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Development of the
embryo
Chapter 17



Zygote
Zona pellucida
Blastocyst
Inner cell mass
Primary germ
layers
Endoderm
Mesoderm
Ectoderm






Implantation
Embryo
Yolk sac
Amnion
Chorion
Placenta
Ovulation
Nucleus Medical Media
https://www.youtube.com/watch?v=nLmg4wS
HdxQ
Fertilisation & the formation of the zygote
Nucleus Medical Media
https://www.youtube.com/watch?v=_5OvgQW
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Weeks 0-1

Blastocyst
Weeks 1-8

Embryo
Week 9

The start of the
foetal period


Fertilisation takes place
in the upper part of the
Fallopian tube.
When an ovum is
fertilised it becomes a
diploid zygote.
Zygote - The diploid cell
resulting from the fusion
of a sperm and an egg
Wellcome photo library


The zygote divides
by mitosis to form
a ball of cells
known as the
morula.
The morula travels
down the Fallopian
tube and into the
uterus.
Yorgos Nikas, Wellcome Images
Twins
Identical twins


The cells of the morula continue to divide and
about five days after fertilisation have formed
a hollow ball of cells known as the blastocyst.
The blastocyst consists of about 100 cells.
The outer cell layer will form the placenta,
and a cluster of cells on the interior - the
inner cell mass - forms the embryo.
The clump of cells
within the blastocyst
is the inner cell
mass, which goes on
to form the
embryonic tissue.
These cells can be
collected and
cultured to generate
embryonic stem
cells.
Wellcome Images – Jenny Nichols
A SEM of a six-day old human blastocyst
implanting into the endometrium
Wellcome Images - Yorgos Nikas

Colour-enhanced
image of a human
embryo at the
blastocyst stage
opened to reveal
the inner cell
mass. These cells
go on to form the
embryo and can be
harvested and
cultured to give
rise to embryonic
stem (ES) cells.
Wellcome Images - Yorgos Nikas
Yorgos Nikas, Wellcome Images
Blastocyst
Day 7
Week 3
ENDOMETRIUM
Inner cell
mass
Developing
villus
Day 9
Yolk sac
Blastodisc
Day 14
Week 4
Developing
placenta
ENDOMETRIUM
Chorion
Amnion
Embryo
Yolk sac
Allantois
Chorionic villi
Umbilicus
Space filled
with mother’s
blood
Mother’s
blood vessels
Umbilical arteries
Umbilical vein
Trophoblast
future placenta
Blastocoele
Blastodisc
future foetus
Inner cell
mass
ENDOMETRIUM
Amniotic
cavity
Amnion membrane
 Surrounds the embryo and contains the
amniotic fluid
Chorion membrane
 Chorionic villi grow into the uterine wall
 A mass of capillaries that are in contact with
the maternal blood pool
 Becomes the placenta
 During
the second week of
development three layers of cells
form from which the various
organs of the body will be
derived.
 The three layers are the ectoderm,
mesoderm and endoderm – these
form the embryonic disc.
Ectoderm
Endoderm
pancreas, liver,
bladder, thymus &
thyroid gland
Skin, hair etc.
Nervous tissue
Lining of
respiratory &
digestive systems
Mesoderm
Skeletal, muscular,
cardiovascular,
lymphatic, urinary &
reproductive
systems
Gray’s Anatomy
Week
Key events
1
Fertilisation
Zygote
Morula
Blastocyst (cavity forms)
Blastocyst “hatches” (sheds zona pellucida)
Implantation
2
Bilaminar (two layer) germ disc forms
Amniotic cavity & yolk sac appear
Implantation completed
Trophoblast (future placenta) starts to develop
3
Trilaminar (three layer – ectoderm, mesoderm & endoderm)
forms
Primitive groove & notochord (future backbone) appear
Blood and vascular system start to develop (heart begins to
beat around day 21)
a
c
b
d
Wellcome Images – Stephen Elliman
Eyes
Limb buds
Segmented
body
(somites:
future
skeletal
muscles)
Wellcome Photo Library, Wellcome Images
Wellcome Photo Library
Week
Key events
4
Embryo can be distinguished as a cylindrical, pear-shaped
structure, 1.5-3.5 mm long
Neural groove (future CNS) forms
4 to 12 somites (body segments – future skeletal muscles) visible
Many major organs begin to develop
5
Recognisable as an embryo with head, face, tail and limb buds
2.5-7 mm long
Has more than 30 somites
Sense organs (eyes & ears) start to develop
6
Umbilical cord starts to form
Gut & liver start to develop
Cartilaginous skeleton begins to develop
Week
Key events
7
Embryo recognisably human
Head becomes rounded
Eyes conspicuous
Hands & feet develop
Trunk straightens
Tail disappears
Ossification of skeleton begins
8
27-31 mm long
Face & limbs well developed
First movements of the extremities
Over 90% of the structures of the adult body are
already established
The gender of the baby can be determined using
chorionic villus sampling
Blastocyst implantation & development
https://www.youtube.com/watch?v=bIdJOiXpp
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Embryonic development: first trimester
https://www.youtube.com/watch?v=BsHSebVm
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