Human Embryo
Development
Extended Study
Learning Objectives
 List the sequence of development of an
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embryo
Explain the term fertilized egg
Explain the term blastocyst
Explain the term amnion
Explain how the placenta is formed
Explain how the embryo develops up to the
third month of gestation
Sequence of
development from
fertilised egg
Early stages
Sequence of development from
fertilised egg
 The zygote
contains 46
chromosomes,
twenty three from
the egg and 23
from the sperm
It divides rapidly by mitosis to produce 2 cells,
then 4, then 8, 16 etc. and continues to divide
 At this point the
developing
individual is referred
to as the morula
 Around 5 days after
fertilisation the
morula forms a
hollow ball of cells
called the blastocyst
 The outer layer of
the blastocyst forms
the trophoblast. This
will later develop
into the layer of
membranes that
surround the
embryo (placenta
and amnion)
Trophoblast
 The inner cells (called
the inner cell mass) of
the blastocyst will
eventually form the
embryo. These cells are
not yet specialised.
They have a
phenomenal ability to
differentiate – divide to
give rise to many
different types of tissue
Inner cell mass
Stem cells
 Huge research potential to renew or repair
damaged body parts.
Learning check
 How many chromosomes in a fertilised egg?
 Is a fertilised egg haploid or diploid?
 What is the developing individual referred to
when it is made up of 8 cells?
 What is it referred to after a number of days?
 What is unusual about the cells of the inner
cell mass?
The morula/blastocyst is pushed along the fallopian
tube until it enters the uterus
 Here it will implant
into the uterus wall.
The endometrium
now provides
nourishment for the
developing
blastocyst
 Connections with
the mother will begin
to form (placenta
and umbilical cord)
This point marks the beginning of pregnancy
Sequence of
development from
fertilised egg
Development of the embryo
 About 10 days after fertilisation the inner cell
mass forms the embryonic disc
 This usually consists of three layers called
germ layers
 Ectoderm (outside)
 Mesoderm (middle)
 Endoderm (inside)
 Each of these layers gives rise to specific
structures in the developing embryo
 In humans the mesoderm is split by a layer
called the Coelom
 This allows space for more complex organs
such as heart, lungs and kidneys to develop
Ectoderm – skin, nervous system
Coelom – heart, lungs
Mesoderm – muscles, skeleton
Endoderm – inner lining of
digestive system
The Amnion
The Amnion
 When first formed the amnion is in contact
with the embryo, but at about the fourth or
fifth week fluid begins to accumulate within it
(amniotic fluid)
 The primary function of the amnion and
amniotic fluid is the protection of the embryo
for its future development
Four to five weeks after fertilisation
 The heart forms and
starts to beat
 The brain also
develops
 The limbs have
started to form
By the 6th week
 Eyes are visible
 The mouth, nose
and ears are
forming
 The skeleton is at
the early stages of
development
By the 8th week
 the major body
organs are formed
 Sex glands have
developed into
testes or ovaries
 Bone is beginning to
replace cartilage
By the 8th week
 At this stage the
embryo has taken on
a recognisably
human from.
 From this point it is
referred to as a
foetus
 The foetus continues
to grow. No new
organs are formed
from this point
By the 12th week (3 months)
 The nerves and
muscle become coordinated allowing
the arms and legs to
move
 The feotus sucks its
thumb, urinates and
even releases
feaces into the
amniotic fluid
By the 12th week (3 months)
 The gender of the
foetus can be seen
in scans
 The time from
fertilisation to birth
(the gestation
period) lasts about
38 weeks (9
months)
Learning Check
 What are germ layers?
 Name them
 What features have already appeared by the
fifth week?
 At what stage is the developing individual
referred to as a feotus?
Placenta formation
Placenta formation
Placenta formation
 The placenta forms
from a combination of
the tissues of the
uterus and the
embryo
 Soon after
implantation a
membrane called the
chorion completely
surrounds the amnion
and embryo
 The chorionic villi
emerge from the
chorion and invade
the endometrium
allowing the transfer
of nutrients from
maternal blood to
fetal blood
 This combination of the chorionic villi and the
endometrium will eventually form the placenta
which becomes fully operational about three
months into the pregnancy
The Placenta
Placenta
Chorion
Embryo
Mother’s blood
Wastes, Carbon Dioxide, Water
Nutrients, Oxygen, antibodies
Mother
Embryo
Amnion
Amniotic fluid
Umbilical cord
Embryo’s blood
 Placenta allows gases, nutrients, waste,
antibodies, some drugs, hormones and
micro-organisms to be exchanged between
mother and baby
 Placenta also produces hormones
 Blood supplies of mother and embryo do not
mix
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Blood types may not be compatible
Mother’s blood pressure might damage
embryo
 Umbilical cord
connects the
embryo with the
placenta
 it takes blood from
the embryo to the
placenta and back
again
 It must be cut at
birth to separate
mother and baby
45 seconds after birth!
Syllabus
Learning check
 Name the structures that move into the
endometrium and eventually become part of
the placenta
 At what point does the placenta become fully
operational?
 Why is it important that the blood of the
mother and baby do not mix?
Depth of treatment
 Sequence of development from fertilised egg,
morula, blastocyst, existence of amnion,
placenta formation from embroyonic and
uterine tissue.
 Development of embryo up to third month.