Animal Development
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
received from neighboring cells.
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The stages of animal development:
Fertilization - sperm finding and penetrating egg, followed by
fusion of sperm and egg nuclei
Cell cleavage - the single celled zygote becomes many
undifferentiated cells by mitotic divisions
Blastulation - becoming a hollow ball of undifferentiated cells
Gastrulation - invagination of cells produces the three primary
tissue layers : endoderm, mesoderm, and ectoderm
Neurulation - folding of ectoderm produces the nervous system
Cell Migration - some cells move to specific locations in the
embryo and give rise to specific tissues
Organogenesis and Growth - tissues associate to become
organs and the embryo increases greatly in size
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Fertilization - sperm penetration of the egg, fusion of the sperm
and egg 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
digesting the outer layer of
the egg
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The egg has an outer protective layer that prevents immediate entry
of the sperm. Sperm must digest their way into the egg.
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Eggs release
small amounts of
Ca. Sperm swim
toward calcium
sources.
When a sperm
finds an egg, its
acrosome opens
and begins to
digest the “jelly
coat”of the egg.
Many sperm must reach the egg to thin the jelly coat sufficiently
for a single sperm to reach the vitelline membrane. Once a sperm
reaches the vitelline membrane the egg becomes activated.
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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 outer surface of
the egg
Also, cortical granules (vesicles just inside the egg plasma
membrane) release their contents into the space between the
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.
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After the sperm nucleus enters the egg, nuclear fusion occurs,
creating a diploid nucleus. The egg then begins cleavage.
Cleavage is a series of cell divisions that occur in a specific
pattern.
Eggs have a polarity - the upper end is called the animal pole
and the lower end is called the vegetal pole
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In organisms with relatively little yolk in their
eggs, cleavage results in complete division of
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.
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Eventually, the embryo consists of a large amount of cells.
Within those cells a hollow space forms - the blastocoel.
At this embryonic stage the embryo is called a blastula.
Blastocoel
A lancelet
A mammal
A bird
Lancelets are primitive fish-like
relatives of the vertebrates with
holoblastic cleavage
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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
the cells of the vegetal pole. Some of those cells begin to 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
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become ectoderm.
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
and then descend into the interior
of the embryo.
Those cells in contact with the
yolk become endoderm. Those
that move into the space above
the endoderm become
mesoderm. The cells that remain
on the outside of the embryo
become ectoderm.
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In mammals, the inner cell mass is similar to the blastodisc of
birds and reptiles. The amniotic cavity forms in the upper part of
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 ectoderm. The cells that remain on the
outside of the embryo become ectoderm.
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In all chordates (which includes vertebrates) some of the
mesoderm becomes organized into the rod-like notochord. The
ectoderm above the notochord, the neural ectoderm, begins to fold
into the neural tube. This process is called neurulation.
The ectoderm
lining the neural
tube gives rise
to the entire
nervous system.
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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.
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Overlying ectoderm is induced by the optic stalk of the
developing nervous system to become the lens and outer layers
of the eye.
Cells receive information from their neighbors and use that
information to regulate genes and take on new properties.
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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.
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Concentration provides information about distance from the
organizer and thus cell position in the embryo.
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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. This process is called determination.
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
place, or if they are transplanted anywhere else in the embryo.
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
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cell’s ultimate fate is sealed.
Differentiation was once thought to be irreversible. It is now
known that the nuclei of some cell types can be de-differentiated.
This is the basis for cloning.
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Totipotency is not found in all animals. The relatives of
vertebrates (which includes the invertebrate starfish and sea
urchins) do, but all other invertebrates (arthropods, molluscs,
annelids) do not have totipotent 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
determination through the positioning of cells and communication
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.
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In organisms with mosaic development, 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 cell will give rise to the head or the tail or the left
appendages or the right appendages. Lack of those factors leads
to improper development and missing body parts.
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In spite of major differences in the way cell fates are determined in
organisms with regulative and mosaic development, the same sets
of regulatory genes appear function in each to determine the
overall body plan of the organism.
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