Animal Development
De elopment
Development is a highly regulated process. Embryonic cells
take on the structure and function of adult cells as a result of
chemical messages packaged in the egg or chemical messages
1
received from neighboring cells.
The stages of animal development:
Fertilization - sperm finding and penetrating egg, followed by
fusion of sperm and egg nuclei
Cell cleavage - the
h single
i l celled
ll d zygote becomes
b
many
undifferentiated cells by mitotic divisions
Bl t l ti - becoming
Blastulation
b
i a hollow
h ll ball
b ll off undifferentiated
diff
ti t d cells
ll
Gastrulation - invagination of cells produces the three primary
tissue layers : endoderm,
endoderm mesoderm,
mesoderm and ectoderm
Neurulation - folding of ectoderm produces the nervous system
Cell
C
ll Migration
Mi
i - some cells
ll move to specific
ifi locations
l
i
in
i the
h
embryo and give rise to specific tissues
Organogenesis and Growth - tissues associate to become
organs and the embryo increases greatly in size
2
3
4
Fertilization - sperm penetration of the egg, fusion of the sperm
and egg nucleus,
nucleus and the prevention of further sperm penetration
Sperm cells are specialized
for mobility - a large
flagellum powered by large
mitochondria
Sperm cells are specialized
for delivering a nucleus to
the egg
Sperm cells are specialized
for penetrating the egg - the
acrosome is a vesicle filled
with enzymes specialized for
di ti the
digesting
th outer
t layer
l
off
the egg
5
The egg has an outer protective layer that prevents immediate entry
of the sperm. Sperm must digest their way into the egg.
6
Eggs release
small amounts of
Ca++. Sperm
swim toward
calcium sources.
When a sperm
fi d an egg, its
finds
it
acrosome opens
and begins to
digest the “jelly
coat”of the egg.
Many sperm must reach the egg to thin the jelly coat sufficiently
g sperm
p
to reach the vitelline membrane. Once a sperm
p
for a single
reaches the vitelline membrane the egg becomes activated.
7
Once the egg is
activated - the
membranes of the
egg swell around the
sperm head and
draw it in. The
flagellum is left on
the
h outer surface
f
off
the egg
Also, cortical granules (vesicles just inside the egg plasma
membrane)
b
) release
l
their
th i contents
t t into
i t the
th space between
b t
the
th
plasma membrane and vitelline membrane, creating the
fertilization membrane and the egg releases massive amount
of Ca. The fertilization membrane and Ca release prevent
polyspermy, the entry of additional sperm.
8
After the sperm nucleus enters the egg, nuclear fusion occurs,
creating
i a diploid
di l id nucleus.
l
The
Th egg then
h begins
b i cleavage.
l
Cleavage
g is a series of cell divisions that occur in a specific
p
pattern.
Eggs have
E
h
a polarity
l it - the
th upper endd is
i called
ll d the
th animal
i l pole
l
and the lower end is called the vegetal pole
9
In organisms with relatively little yolk in their
eggs cleavage results in complete division of
eggs,
the egg. This is called holoblastic cleavage
and the cells are called blastomeres.
blastomeres
In organisms with large
amounts of yolk in their
eggs, cleavage occurs
only at the animal pole,
and the initial cleavages
are incomplete. This is
called meroblastic
cleavage.
l
10
Eventually, the embryo consists of a large amount of cells.
Within those cells a hollow space forms - the blastocoel.
blastocoel
At this embryonic stage the embryo is called a blastula.
Blastocoel
A llancelet
l t
A mammal
A bird
Lancelets are primitive fish-like
relatives
l ti
off the
th vertebrates
t b t with
ith
holoblastic cleavage
11
The next stage in development is gastrulation - the formation the
primary tissue layers. In all animals this involves the movement
of cells within the embryo.
In amphibians, cells at the animal pole cells move down and over
th cells
the
ll off the
th vegetal
t l pole.
l Some
S
off those
th
cells
ll begin
b i to
t move
into the interior of the embryo at the dorsal lip of the blastopore.
The first cells to move in contribute to endoderm. The later cells
become mesoderm, and the cells that remain on the exterior
12
become ectoderm.
Frog Development
Cell size difference
Yolk Plug
In birds and reptiles, the cells are
found only in a blastodisc at the
animal pole. The cells move
toward the midline of the embryo
andd th
then ddescend
d into
i t the
th interior
i t i
of the embryo.
Those cells in contact with the
yolk become endoderm. Those
that move into the space above
the
h endoderm
d d
b
become
mesoderm. The cells that remain
y
on the outside of the embryo
become ectoderm.
18
In mammals, the inner cell mass is similar to the blastodisc of
bi d and
birds
d reptiles.
il
The
Th amniotic
i i cavity
i forms
f
in
i the
h upper part off
the embryo and cells of the inner cell mass begin to move toward
their midline. The lower cells of the inner cell mass become
endoderm. The cells that move into the space above the
endoderm cells become mesoderm. The cells that remain on the
outside of the embryo become ectoderm.
ectoderm
19
In all chordates (which includes vertebrates) some of the
mesoderm
d
becomes
b
organized
i d into
i
the
h rod-like
d lik notochord.
t h d The
Th
ectoderm above the notochord, the neural ectoderm, begins to fold
into the neural tube. This pprocess is called neurulation.
The ectoderm
lining the neural
tube gives rise
to the entire
nervous system.
20
Chicken Development
How do cells know what tissues to become?
They receive information from neighboring cells.
Mesodermal
cells that
become
notochord
induce the
formation of a
neural fold in
the ectoderm
above them.
This is called
embryonic
induction.
27
Overlying ectoderm is induced by the optic stalk of the
d l i nervous system to become
developing
b
the
h lens
l
andd outer layers
l
of the eye.
Cells receive information from their neighbors and use that
i f
information
ti to
t regulate
l t genes andd take
t k on new properties.
ti
28
Some cells serve as organizers for the rest of the embryo.
Organizers produce chemical signals called morphogens that
diffuse to other cells and cause them to take a specific course in
development. The concentration of a morphogen highest for cells
near the organizer. Concentration provides information about
distance from the organizer and thus cell position in the embryo.
29
Concentration provides information about distance from the
organizer
i andd thus
h cell
ll position
i i in
i the
h embryo.
b
30
Early blastomeres of vertebrates and relatives are totipotent - any
single cell can give rise to an entire fully formed embryo. This is
the basis for identical twinning in humans.
As cells receive chemical signals from their neighbors they adopt
a fate.
f t This
Thi process is
i called
ll d determination.
d t
i ti
Before the notochord begins to form, ectodermal cells lying in the
position where the neural tube will form can be transplanted
anywhere in the embryo and become the same type of ectoderm
as the neighboring cells. Once the notochord forms, the
ectodermal cells above the notochord become neural ectoderm.
The neural ectoderm cells will become nervous tissue if left in
pplace,, or if theyy are transplanted
p
anywhere
y
else in the embryo.
y
After a cell is determined, it can begin differentiation - taking on
the characteristics of the cell type that it is destined to become.
Sometimes, differentiation does not begin immediately, but the
31
cell’s ultimate fate is sealed.
Differentiation was once thought to be irreversible. It is now
k
known
that
h the
h nuclei
l i off some cell
ll types can be
b de-differentiated.
d diff
i d
This is the basis for cloning.
32
Totipotency is not found in all animals. The relatives of
vertebrates
b
( hi h includes
(which
i l d the
h invertebrate
i
b
starfish
fi h andd sea
urchins) do, but all other invertebrates (arthropods, molluscs,
annelids)) do not have totipotent
p
blastomeres.
The different developmental patterns are called regulative
development (vertebrates and relatives) and mosaic development
(all other invertebrates).
Regulative development is characterized by totipotency and
d t
determination
i ti through
th
h the
th positioning
iti i off cells
ll andd communication
i ti
between neighboring cells. In organisms with regulative
development, in early development blastomeres can be removed
or repositioned with no effect on the developing embryo. The
neighbors of the missing cells compensate and take on the roles
that the missing cells would have adopted.
adopted
33
In organisms
g
with mosaic development,
p
, the fate of cells is
determined by developmental factors that are positioned in the
egg during formation of the egg.
If a blastomere is removed from the embryo, the factors that will
determine the fate of its descendant cells will be missing from the
embryo - and the cell types that those factors determine will be
missing from the embryo. (Neighboring cells do not compensate
for missing cells.) Important factors may determine whether a
particular
ti l cell
ll will
ill give
i rise
i to
t the
th head
h d or the
th tail
t il or the
th left
l ft
appendages or the right appendages. Lack of those factors leads
to improper development and missing body parts.
34
In spite of major differences in the way cell fates are determined in
organisms with regulative and mosaic development,
development the same sets
of regulatory genes appear function in each to determine the
overall body plan of the organism.
35