
we had learned that in the seventh day the blastocyst implants superficially in the endometrium
(in the tenth day it become completely implanted )also the trophoblast had turned to
cytotrophoblast and syncytiotrophoblast and the blastocyst cavity still existing, but also the
amniotic cavity formed above the epiblast.
Embryology 8
The eighth day
Blastocyst development
 Blastocyst is partially embedded in the endometrial stroma (partial
implantation). In the area over the embryoblast, the trophoblast has
differentiated into two layers:
1-cytotrophoblast (which have cell membranes "obvious boundary "
,mononucleated and divide by mitotic division).
2-syncytiotrophoblast (which multinucleated zone without distinct cell
boundaries and no divisions)
 Mitotic figures are found in the cytotrophoblast but not in the
syncytiotrophoblast.Thus, cells in the cytotrophoblast divide and migrate into
the syncytiotrophoblast, where they fuse and lose their individual cell
membranes, the syncytiotrophoblast secrete HCG hormone that used in the
pregnancy test
 the implantation took place in the superficial layer (surface epithelium)by
penetration of the embryo in the seventh day so the surface epithelium contains
an opening that will be closed by fibrin clot at the end of implantation then the
surface epithelium regenerates and forms complete surface .
 the part of the embryo that penetrates the endometrium called embryonic pole
and the opposite pole called abembryonic pole.
 By the end of the thirteenth and the fourteenth days (the second week) the
whole embryo has become inside the endometrium and the changes surround
all its parts.
Embryology 8
The changes in the endometrium in the 8th day:
 The small blood vessels dilate and encourage also the glands of the uterine to
become fully secretive and to secrete the glycogen and other proteins to nourish
the embryo. The endometrial stroma adjacent to the implantation site is
edematous and highly vascular. The large, tortuous glands secrete abundant
glycogen and mucus.
 Cells of the inner cell mass or embryoblast also differentiate into two layer
(a) a layer of small cuboidal cells adjacent to the blastocyst cavity, known as the
hypoblast layer,
(b) a layer of high columnar cells adjacent to the amniotic cavity, the epiblast
layer that differentiate to the ectoderm layer in the future.
 The three germ layers of the embryo that will form in the future are:
1-the ectoderm (external layer)
2-the mesoderm (middle layer)
3-the endoderm (internal layer)
 And these three layers will differentiate to form the all different organs of the body.
Like the connective tissues (blood, lymph, bones and cartilage) which originate from
the mesoderm layer.
 The epiblast also called primary ectoderm and the hypoblast called primary
endoderm these two layers form the bilaminar germ disc, notice that the primary
mesoderm layer has not formed yet.
 At the same time, a small cavity appears within or above the epiblast. This cavity
enlarges to become the amniotic cavity.
 Epiblast (columnar cells) cells adjacent to the cytotrophoblast are called
amnioblasts(flattened cells); together amnioblast with the rest of the epiblast, they
line the amniotic cavity and this amniotic cavity formed inside the epiblast and in
the future it will enlarge and contain the amniotic fluid that surrounds the embryo
completely with the amniotic membrane (from the amnioblast cells).
Embryology 8
The Ninth day
Blastocyst Development
 The amniotic cavity gradually enlarges and the amnioblast cells with epiblast become
apparent. The hypoblast cells proliferate especially at the edges and continue downwards as
primary endodermal cells.
 The blastocyst cavity turns to exocoelomic cavity, or primitive (primary) yolk sac and its
function is nourishment to the embryo. It acquired this name because its lined by
exocoelomic (Heuser's) membrane which proliferated "the membrane" from the hypoblast
cells. So the lining epithelium of the exocoelomic cavity is the hypoblast cells from above and
the exocoelomic (Heuser's) membrane from the sides.
 At the place of penetration, a fibrin coagulum has formed and the whole embryo is now
inside the endometrium, so the implantation occurred fully either by the end of the ninth
day or the beginning of the tenth day.
 The epithelial surface of the endometrium enters mitotic division to make fully healing in the
place of implantation and the cytotrophoblasts all around the blastocyst enter mitotic
division and form syncytiotrophoblast even at the abembryonic pole, as well as trophoblastic
lacunae(called like this because the syncytiotrophoblast originate from trophoblast) formed
within syncytiotrophoblast ,these lacunae will fuse with the uterine blood vessels to form
the uteroplacental circulation, note that in the future the syncytiotrophoblast will grow up
to form placenta.
Embryology 8
The changes in the endometrium in the 9th day:
 The epithelial surface of the endometrium enters mitotic division to make fully
healing in the place of implantation and the blood vessels become dilated , enlarged
and near to synsytiotrophpblast.
 The blastocyst is more deeply embedded in the endometrium, and the penetration
defect in the surface epithelium is closed by a fibrin coagulum
 The trophoblast shows considerable progress in development, particularly at the
embryonic pole, where vacuoles appear in the syncytium(synsytiotrophpblast).
 When these vacuoles fuse, they form large lacunae, and this phase of trophoblast
development is thus known as the lacunar stage
 At the abembryonic pole, meanwhile, flattened cells probably originating from the
hypoblast form a thin membrane, the exocoelomic (Heuser’s) membrane, that lines
the inner surface of the cytotrophoblast
 This membrane, together with the hypoblast, forms the lining of the exocoelomic
cavity, or primitive yolk sac.
The tenth day
The blastocyst is completely implanted.
The endometrium divides to fully close implantation place with epithelium.
The eleventh and the twelfth days
The extraembryonic coelom (also called primary mesoderm) forms in the 11th day and it's called
extra embryonic because it is out of the bilaminar germ disc.
This extraembryonic ceolom is extraembryonic mesoderm and the Heuser's membrane will be
replaced by primary endoderm cells extraembryonic endoderm and the primary yolk sac will
turn to a secondary yolk sac which smaller in the size.
Embryology 8
The changes in the endometrium in the 11th & 12th days:
Trophoblastic lacunae will be formed at abembryonic pole and the syncytiotrophoblast will
penetrate large area of the endometrium ,also the dilated and encouraged blood vessels grow
up to maternal sinusoids at the embryonic pole
Sinusoids: large spaces filled with maternal bloods in the endometrium
These sinusoids in the maternal side ,but in the embryonic side the trophblastic lacunae enlarge
and do communication with sinusoids in the 12th day
This communication called uteroplacental circulation functions as a nourishment.
By the 11th to 12th day of development, the blastocyst is completely embedded in the
endometrial stroma and the surface epithelium almost entirely covers the original defect in the
uterine wall
The blastocyst now produces a slight protrusion into the lumen of the uterus.
The trophoblast is characterized by lacunar spaces in the syncytium that form an
intercommunicating network.
This network is particularly evident at the embryonic pole; at the abembryonic pole, the
trophoblast still consists mainly of cytotrophoblastic cells
Concurrently, cells of the syncytiotrophoblast penetrate deeper into the stroma and erode the
endothelial lining of the maternal capillaries.
These capillaries, which are congested and dilated, are known as sinusoids.
The syncytial lacunae become continuous with the sinusoids and maternal blood enters the
lacunar system
As the trophoblast continues to erode more and more sinusoids, maternal blood begins to flow
through the trophoblastic system, establishing the uteroplacental circulation.
In the meantime, a new population of cells appears between the inner surface of the
cytotrophoblast and the outer surface of the exocoelomic there is very important
event including the proliferating of the cells.
Embryology 8
These cells, derived from yolk sac cells, form a fine, loose connective tissue called, the
extraembryonic mesoderm, which eventually fills all of the space between the cytotrophoblast
externally and the amnion and exocoelomic membrane internally
Soon, large cavities develop in the extraembryonic mesoderm, and when these become
confluent, they form a new space known as the extraembryonic coelom, or chorionic cavity
This space surrounds the primitive yolk sac and amniotic cavity except where the germ disc is
connected to the trophoblast by the connecting stalk also,it surrounds the bilaminar germ disc.
The connecting stalk is the part that will form in the future the umbilical cord
Nobody exactly know the origin of the extraembryonic mesoderm but the some supposed it
has come from the edges of the bilaminar germ disc but actually there is genome box in the
embryonic cells programmed by the time so in the 12th or 13th day it is translated and form
the extraembryonic mesoderm layer
The extraembryonic mesoderm lining the cytotrophoblast and amnion is called the
extraembryonic somatopleuric mesoderm
the lining covering the yolk sac is known as the extraembryonic splanchnopleuric
Growth of the bilaminar disc is relatively slow compared with that of the trophoblast
the disc remains very small (0.1–0.2 mm).
Embryology 8
Cells of the endometrium, meanwhile, become polyhedral and loaded with glycogen and lipids;
intercellular spaces are filled with extravasate, and the tissue is edematous.(the endometrium
becomes edematous by the fluids ,glycogen and other proteins)
These changes, known as the decidua reaction (the changes of the endometrium), at first are
confined to the area immediately surrounding the implantation site but soon occur throughout
the endometrium.
The decidua is three types:
1- basalis that forms placenta and it's the most important one.
**Pleural is the adjective of pleura which surround the organs like the parietal(somatic) and
visceral(splanchnic) pleura of the lungs.
The thirteenth day:
Blastocyst development:
The connecting stalk is the only part of the embryo out the chorion ,it's at the side of the tail of
the embryo.
The extraembryonic coelom (chorionic cavity) enlarges and the primary yolk sac decreases in
size forming secondary yolk sac.
Embryology 8
The Heuser's membrane turns to extraembryonic endoderm lines the secondary yolk
sac,peripherally to the extraembryonic endoderm is extraembryonic splanchnopleuric
The extracoelomic cyst has no importance it just produced by changing the yolk sac from
primary to secondary
The cytotrophoblast makes primary villi which is protrusion the cytotrophoblast through the
synsytium at the embryonic pole then secondary villi formed and protrude the synsytium in the
endometrium also there is tertiary villi and finally it form the placenta.
The changes in the endometrium in the 13th day:
It become edematous and filled with lipids, glycogen and proteins
The trophoblastic lacunae began in the embryonic pole and it will extend to reach the
abembryonic pole.
Decidual reaction still exists.
By the 13th day of development, the surface defect in the endometrium has usually healed.
Occasionally, however, bleeding occurs at the implantation site as a result of increased blood
flow into the lacunar spaces.
Because this bleeding occurs near the 28th day of the menstrual cycle, it may be confused with
normal menstrual bleeding and, therefore, cause inaccuracy in determining the expected
delivery date(some pregnant ladies have in the 28th day of menstrual cycle (the thirteenth day
of pregnancy)have bleeding as the menstrual cycle caused by the bulky amount of blood in
lacunar spaces that results in miscalculation the delivery date) .
The trophoblast is characterized by villous structures.
Cells of the cytotrophoblast proliferate locally and penetrate into the syncytiotrophoblast,
forming cellular columns surrounded by syncytium
Cellular columns with the syncytial covering are known as primary villi.
In the meantime, the hypoblast produces additional cells (extraembryonic endoderm cells)that
migrate along the inside of the exocoelomic membrane
These cells proliferate and gradually form a new cavity within the exocoelomic cavity.
Embryology 8
This new cavity is known as the secondary yolk sac or definitive yolk sac
This yolk sac is much smaller than the original exocoelomic cavity, or primitive yolk sac
During its formation, large portions of the exocoelomic cavity are pinched off. These portions
are represented by exocoelomic cysts, which are often found in the extraembryonic coelom or
chorionic cavity.
The beginning of the fourteenth day:
Meanwhile, the extraembryonic coelom expands and forms a large cavity, the chorionic cavity.
The extraembryonic mesoderm lining the inside of the cytotrophoblast is then known as the
chorionic plate (including the cytotrophoblast and extraembryonic).
The only place where extraembryonic mesoderm traverses the chorionic cavity is in the
connecting stalk
With development of blood vessels, the stalk becomes the umbilical cord.
Pregnancy tests
The syncytiotrophoblast is responsible for hormone production including human chorionic
gonadotropin (HCG).
By the end of the second week, quantities of this hormone are sufficient to be detected by
radioimmunoassay, which serve as the basis for pregnancy testing.
Abnormal implantation:
Abnormal implantation sites sometimes occur even within the uterus.
Occasionally the blastocyst implants close to the internal opening os (opening) of the
the placenta bridges the opening (placenta previa) and causes severe, even lifethreatening bleeding in the second part of pregnancy and during delivery so the baby
born by caesarean surgery.
Embryology 8
Occasionally, implantation takes place outside the uterus, resulting in extrauterine
pregnancy, or ectopic pregnancy.
Ectopic pregnancies may occur at any place in the abdominal cavity, ovary, or uterine
However, 95% of ectopic pregnancies occur in the uterine tube, and most of these
are in the ampulla
In the abdominal cavity, the blastocyst most frequently attaches itself to the
peritoneal lining of the rectouterine cavity, or Douglas’ pouch (these cases is not
Sometimes the blastocyst develops in the ovary proper, causing a primary ovarian
In most ectopic pregnancies, the embryo dies about the second month of gestation,
causing severe hemorrhaging and abdominal pain in the mother .
Abnormal blastocysts are common.
It is likely that most abnormal blastocysts would not have produced any sign of
pregnancy because their trophoblast was so inferior that the corpus luteum could
not have persisted.
In some cases, however, the trophoblast develops and forms placental membranes,
although little or no embryonic tissue is present.
Such a condition is known as a hydatidiformmole. Moles secrete high levels of hCG and may
produce benign or malignant (invasive mole, choriocarcinoma.
Good luck
Afnan Emad