Download Bilaminar Germ Disc Lecture 2013

INDI-555
Anatomy and Pathophysiology
Bilaminar Germ Disc
Carlos A C Baptista, MD., PhD. MPH
Department of Neurosciences
Rule of Twos
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2 germ layers
2 tissues from trophoblast
2 times blastocyst cavity remodeled
2 new cavities appear
2 layers from extraembryonic
mesoderm
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Formation of the Bilaminar
Germ Disc
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1. Differentiation of the inner cell
mass
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2. Differentiation of the trophoblast
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a. epiblast
b. hypoblast
a. cytotrophoblast
b. syncytiotrophoblast
3. Formation of the amniotic cavity
4. Formation of the yolk sac
2/25/2013
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Cleavage and Blastocyst Formation
Blastocyst
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A blastocyst is formed by accumulation of fluid
between the blastomeres. The cells form two
distinct groups:
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The trophoblast – a single layer of cells surrounding
the blastocyst cavity. It will eventually form the
placenta and membranes.
The embryoblast – a mass of cells situated within
the trophoblast. It will give rise to the embryo.
As a result of the accumulation of fluid the
blastocyst increases rapidly in size. The zona
pellucida ruptures and disintegrates.
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Cleavage and Blastocyst Formation
embryoblast
Stages in Implantation
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Implantation
Implantation (Day 6-9)
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On Day 8
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Contact with the uterine endometrium
induces trophoblast at the embryonic
plate to proliferate
Cells lose their cell membrane and
coalesce to form a syncytium.
Cells of the trophoblast (forming the
wall of the blastocyst) retain their cell
membrane.
Day 8
On approximately day 8 the trophoblast
differentiates into two distinct layers:
 Cytotrophoblast
 Syncytiotrophoblast
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Day 8
Cytotrophoblast :
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The inner layer
Consists of a single layer of cuboidal
cells
Is the source of dividing cells
Syncytiotrophoblast
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Day 8 - Syncytiotrophoblast
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The outer layer
Consists of a mass of multinucleated
cytoplasm with irregular finger-like
processes
Formed by coalescence of cells derived
from the cytotrophoblast
Does not contain mitotic figures
Differentiation of the syncytiotrophoblast
begins at the embryonic pole and spreads
over the blastocyst.
Cytotrophoblast
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Day 8
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The syncytiotrophoblast has two important
secretory functions:
1. Secretion of hydrolytic enzymes
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essential for erosion and penetration of the
endometrium
2. Secretion of Human Chorionic Gonadotrophin
(HCG)
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It has properties of LH
It is essential for maintenance of the corpus luteum,
which enlarges to form a corpus luteum of pregnancy
It is essential for maintenance of pregnancy
The embryo’s signal saying “Hi Mum! I’m here!”
Hydrolytic Enzymes
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Embryo becomes completely implanted
in the endometrium largely as a result
of the activities of the highly invasive
syncytiotrophoblast.
Hydrolic enzymes break down the
extracellular matrix between the
endometrial cells.
The syncytiotrophoblast gradually
envelops the blastocyst.
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Day 8
Day 9
Acellular material
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Day 8
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The embryoblast differentiates
(splits) into two germ layers to form
a bilaminar embryo (or germ disc):
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Epiblast – a columnar epithelium
adjacent to the trophoblast (external)
(primary ectoderm)
Hypoblast – a cubical epithelium
adjacent to the blastocyst cavity
(internal) (primary endoderm)
Epiblast and Hypoblast
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Day 8
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Two cavities are also formed:
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Amniotic cavity lined by the amniotic
membrane, a thin layer of cells derived
and growing out from epiblast
Primary yolk sac lined by Heuser’s
membrane, a thin layer of cells derived
and growing out from the hypoblast
Amniotic Cavity
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On day 8 fluid begins to collect
between cells of the epiblast
A layer of epiblast cells is gradually
displaced toward the embryonic
pole.
The layer differentiate into a thin
membrane separating the new
cavity from the cytotrophoblast.
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Day 8
Amniotic Membrane
epiblast
Amniotic
cavity
amnioblast
Amniotic
membrane
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Yolk Sac Formation
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Two successive membrane migrate
out from the hypoblast to line the
blastocyst cavity, transforming it first
into a primary yolk sac and then into
the definitive yolk sac. (Heuser’s
Membrane)
Novel space created (chorionic
cavity)
Outer wall of the blastocyst is called
chorion
Day 9
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Yolk Sac Formation
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Separation of Heuser’s membrane from the
cytotrophoblast gives rise to two new cavities:
The secondary (definitive) yolk sac shrinks
away from the cytotrophoblast and becomes
re-lined by a new layer of cells derived from
the hypoblast
The chorionic cavity or extra-embryonic
coelom forms between the lining of the yolk
sac and the cytotrophoblast. It contains cells –
the extra-embryonic mesoderm- of uncertain
origin.
Extraembryonic Mesoderm
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On days 10-11, an acellular
extraembryonic reticulum forms
between Heuser’s membrane and
cytotrophoblast.
On days 11-12, the reticulum is
rapidly invaded by extraembryonic
mesoderm
It originates from the epiblast (?)
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Extraembryonic Mesoderm
Extraembryonic Mesoderm
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Extraembryonic Mesoderm
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By day 12 it becomes organized:
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Coat the outside of Houser’s
membrane
Lining the inside of cytotrophoblast
Lacunae appear in the
extraembryonic reticulum between
the layers and coalesce to form the
chorionic cavity.
Day 12
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Chorionic Cavity
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The chorionic cavity expands greatly by
accumulation of fluid within it. As a result :
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The chorionic cavity becomes the dominant
cavity
the amniotic cavity and yolk sac become
progressively smaller.
The chorionic cavity is lined by extraembryonic mesoderm
The cytotrophoblast is lined by a layer of
extra-embryonic mesoderm.
Day 8
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The chorion is the three-layered
membrane surrounding the chorionic
cavity.
It consists of cyncytiotrophoblast,
cytotrophoblast and extra-embryonic
mesoderm.
Both the amnion and the yolk sac are
covered externally by a layer of extraembryonic mesoderm.
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Chorionic Cavity
Chorionic Cavity
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Definitive Yolk Sac
New wave of proliferation in the hypoblast produces a new membrane that
Migrate over the inside of the extraembryonic mesoderm.
Primary Yolk Sac
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Yolk Sac
Uteroplacental Circulation
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First week embryo obtains
nutrients and eliminates wastes by
simple diffusion.
Growth of embryo required more
sophisticated system.
Uteroplacental circulation is born
Begins to form at day 9
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Uteroplacental Circulation
Day 11-13
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Vacuoles called trophoblastic lacunae
open within the syncytiotrophoblast.
Maternal sinusoids anastomose with
the trophoblastic lacunae.
Cytotrophoblast proliferate and grow.
Protrusions induced by the
extraembryonic mesoderm (?)
Primary stem villi
Day 11-13
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Uteroplacental Circulation
Day 16
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Extraembryonic mesoderm
associated with the
cytotrophoblast penetrated the
core of the primary stem villi, thus
transforming them into secondary
stem villi
Day 16
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Uteroplacental Circulation
End of third week
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Villous mesoderm has given rise
to blood vessels that connect with
the vessels forming in the embryo
proper establishing a working
uteroplacental circulation.
Tertiary stem villi (villi containing
differentiated blood vessels)
Day 21
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Gas and Nutrients Diffusion
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Cross four tissue layers:
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Endothelium of the villus capillaries
Loose connective tissue in the core
of the villus
A layer of cytotrophoblast
A layer of syncytiotrophoblast
Implantation
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Three Layers Formation
Cell and Tissue Lineage
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Sites of Ectopic Implantation
Tubal (isthmic)
Interstitial
Tubal
(ampullar)
Infundibular
(ostial)
Ovarian
cervical
Unruptured Tubal Pregnancy
Villi invading
tubal wall
Chorion
Amnion
hemorrage
lumen
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Tubal
Pregnancy
Uterus
Intraperitoneal rupture
Of uterine tube
Tubal Pregnancy
Uterus
Ovarian Ligament
Ovary
Rupture into Broad Ligament
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Tubal Pregnancy
Spontaneous Tubal
Abortion
Abdominal Pregnancy
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Interstitial Pregnancy
Uterus
Tube
Ovarian Pregnancy
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