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Developmental Stages
Ø Early Development
•  Fertilization
•  Cleavage
•  Gastrulation
•  Neurulation
Ø Later Development
•  Organogenesis
•  Larval molts
•  Metamorphosis
•  Aging
Ø Rapid cell divisions
•  Drosophila 8 , Xenopus
20
– no transcription
– S and M phases only
•  mammals 12-24 hr, chick
6-12 hr
– transcription occurs
Ø Produces a hollow
embryo
•  blastocoel
Ø mesoderm induction
•  signals from endoderm
induce ectodermal cells
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Birds, Reptiles, & Fishes Eggs
Telolecithal eggs that completely support
embryogenesis; undergo meroblastic
cleavage
In contrast to holoblastic cleavage, where the
entire zygote divides into blastomeres,
meroblastic cleavage leaves a large portion
of the zygote uncleaved. There are 2 types of
meroblastic cleavage, discoidal & superficial.
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Birds, Reptiles, & Fishes Eggs
Discoidal: Birds &
reptiles: 1st cleavage is
meridional. Starts at the
animal pole but does not
progress far. The 2nd and
3rd cleavages are also
m e r i d i o n a l . T h e 4th
cleavage is equatorial,
and it creates a layer of
small cells on top of the
huge uncleaved area
below (yolk).
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Birds, Reptiles, & Fishes Eggs
Blastoderm: when
cleavage progressed
such that there are
many blastomeres in
the animal pole;
blastoderm. Chicken
eggs have a
blastoderm of
60,000 cells when
the egg is laid.
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Birds, Reptiles, & Fishes Eggs
The next step eggs is formation of the upper &
lower blastoderm.
Epiblast: (epi = upon) is the upper layer & forms
the embryo proper.
Hypoblast: (hypo = under) is the bottom layer that
will form extra embryonic endoderm that
surrounds the yolk.
What is the counterpart in mammals?
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Birds, Reptiles, & Fishes Eggs
Blastocoel: lies between the 2 layers.
Subgerminal space: lies between the hypoblast &
yolk.
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The mitotic spindle determines the
orientation of the cleavage plane
Blastomeres can cleave either
equatorially or meridionally.
Cytokinesis usually directly
follows mitosis, except for
superficial cleavage.
How does a blastomere know to divide
meridionally or equatorially?
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The mitotic spindle determines the
orientation of the cleavage plane
Mitotic spindles work to keep the cell round
in shape.
Experiment: It is possible to control how tightly
blastomeres adhere by changing the
concentration of calcium. High calcium
concentrations cause more cell – cell
attachment. Low calcium causes minimal
attachment. The effect is likely mediated by
adhesion molecules such as cadherin.
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The mitotic spindle determines the
orientation of the cleavage plane
When blastomeres adhere they have a
longer axis, and the mitotic spindle is
almost always oriented parallel to this axis.
As the cell becomes more spherical in low
calcium medium, the mitotic spindle
orientation starts to become random.
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Mammalian eggs have rotational cleavage
that is holoblastic
The mammalian egg is a little slow. It
begins to cleave in the oviduct and
continues until it implants in the wall of
the uterus (1 cleavage / 24 hr).
Asynchronous cleavage: mammalian
embryos are unusual in that they have
asynchronous cleavage. Not all
blastomeres divide at the same time.
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Mammalian eggs have rotational cleavage
that is holoblastic
The first cleavage is meridional, and the
second cleavage is rotational. The 2
blastomeres divide in different planes
(one is equatorial and one is meridional.
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Mammalian embryos undergo compaction
at the 8 cell stage
At first, the blastomeres of mammalian
embryos have a loose arrangement, and touch
only at the basal surfaces.
After compaction, blastomeres adhere tightly,
maximizing the area of contact.
During compaction, each blastomere
undergoes polarization. Tight junctions
develop on the outer surface, allowing proteins
to specialize. Cells take up fluids from uterine
environment & secrete into the blastocoel. 12
Mammalian embryos undergo compaction
at the 8 cell stage
Gap junctions form on the outer cells to aid in
intercellular communication.
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Blastula
„ Morula;
16-cell
stage, embryo
enclosed in the
zona pellucida.
-
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A blastocoel develops as cleavage
proceeds to the 32-64 cell stage
After compaction at the 8-16 cell stage, there
are 2 types of blastomeres. Outside
blastomeres are tightly joined and number
about 9-14. They surround 2-7 inside
blastomeres that are loosely joined.
Cavitation: the outside blastomeres start to take
up fluid from the uterus and pump it into the
center, creating the blastocoel. The blastocyst
is the hallmark of early embryonic development
in mammals.
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A blastocoel develops as cleavage
proceeds to the 32-64 cell stage
Trophoblast: a structure
consisting of outside
blastomeres, this contributes to
forming the placenta.
Inner cell
mass:
this gives
rise to
the
embryo,
&
develops
from the
inside
blastome
res
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Embryonic stem cells can be cultured
from the inner cell mass
•  C e l l s i n t h e i n n e r c e l l m a s s a r e
undifferentiated, they multiply indefinitely, & are
known as embryonic stem cells.
•  Stem cells are totipotent = they have the
potential to form any tissue. These cells are of
great scientific and medical importance.
•  They can be removed from the embryo, genes
can be introduced into the cells.
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Embryonic stem cells can be cultured
from the inner cell mass
Interestingly, if you inject adult, differentiated
cells back into the environment of the morula or
blastula, they become undifferentiated, and they
can re-differentiate to form many parts of the
body.
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Development of monozygotic or
identical twins
Monozygotic twins
develop from one
zygote by splitting at
various stages of
development (from
the 2 cell to the
blastocyst stage).
The stage of splitting
effects the overall
structure of the
embryo and
extraembryonic
membranes.
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Development of monozygotic or
identical twins
What are conjoined twins
and how do they arise?
Where do fraternal twins
come from?
None identical
twins
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Implantation
„ After
fertilization, it takes 2-3 days for the zygote to
travel to uterus.
„  Blastulation
„  Blastula
à embryo arrives in uterus(4)
hatches from zona pellucida and contact
uterus.
„  Blastocyst
surrounded by ZP prevents premature
implantation and ectopic pregnancy.
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Implantation
„ Hatching
lyses of ZP
• 
involves à a trypsin-like protease
--Trophoblast cells secrete proteases which
degrades the endometrial wall and blastocyst
embeds
About 7 days after fertilization, embryo implants
(progesterone is the important hormone) in the
uterus by releasing enzyme from trophoblasts .
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Implantation
• 
Enzymes degrade endometrium and implants
blastocyst (stage of embryonic development)
•  Failure of implantation & placentation
common cause of spontaneous abortion (70%
of all pregnancies)
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Implantation
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Implantation
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Implantation
(implanta
tion
chamber)
Invasive
Implant.
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