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Developmental Biology BY1101
P. Murphy
Lecture 6
The first steps to forming a new organism
Descriptive embryology IV
·How to produce an embryo on dry land
-specialisations of amniotes
·How mammals develop
-specialisations of mammals
We started by recapping once again the series of processes involved in the formation of
an organism;
fertilisation, cleavage, gastrulation, neurulation and organogenesis,
emphasising that they are not separate isolated events in the embryo but a continuous
series of overlapping processes. For example gastrulation is not fully complete before
neurulation and other organogenesis events start.
So you saw that cleavage, gastrulation and organogenesis in vertebrate embryos produce
a body design that is very similar.
But among these organisms, two are quite different from the others in the way they
develop- which ones and why?
The chick and mouse are amniotes.
Reptiles, including birds, and mammals left an aquatic life and must produce their young
on dry land.
Their eggs and embryos have special characteristics that reflect this- they are
Ancestors of reptiles, birds and mammals shifted from an aquatic (fish) or semi aquatic
(amphibian) life style to a fully terrestrial one.
An important task for this to be successful is laying an egg and, nourishing and protecting
an embryo on dry land.
This was achieved through two different innovations in different groups of animals
1. Evolution of a protective egg shell on externally laid eggs (reptiles, birds and a few
mammals (monotremes)
2. Evolution of a uterus and placenta to facilitate internal development inside the
mother’s body in mammals.
Reptiles, including birds,
and monotreme mammals
→ protective egg shell
Marsupial and eutherian
(placental) mammals
→ uterus, placenta and internal
In both cases, specialized protective membranes are needed to enclose the embryo.
By studying these extra-embryonic membranes we see similarities in their design across
all of these animals
The group is therefore called after one of the extra-embryonic membranes- the
amnion- ⇒ amniotes. This supports the belief that mammals as well as birds and
other present-day reptiles evolved from early reptile-like vertebrates that left the
water ⇒ there was a common ancestor for reptiles and mammals.
This is an example of how studying the embryo shows us evolutionary connections
between the species
In recent research, discovery of the genes that guide development has reinforced the
connections between evolution and development and how much we can learn about
evolution from the study of development -more than anyone had imagined possible
More about this in lecture 12.
Back to organogenesis in the chick
Notice in Figure 47.14 below that the germ layers extend outside the future body of the
These will form the extra-embryonic membranes
There are 4 membranes as shown in the figure below (labels boxed in red)
Amnion- surrounds the embryo, forms fluid filled sac that bathes the embryo. Made from
ectoderm and mesoderm (notice the colour coding- blue=ectoderm, red=mesoderm,
Chorion: outer membrane- exchanges gasses between embryo and surrounding air. O2
and CO2 diffuse freely across the egg shell.
Yolk sac: encloses the yolk- blood vessels develop in the yolk sac to bring nutrients to
the embryo. The first site of blood production.
Allantois: waste disposal sac- metabolic wastes. Also performs gas exchange.
Membranes named in boxes
These 4 membranes provide a more complex life support system for the embryo
Mammalian evolution
Amphibian like → reptile like ~300 million years ago (mya)
Very few predators on dry land at that time so once the architecture of the external egg
evolved- it proved to be a very successful strategy
→ early reptilians multiplied and diversified leading to a huge number of lineages,
although some were not successful (e.g. dinosaurs).
Then 100 mya, the situation was very different. With multiplication and diversification of
early reptile species there were then many predators on land which meant an external egg
became vulnerable.
Another evolutionary innovation that was a big advantage at that time was endothermy
(warm bloodedness)- now enzymes could work better and biological processes could be
more efficient. Ideally it would be best to incubate the embryo as well→ with incubation the incubator parent was also more vulnerable to predation.
→ and this also reduced the time available to the incubating parent for foraging.
So scene was ripe for a new innovation – viviparity or live birth.
Ancestral reptile
Nest in
inaccessible places
e.g. duck-billed
platypus, Echidna
ancestral mammals
still laid large yolky eggs
How do we know?
Living examples – monotremes
use elaborate means of
protecting the egg
Marsupials and placental mammals
Viviparity- giving birth to live young
The advantages of viviparity
•Mother can still hunt, forage and remain active
The evolution of viviparity was facilitated by the earlier evolution of the extra-embryonic
membranes- these membranes were “hijacked” – or adapted- for use in the mammal.
A new structure was needed for success of viviparity- the placenta.
The placenta: exchange organ between mother and fetus.
C O2
Urea in H2O
Waste of metabolism
Hormones, drugs, viruses
The placenta is made up partly from extra-embryonic membranes and partly from the
wall of the mother’s uterus
A huge variety of placental designs have evolved in different mammalian species so it is
difficult to generalise. Different extra-embryonic membranes are used to varying degrees
in the different designs- some use the chorion (humans)- others the yolk sac or allantois.
The big issue with internal development is –
how to prevent immune rejection of the embryo?
Consider that the embryo is like transplanted foreign tissue within the body of the
The placenta also acts as a barrier for the immune response
Marsupials avoided the problem of immune rejection by birth of the embryo at a very
early stage in development and incubation in a pouch – they did not evolve a placenta.
(The bandicoot has a very simple placenta-like structure- the yolk sac attaches onto the
uterine wall- but not a true placenta)
Mammalian Development
Here we followed the sequence of events through fertilization, cleavage, implantation,
and gastrulation to highlight the special features of mammalian development- especially
The small non-yolky egg is fertilised in the oviduct and it cleaves as it moves down into
the uterus
Cleavage is holoblastic (division of the whole egg – as apposed to meroblastic as in the
chick where the yolky part of the egg does not divide) and there is no apparent
polarisation of the egg and morula which forms a ball of uniform cells
Cleavage is relatively slow with the first cell division after 36 hours in the human.
Further cleavage produces a hollow blastocyst, thickened on one side which is called the
Inner Cell Mass (ICM)
The inner cell mass (ICM) will give rise entirely to the embryo and most extra-embryonic
The larger cells around the outside form the Trophoblast- these have the job of effecting
implantation- they give rise to the chorion and contribute to the placenta.
ImplantationSee Campbell and Reece Fgure 47.12
The cells of the trophoblast attach to the uterine wall- the trophoblast secretes enzymes
that break down molecules of the endometrium and the embryo enters the tissue of the
uterine wall.
The trophoblast thickens and sends finger like projections into blood rich maternal tissue
so the embryo becomes bathed in blood.
The ICM forms a flat disk of cells made up of two layers- the epiblast above the
hypoblast – the embryo will arise entirely from the upper layer, the epiblast -so the shape
of the mammalian embryo is now similar to the chick embryo at an equivalent stage- a
flat disc.
→ gastrulation starts. Again very similar to chick- epiblast cells move underneath
through a primitive streak - emerging as mesoderm and endoderm
The three germ layers of the embryo form from gastrulation of the epiblast.
An examination of the extra-embryonic membranes in the mammal:
Amnion- encloses the embryo in a fluid filled sack- this ruptures just before birth
(referred to as breaking of waters)
Chorion- contributes to the placenta in humans- outer membrane.
Yolk sac- no yolk but retains the name from its ancestral role- this structure is said to be
“homologous” to the structure in birds (see explanation of term homologous below). It is
similar in another of its functions across the group- it is the site of first blood production.
Allantois- incorporated into the umbilical cord.
As you see there are similarities in the structures and roles of the 4 extra-embryonic
membranes across birds and mammals and it is clear that both sets of structures arose
from the same structures in a common reptilian ancestor.
NB The extra-embryonic membranes of shelled eggs, where embryos are nourished
with yolk, were conserved as mammals diverged from reptiles in the course of
evolution-the membranes became modified and adapted to supporting life within the
uterus of the mother.
ASIDE- definition of homologous
Homologous - common origin in an evolutionary ancestor
Homologous structures: structures in two organisms that arose from a single structure
(the same structure) in a common ancestor e.g. yolk sac in birds and mammals, the limbs
of mice and humans.
The limbs of insects and humans are not homologous.
Homologous genes: genes in two organisms that arose from the same gene in a common
Key concepts in lecture 6
1. The first ancestors of reptiles, birds and mammals shifted from an aquatic (fish) or
semi aquatic (amphibian) life style to a fully terrestrial one and must produce their
young on dry land. An important task for this to be successful is laying an egg
and, nourishing and protecting an embryo on dry land.
2. This was achieved through two different innovations in different groups of
animals, collectively referred to as amniotes
a. →Evolution of a protective egg shell on externally laid eggs (reptiles,
birds and a few mammals (monotremes) or
b. →Evolution of a uterus and placenta to facilitate internal development
inside the mother’s body in mammals.
3. In both cases, protective membranes are needed to enclose the embryo. There are
4 extra-embryonic membranes: amnion, chorion, yolk sac and allantois. These 4
membranes provide a more complex life support system for the amniote embryo.
4. By studying these extra-embryonic membranes we see similarities in their design
across all of these animals. Each membrane evolved specific functions related to
their structure
5. We believe that 300mya the first reptiles emerged from amphibian ancestors.
100mya ancestral reptiles living at that time gave rise to birds and to ancestral
mammals. Birds evolved along one lineage continuing to lay external eggs,
incubating them in a nest and protecting them by making the nest inaccessible.
Ancestral mammals still laid external yolky eggs but most mammals have
subsequently evolved viviparity.
6. Viviparity (live birth) evolved with the advantages of protecting and nourishing
the embryo inside the mother’s body, incubating the embryo at body temperature
while liberating the mother to move and forage.
7. Evolution of the placenta facilitated internal incubation of the embryo inside the
mother’s body and live birth. The placenta is the exchange organ between mother
and fetus and facilitates passage of important molecules in both directions (know
the molecules and direction) as well as a barrier to immune rejection of the fetus
by the mother’s body.
8. If we examine the processes involved in mammalian development we see a
number of specialisations, specific to mammals, including implantation and the
adaptation of extra-embryonic membranes to form the placenta. The embryo itself
develops from the epiblast, a layer of cells which remain undifferentiated until
implantation has been achieved.
Lecture 6: Learning outcomes: you should be able to…
A) Describe the importance of the evolutionary step of moving entirely to a terrestrial
mode of life.
B) Describe the extra-embryonic membranes that became an important feature of
amniotes, using the chick as an example and how this relates to success of terrestrial life.
C) Describe the further specialisations needed for internal development seen in mammals
and how these features relate to extra-embryonic membranes.
D) Describe the special features of development of a mammalian embryo including
implantation and delayed development of the embryo proper.
Key terms to be familiar with: amniote, extra-embryonic membranes, amnion, chorion,
yolk sac, allantois, endothermy, viviparity, placenta, immune rejection, implantation,
blastocyst, trophoblast, inner cell mass, homologous.