Twenty Two Flashcards
Describe fertilization up to the point of the formation of the bilaminar germ disc.
About a week after fertilization, the blastocyst attaches to the endometrium, and starts
embedding into the stroma of the endometrium by sending out little fingers or blobs called
SYNCYTIOTROPHOBLASTS. This syncytiotrophoblast is basically a mass of what used to
be CYTOTROPHOBLASTS that have lost their cell membranes and mitotic figures…kind of
an amorphous invading blob of goo. The cells on the opposite side of the invading blastocyst
retain their original structure and are still called cytotrophoblasts. After about 2-3 days, the
blastocyst is completely embedded in the endometrium. Proteolytic enzymes control this
whole invasion process. Note that the blastocyst has this lump of cells in the area closest to the
uterus called the INNER CELL MASS. This mass is getting ready to form a little person by
forming itself into two parts. The HYPOBLAST layer, and the EPIBLAST layer.
So. What we have so far is an
invading blob of
syncytiotrophoblasts, an epiblast and
a hypoblast layer, and
cytotrophoblasts surrounding the
blastocyst cavity, (which is soon to
go). This is shown at the left.
The epiblast and hypoblast form the
BILAMINAR GERM DISC.
Also note that as the embryo
embeds more deeply, the
whole thing gets surrounded
by this syncytiotrophoblast
layer. This is shown on the
next diagram.
What is the epiblast? Where is it located? What is it like? Same questions for hypoblast?
Back to the bilaminar germ
disc. This is differentiated by
around day 8. The epiblast is
a layer of columnar cells and
is also referred to as
the PRIMARY ECTODERM. The hypoblast is the layer of cuboidal cells closest to the
doomed blastocyst cavity. It is also called the PRIMARY ENDODERM. There is also a basement membrane between these two layers.
When does the amniotic cavity develop? What is it like initially?
On about day 8 (see above diagram), the amniotic cavity starts to develop. This is the big one
because eventually, this is the one the baby will be swimming in. DON’T CONFUSE this with
the chorionic cavity (later), which is huge at first, but ends up being obliterated as well.
Note that there is a thin layer of cells surrounding the new amniotic cavity called
AMNIOBLASTS. This gives rise to the amniotic membranes which when ruptured, give rise
to the saying “she broke her water”. We will follow the development of these membranes so
just pay close attention. It is easy to get lost in all of these cavities.
Describe the development of the exocoelemic cavity (another name?) and the extraembryonic reticulum.
Also on day 8, the hypoblast gives rise to a new membrane called the EXOCOELOMIC
MEMBRANE or HUESER’S MEMBRANE. If you look in the ABOVE diagram, you will see
the arrows of this membrane lining the
inside of the blastocyst cavity. This
cavity, once it is finally lined with
Heuser’s membrane is now called the
PRIMARY YOLK SAC or the
EXOCOELOMIC CAVITY. To
make things even more fun, a thick,
loosely reticular layer of acellular
material gets secreted between this
new Heuser’s membrane and the
cytotrophoblasts that were originally
the wall of the blastocyst cavity. This
is the EXTRAEMBYONIC
RETICULUM.
Describe the development of the chorionic cavity.
On about day 12 or 13, some new cells arise from the epiblast, and these are called the
EXTRAEMBRYONIC MESODERM cells. What these cells do (and remember they start near
the bilaminar disc) is they migrate through that thick extraembryonic reticulum that we just
talked about. The cells kind of take a split pathway: one part migrates down the outside of the
extraembryonic reticulum along the inside of the cytotrophoblasts, and the other migrates
along the outside of the primary yolk sac, along Heuser’s membrane. Eventually the
extraembryonic reticulum breaks down and becomes fluid filled, thus forming the
CHORIONIC CAVITY. Note again that this cavity is between the cytotrophoblasts and the
yolk sac. Eventually this cavity is going to get so big, it is going to pinch off the embryo leave
it hanging in the chorionic cavity by a CONNECTING STALK. This outer layer of
extraembryonic mesoderm will ultimately become blood vessels, as we will see in a bit.
Describe the development of the definitive yolk sac. Where does hematopoesis occur? What is Meckels diverticulum?
Also around day 12, the hypoblast starts proliferating again and sends a new wave of cuboidal
cells along the inner surface of the extraembryonic mesoderm (remember that that was the old
blastocyst cavity). What this ultimately will do is makes a new cavity, called the DEFINITIVE
or SECONDARY YOLK SAC. If you follow along the diagram, you can see how this
happens. Basically, these cells migrate along the extraembryonic mesoderm, and somehow
pinch the old primary yolk sac (old blastocyst cavity) off in the middle and send the left over
part to the opposite side of the embryo as exocoelomic vesicles. What is still stuck to the
embryo is the definitive yolk sac. The extraembryonic layer of this yolk sac is where the
HEMATOPOIESIS takes place. Yes, that is right, that is where blood is formed, in the wall of
the definitive yolk sac. Also the germ cells that become the eggs and the sperm form in the
yolk sac. Usually this yolk sac disappears, but may persist as the anomalous MECKEL’S
DIVERTICULUM.
Why does the formation of the placenta become necessary? Describe the development of trophoblastic lacunae and maternal sinusoids, as well as the establishment of uteroplacental circulation.
At first, when the embryo is
tiny, it gets all it needs by
simple diffusion. Later on, this
does not cut it so we see a
placenta develop. This all
happens when the embryo is
pretty much burying itself into
the endometrium. So let’s turn
back the clock about 4 days and
see how this happens.
Remember that amorphous
layer called the
syncytiotrophoplast that
surrounds the embryo? Well,
vacuoles appear in this layer
and later coalesce and eventually turn into what are called TROPHOBLASTIC LACUNAE. These are pictured in the
illustration at the top of page 2. You can follow the progression in the prior illustrations, but
what happens is that the syncytiotrophoblast just keep penetrating more deeply into the
endometrium and eventually runs into maternal capillaries. The syncytiotrophoblast eventually
erodes into the endothelial lining of the capillaries, which become congested and dilated.
These are called maternal SINUSOIDS. The lacunae in the syncytiotrophoblast layer become
continuous with these sinusoids, and maternal blood flows through this lacunar system. This
mixing of maternal blood into the lacunae establishes the UTEROPLACENTAL
CIRCULATION.
Describe the development of primary villi, secondary villi, and tertiary villi.
Next, the cytotrophoblasts (see fig A on page 5) start forming these little finger like projections
that grow outward into the syncytiotrophoblasts. These cytotrophoblast columns with the
syncytial covering are called PRIMARY VILLI.
On about day 16, the extraembryonic mesoderm starts to grow outward and penetrate into the
cytotrophoblastic columns. This new structure is called the SECONDARY VILLUS. By the end of the third
week, this mesoderm
in the core of the
secondary villus begins
to differentiate into
blood vessels and once
this process is
completed, become
TERTIARY VILLI.
What is the chorionic plate? Connecting stalk?
These capillaries miraculously hook up with capillaries and vessels in the extraembryonic
mesoderm (which is now called a chorionic plate) and connecting stalk (later to become the
umbilical cord) to establish the whole circulatory set up. Later you can see these vessels on the
fetal side of the placenta.
Describe the development of the outer cytotrophoblastic shell. What are stem or anchoring villi? Terminal villi?
If you look carefully at
the diagrams, especially
the one to the left here,
you will see that the
cytotrophoblastic
columns have not only
hollowed out to become
the tertiary villi, but have
also kept growing up and
away from the embryo
and started branching out,
ultimately connecting
into a thin OUTER
CYTOTROPHOBLAST
SHELL. This eventually
surrounds the entire
embryo and is what
attaches the embryo and all the layers onto the endometrium of mom. Eventually, there are
villi that extend from the chorionic plate all the way through to the decidua basalis discussed
below). These are called STEM or ANCHORING VILLI. There will be a lot of branching
from these stem villi called TERMINAL villi, and this is where most of nutrient exchange
What are the decidua basalis, the decidua capsularis, and the decidua parietalis?
The decidua is basically the endometrium of a pregnant uterus. After childbirth, this layer will
separate from the rest of the uterus and slough away. This is called lochia and you will learn
about it and see it during your OB/GYN rotation.
There are three regions of the deciduas that have special names:
DECIDUA BASILIS-the maternal component of the placenta, or endometrium that is directly
beneath the placenta
DECIDUA CAPSULARIS-encapsulates the embryo
DECIDUA PARIETALIS (VERA) –the rest of the endometrium on the uterus
Describe the development of the villous chorion and the smooth chorion.
Also note that at about 8 weeks, the chorionic villi that were once surrounding the entire
embryo are starting to disappear in the area of the deciduas capsularis. This chorion becomes
the CHORION LAEVE or SMOOTH CHORION. Remember that the chorion was the space
that was more-or-less the original blastocyst cavity. It is easy to confuse this with amniotic
cavity. Also note that the chorion at the site of the placenta is called the VILLOUS chorion or
CHORION FRONDOSUM (bushy chorion). The stem villi branch as previously mentioned
and this significantly causes the placenta to become thicker.
Describe the development of the amniochorionic membrane.
Eventually with growth, the decidua capsularis degenerates and the smooth chorion laeve
comes into direct contact with the decidua parietalis on the opposite wall of the uterus. The
two fuse and obliterate the uterine cavity. So basically it is the chorion frondosum and the
decidua basalis that make up the actual placenta. Also, the chorionic cavity is obliterated as
well by the development of the amniotic cavity. The amnion and smooth chorion fuse to form
an AMNIOCHORIONIC MEMBRANE, and it is this membrane that causes panic when “her
water breaks!”
Describe what happens with the chorionic villi at about 20 weeks.
Also note that chorionic villi
change too. At first there are
four layers: (outer to inner)
- syncytiotrophoblasts
- cytotrophoblasts
- Connective tissue in
the villus
- Endothelium of the
fetal capillaries
At about 20 weeks, the
cytotrophoblast and at times
the connective tissue
disappear, leaving only the
endothelium and syn-
cytiotrophoblasts separating
maternal and fetal
circulations.
Describe the development of the decidual plate and the chorionic plate. What are they?
By about the fourth
month, the placenta has
two components: the
fetal side, which is the
chorion frondosum, later
to be called DECIDUAL
PLATE and the maternal
side (endometrial) which
is the decidua basalis.
(See figure, top of page 8
and to the left). What the
baby sees is the chorionic
plate. Between the
chorionic plate and decidual plates are the villi, the “meat” of the placenta and the intervillous
spaces that are filled with maternal blood. These are pictured at the top of the page.
