FERTILIZATION
Sperm finds egg from chemotaxins
Fertilization occurs in the ampulla (region that is slightly
dilated and curves over the ovary).
Sperm binds to zona pellucida of secondary oocyte arrested in
metaphase of meiosis II and triggers the acrosome reaction.
Acrosome Reaction—acrosomal enzymes made from the
sperm break down the zona pellucida and allow sperm to
penetrate the gelly layer
Sperm penetrates/binds oocyte membrane
Cortical Reaction—the oocyte releases granules that harden
the zona pellucida preventing more sperm from tunneling
through and destroys sperm receptors. *this prevents
polyspermy.
Oocyte completes meiosis II and forms
second polar body, resulting in mature
oocyte whose nucleus becomes the
female pronucleus. The nucleus of sperm
becomes the male pronucleus. The
sperm loses its tail and mitochondria.
The two nuclei fuse forming a zygote
CLEAVAGE
Zygote divides repeatedlycells formed
are called blastomeres.
After 3rd cleavage compaction occurs—in which the blastomeres form a compact ball of cells which needs to happen in
order for the formation of the morula
Morula—12-32 blastomere stage which has an inner cell mass and an outer cell mass.
Inner cell mass eventually becomes embryoblast which will become the embryo proper
outer cell mass eventually becomes trophoblast which will become the placenta
BLASTOCYST FORMATION
(Blastocyst = mammalian term for blastula)
Morula enters uterus and the blastomeres begin secreting fluid that forms the blastocyst cavity (aka blastocoel)
Inner cell mass now called the embryoblast (aka embryonic pole)
Outer cell mass becomes trophoblast
Zone Pellucida is shed (to let the blastocyst grow more)
Nutrient source = Uterine Gland
IMPLANTATION—End of the first week
Blastocyst attaches to the posterior superior wall of uterus with
the inner cell mass side against the uterine wall
Trophoblast differentiate and form:
Cytotrophoblast (inner layer)
Syncytiotrophoblast—(outer layer)multinucleated/fused
cells; invade the CT of endometrium using enzymes to erode
maternal tissue.
Cytotrophoblasts feed into the syncytiotrophoblasts and
fuse with them. (resupply them)
Syncytiotrophoblasts made hCG
Embryoblast begin differentiating into two layers..
Endometrial Changes During Implantation endometrial cells swell from accumulation of glycogen and lipid droplets
become decidua (what you call the endometrium during pregnancy). It wraps around the implanted embryo—important
because these cells secrete IL-2 which prevents the mother from thinking the embryo is a foreign pathogen.
Decidua Basalis—between embryo and myometrium; supplies nutrients to embryo and will become the maternal part of
the placenta
Decidua Parietalis—the endometrium in the part of the uterus occupied by the implanted embryo
Decidua Casularis—covers the embryo
SECOND WEEK—Embryonic Disc Formation
Part 1:
Inner Layers—fluid accumulates in the epiblast and fuses to form the amniotic cavity that splits the epiblast into two
layers, with a layer of amnion/amnioblast lining the upper region (separates the cavity from the cytotrophoblasts. At
this point the epiblast has completely differentiated into the
hypoblast (cuboidal cells next to blastocystic cavity)
epiblast (a layer of high columnar cells adjacent to amniotic cavity)
Bilaminar/Embryonic Disc = epiblast + hypoblast layered one on top of each other
Nutrient source = eroded maternal tissue
Outer Layers—syncytiotrophoblasts continue to invade endometrium.
Cytotrophoblast divide mitotically, Syncytiotrophoblasts DO NOT.
Nutrient Source = engulfed dying cells
Hypoblast
Primary Yolk Sac
Forming Extra Embryonic Mesoderm
Part 2:
Hypoblast cells from the edge of the bilaminar disk migrate along the inner side
of the blastocyst cavity and thereby flatten out. This new layer of hypoblast cells
is called Heuser’s Membrane. The cavity it surrounds is now called the primary
yolk sac.
Extra embryonic reticulum (acellular) forms between the cytotrophoblasts and
Heuser’s membrane, separating the primary yolk sac from the cytotrophoblast.
Epiblast proliferate and form the cells called the extraembryonic mesoderm that
lines the cytotrophoblast, covers the amnion, (and eventually forms the
connecting stalk.)
After Day 12Vacuoles fuse to form large
lacuna = Lacunar Stage
Nutrient Source = Lacuna becomes filled with a
mixture of maternal blood (with hCG) from
ruptured capillaries and dead cells; this diffuses
into embryo.
Primary Chorionic Villi—formed by
cytotrophoblast protrude into the
syncytiotrophoblast. Eventually the Villi will be
made from chorionic cavity and sac.
Part 3:
Fluid filled spaces appear in the extraembryonic
reticulum and come together to form the fluid
filled chorionic cavity. This cavity gets larger and
becomes the dominant cavity of the embryo. The
outer layers of the embryo are now collectively
called the chorion—made up of:
1. syncytiotrophoblast
2. cytotrophoblast
3. extraembryonic mesoderm
As the chorionic cavity becomes larger, the
primary yolk sac shrinks, and hypoblast
proliferate and form a new wave of cells to line the yolk sac, causing it to push down towards the opposite pole. With
this new lining, the primary yolk sac becomes the secondary/definitive yolk sac.
Nutrient Source = endometrial capillaries connect
to lacunar networks = primordial uteroplacental
circulation
Mesoderm: the extraembryonic mesoderm is
called different names depending on what part it is
surrounding. The part surrounding the yolk sac is
called extraembryonic splanchnic mesoderm and
the part surrounding the chorionic cavity and
amnion is called the extraembryonic somatic
mesoderm.
AFTER 2 WEEKS: thick area of hypoblast ion the edges of the bilaminar disc forms the prechordal plate which is where
the mouth will eventually form.
Summary
First Week
Fertilization
Division-Blastocyst Formation
Zona Pellucida degenerates
Implantation—Attachment, Invasion
Trophoblast proliferationcytotrophoblast +
syncytiotrophoblast
Second Week
Syncytiotrophoblast continue to invade endometrium
Blood-filled lacuna form
Blastocyst completely in epithelium (closing pug forms)
Lacunar Network forms
Uteroplacental circulation forms
Defect in endometrial epithelium repaired
Primary Chorionic villi develop