WEEK 1

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WEEK 1
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Week 1

• Passage of sperm through corona radiata
  surrounding the zona pellucida
• Hyaluronidase from sperm acrosome
• Changes the makeup of zona pellucida
• Zona reaction occurs lysosomal enzymes released
  by cortical granules near plasma membrane of
  oocyte Esterases, acrosin, and neuramidase
  released from acrosome
• Completion of second meiotic divison occurs with
  formation of male and female pronucleus

FERTILIZATION
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CLEAVAGE
Week 1
Blastomere 2-8 cells Morula 12-15 cells (inner
and outer cell mass) compaction takes
place at beginning of this stage Blastocyst
inner cell mass and outer trophoblast layer
degrading of zona pellucida (Z.P. prevents
sticking prior to uterus)
blastocyst attaches to endometrium (Day 6)
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-Week 1 Trophoblast (outer cell layer)
Day 6breaks down into two layers
-Syncytiotrophoblast project into endometrium
outer cell layer -
Cytotrophoblast inner cell layer -
Hypoblast forms from embryblast (inner
cell) layer -Day 7 almost
delaminate roof of blastocystic cavity
responsible for formation of primitive yolk
sac PRIMITIVE ENDODERM
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WEEK 2
Completion of Implatation Formation of Amniotic
Cavity, Embryonic Disc, and Yolk Sac
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Week 2 Epiblast (formerly embryoblast - inner
cell mass) -Proliferate cells that make up
the lining of the amniotic cavity
floor of amnion -PRIMITIVE ECTODERM
-columnar cells Exocoelomic cavity (formerly
blastocystic cavity) -roof consists of
hypoblast (primary endoderm) -become primary
yolk sac -exocoelomic membrane lines this
cavity
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Week 2 Extraembryonic mesoderm -Loose CT
-surrounds the amnion and yolk sac
-cavitates to form extraembryonic coelomic
spaces Chorion -encompases the
extraembryonic somatic mesoderm
trophoblast layers Bilaminar embryonic disc
-encompases the epiblast and the hypoblast
-prechordal plate forms w/thickened area
at cranial end
future site of mouth and head region
Chorion
Extraembryonic mesoderm
Day 12
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Week 2 Remnant of primary yolk sac
-formerly part of the secondary yolk sac (known
previously as the primary yolk sac)
Secondary yolk sac
After week 2 there is the presence of 2
layers -amnion -yolk sac
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WEEK 3
Formation of Three Germ Layers and Early Tissue
and Organ Differentiation
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Cranial
Primitive Streak -1st sign of
gastrulation begins at caudal 1/3 end of
embryo -results from proliferation migration
of epiblast cells to the median plane of
embryonic disc - streak ? node and groove ?
pit -gives rise to intraembryonic mesoderm
Caudal
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• Notochordal canal
• -Notochordal process elongates by invagination
  of cells from the primitive pit extending to
  form the notochordal canal (cellular tube)
• -Floor of nodicordal canal (cellular tube)
  fuseswith underlying endoderm of yolk sac so
  that canal is able to communicate with yolk sac

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Notochord Completion -The openings rapidly
become confluent and the floor of the
notochordal canal disappears the remains of the
notochordal process form a flattened, grooved
notochordal plate -Beginning at the cranial end
of the embryo, the notochordal cells proliferate
and the notochordal plate infolds to form the
notochord -The notochord becomes detached from
the endoderm of the yolk sac, which again
becomes a continuous layer -The developing
notochord induces the overlying embryonic
ectoderm to thicken and form the neural plate
Notice the notochord is detached from yolk sac
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Neural Tube Formation - The coelomic spaces
in the lateral mesoderm and caridiac mesoderm
fuse together to form the intraembryonic coelom
which forms the pericardioperitoneal canal (a
horseshoe shaped canal at the cranial end).
- Neural plate appears as thickening of
ectoderm cranial to primitive node - Neural
groove develops in plate, flanked by neural
folds -Tube forms, sinks below ectoderm
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Neural Crest Development and
Role -Neuroectodermal cells (from folding of
neural tube) migrate dorsolaterally to form a
neural crest -Neural crest cells migrate in
various directions and disperse within the
mesenchyme -Give rise to spinal ganglia and the
ganglia of the autonomic nervous system -Ganglia
of cranial nerves V, VII, IX, and X are also
partly derived from neural crest cells
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WEEK 4
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Folding of Embryo -flat disc to cylindrical
embryo (medial and horizontal planes) Head
Fold - 1 -Occurs 1st - Overgrows oropharyngeal
membrane and cardiogenic region and moves them
to the ventral surface of the embryo
incorporating a portion of the yolk sac (foregut)
into the head region Tail Fold - 2 -Occurs
just after head fold but prior to lateral
folding -Tail projects over cloacal membrane
connecting stalk and allantois are moved
ventrally -incorporates part of yolk sac
(hindgut) into embryo Lateral Fold - 3 -last
portion to fold -folding of sides of embryo as a
result of growing spinal cord and
somites -brings part of yolk sac (midgut) into
embryo
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2
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Frontonasal proccess Primitive stomadeum Cardiogen
ic zone Septum transversum
3 pair of pharyngeal arches are visible by 26
days upper limb buds by 26-27 days otic
(ear) pits and lens placodes (eye) visible
lower limb buds by 28 days
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WEEK 5
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• Continued enlargement of head
• Cervical sinus forms
• Mesonephric ridges apparent

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WEEK 6
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• Hand plates and digital rays develop
• Auricular hillocks develop (area of external
  acoustic meatus) ? ear
• Trunk and neck straighten
• Demonstrates reflex to touch

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WEEK 7
25

• Notches appear between the digital rays
• Intestines enter the extraembryonic coelom
• Ossification of the bones of the upper limb
  begins

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WEEK 8
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• Hand digits are separated but webbed
• notching of foot digits appear
• scalp vascular plexus has appeared
• ossification of lower limbs begins
• embryo has distinct human characteristics, head
  still disproportionately large
• neck region is established and eyelids are more
  obvious