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Fertilization
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Scope
1.
2.
3.
Capacitation
Acrosome reaction
Phases of fertilization
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Capacitation
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Functional process & final step in sperm maturation
that takes place in female genital duct and requires
contact with secretions of female oviduct
It involves the destabilization of the acrosomal sperm head
membrane allowing greater binding between sperm and
oocyte. This change is facilitated by the removal of sterols
(e.g. cholesterol) and non-covalently bound glycoproteins.
The result is a more fluid membrane with an increased
permeability to Ca2+
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Capacitation - cont…
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Capacitation represents
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A change in the sperm membrane
A removal of blocking agents
An activation of receptor sites and/or
Acquisition of competence to respond to other agents
A period of conditioning that occurs in the female
reproductive duct
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Acrosome reaction
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Acrosome cap contains acrosin (proteolytic) &
hyaluronidase (mucolytic & zonalysing) enzymes
Penetration of the corona radiata
Penetration of the zona pellucida
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Binds with ZP3 in a human-specific interaction
Release of acrosin facilitates the process
Fusion of sperm-oocyte membrane
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Induces acrosome to release degradation enzymes
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Phases of fertilization
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Consequences of fusion of
gamete cell membranes
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Cortex of oocyte releases granules to the
exterior to prevent entry of other sperm
Zona reaction
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Neuraminidase makes zona pellucida refractory
MII resumes & resulting in extrusion of 2nd polar
body
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Entry of a sperm & formation
of pronuclei
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Formation of pronuclei
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Once formed M & F pronuclei move close to
another
Pronuclear envelopes break up
M & F pronuclei fuse
Zygote results
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Fusion of pronuclei &
formation of zygote
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Consequences of fertilization
1.
2.
3.
4.
5.
Completion of MII of ovum
Restoration of diploid number
Determination of sex
Initiation of cleavage
Appearance in maternal blood of early
pregnancy factor (EPF)
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Events of the first week of life
Cleavage and morula formation
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Cleavage and morula formation
It is a division of cells in the early embryo. The zygotes of many species undergo
rapid cell cycles with no significant growth, producing a cluster of cells the same
size as the original zygote. The different cells derived from cleavage are called
blastomeres and form a compact mass called the morula. Cleavage ends with the
formation of the blastula.
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Cleavage
Cleavage
ZYGOTE
2 cells
4 cells
8 cells
Many cells
(solid ball)
Blastocoel
morula
32 cells
BLASTULA
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(hollow ball)
Cross section 15
of blastula
Segregation of blastomeres into
embryoblast and trophoblast
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Cells of the morula will give rise to
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Embryoblast and
Membranes & placenta
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Implantation/DECIDUAL REACTION
 Blastocyst being naked interacts directly with
endometrium. In the presence of progesterone secretory
cells form called decidual cells i.e. DECIDUAL REACTION
 decidual rxn is the changes of the endometrium of the
uterus preparing it for implantation of the blastocyst
 Endometrial glands enlarge; the uterine wall highly
vascularized and edematous
 By end of first week embryo will implant
 Uterine lining maintained by progesterone from corpus
luteum; Later trophoblast produce hCG(human
chorionic gonadotropin) which supports corpus luteum
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Attachment of blastocyst to endometrial
epithelium - 6d
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Syncytiotrophoblast penetrates endometrial
epithelium - 7d
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Second week of life
1. FORMATION OF THE:EMBRYO DISC,AMNIOTIC CAVITY,
AMNION, & YOLK SAC
2. CHORION FORMATION
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Embryonic disc
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The ICM differentiates into two cell types; a layer of columnar cells
called the epiblast adjacent to the trophoblast, and a layer of
cuboidal cells called the hypoblast adjacent to the Blastocoele
(blastocyst cavity).
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Bilaminar disc…
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The hypoblast lies
beneath the epiblast
and extraembryonic
endoderm (including
the Yolk sac) is derived
from this layer
Cells migrate from the
hypoblast to form
exocoelomic
(Heuser’s) membrane
that encloses the
blastocyst cavity which
will become the primary
yolk sac.
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AMNIOTIC CAVITY & AMNION,
YOLK SAC
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As Implantation continues a small space appears between the
embryoblast & the cytotrophoblast
Flat amniogenic cells called amnioblasts delineate from the epiblast
and surround a new cavity; the amniotic cavity by organizing to
form a membrane, the Amnion.
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The extraembryonic
mesoderm
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During subsequent stages of development, there appears a
population of cells that lie between the cytotrophoblast
and the exocoelomic membrane. These cells give rise to
the extraembryonic mesoderm.
As cavities develop in the extraembryonic mesoderm they
coalesce and a new space is formed called the
extraembryonic coelom (chorionic cavity)
Formation of extraembryonic coelom and yolk sac result into
split of the extraembryonic mesoderm into Extraembryonic
somatic mesoderm & the Extraembryonic splanchnic
mesoderm surround the yolk sac
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The extraembryonic mesoderm…
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The extraembryonic
mesoderm…cont..
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PROCHORDAL PLATE
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Cells of the hypoblast at a localized area cranially change into
columnar cells that is referred to as the prochordal plate an
important organizer of the H&N and a position of the future
mouth.
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Chorion
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The chorion is one of the membranes that exist during
pregnancy between the developing fetus and mother (the
fetal membranes).
The chorion and the amnion together form the amniotic
sac.
It is formed by extraembryonic mesoderm and the two
layers of trophoblast that surround the embryo and other
membranes. The chorionic villi emerge from the chorion,
invade the endometrium, and allow transfer of nutrients
from maternal blood to fetal blood.
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Chorion…
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CHORIONIC VILLI
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The chorion undergoes rapid proliferation and forms numerous
processes, the chorionic villi, which invade and destroy the uterine
decidua and at the same time absorb from it nutritive materials for
the growth of the embryo.
The greater part of the chorion is not in contact with the decidua
capsularis (decidua facing the uterine cavity). Hence this part of the
chorion becomes smooth, and is named the chorion læve; as it
takes no part in the formation of the placenta
On the other hand, the villi on that part of the chorion which is in
contact with the decidua placentalis (basalis) increase greatly in
size and complexity, and hence this part is named the chorion
frondosum.
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twining
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Monochorionic twins are twins that share the same placenta.
It occurs in 0.3% of all pregnancies (in caucasians), and in 75% of
monozygotic (identical) twins, when the split takes place beyond the
third day after fertilization.
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Monochorionic twins
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The remaining 25% of monozygous twins become dichorionic
diamniotic.
The condition may affect any type of multiple birth, resulting in
monochorionic multiples.
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