Transcript Chapter 47 Presentation

Chapter 47
Animal Development
travismulthaupt.com
The Zygote
The question of how a
zygote becomes an
animal has been
asked for centuries.
 As of the 18th century,
preformation was the
prevailing notion. The
idea was that a
preformed miniature
infant (homuniculus)
was contained within
an egg or a sperm.

travismulthaupt.com
The Zygote
More than 2000 years ago: Aristotle
originally posed the idea of epigenesis
to explain embryonic development.
 This idea explained that an animal
gained form gradually from a relatively
formless egg.

travismulthaupt.com
The Zygote
As microscopy improved in the 19th
century, embryologists began to see
that embryonic development took place
in a series of progressive steps and
epigenesis became the favored
explanation.
 The genome of the zygote and
differences in early embryonic cells
determine an organism’s development.

travismulthaupt.com
The Zygote
There are some important events that
regulate animal development:
 1. Fertilization
 2. Cleavage
 3. Gastrulation
 4. Organogenesis

travismulthaupt.com
1. Fertilization
Fertilization is when an egg and a
sperm unite forming a zygote.
 The main function of fertilization is to
produce a diploid egg.
 Egg activation occurs when the sperm
contacts the egg’s surface and initiates
metabolic reactions that trigger the
onset of embryonic development.

travismulthaupt.com
1. Fertilization

In general, when eggs meet sperm, an
acrosomal reaction is triggered.
travismulthaupt.com
1. Fertilization

The process begins when a vesicle at
the tip of the sperm called the acrosome
discharges hydrolytic enzymes.
travismulthaupt.com
1. Fertilization

These enzymes dissolve the egg’s jelly coat
enabling the acrosomal process to penetrate
the it and attach to the egg’s surface.
travismulthaupt.com
1. Fertilization

The sperm binding receptors on the
egg’s surface are attached to the
vitelline layer.
travismulthaupt.com
1. Fertilization

This recognition of the egg’s surface receptors to
sperm is what ensures only sperm from the
same species can penetrate the egg.
travismulthaupt.com
1. Fertilization

The binding of the sperm membrane to
the membrane of the egg changes the
state of the ion channels of the egg.
travismulthaupt.com
Fast Block to Polyspermy

Within 1-3 seconds, depolarization occurs where Na+
ions rush into the egg, and is known as the fast block
to polyspermy. It prevents multiple sperm from
entering the egg.
travismulthaupt.com
Fast Block to Polyspermy

The fast block to polyspermy is shortlived and the membrane polarization
only lasts about one minute.
travismulthaupt.com
Slow Block to Polyspermy

The fusion of the sperm and egg also triggers
a series of changes in the egg that are more
long-lived.
 Sperm binding activates a signal transduction
pathway which causes calcium ions to be
released from the egg’s ER into the cytosol.
travismulthaupt.com
Slow Block to Polyspermy

Release of Ca2+ from the ER occurs 1st
at the site of sperm entry and moves
like a wave over the surface of the egg.
travismulthaupt.com
Slow Block to Polyspermy

The high concentration of calcium initiates the
cortical reaction.
 The reaction triggers the fusion of the egg’s
plasma membrane with numerous vesicles
lying just beneath the membrane.
travismulthaupt.com
Slow Block to Polyspermy
The contents of these vesicles enter the
previtelline space.
 Enzymes in these contents degrade the
proteins holding the vitelline layer to the
plasma membrane.

travismulthaupt.com
Slow Block to Polyspermy
These changes transform the vitelline
layer into the fertilization envelope.
 No more sperm can enter the egg at
this time.

travismulthaupt.com
Slow Block to Polyspermy
This is referred to as the slow block to
polyspermy.
 The sharp rise in Ca2+ also increases
the metabolism of the egg.

travismulthaupt.com
Slow Block to Polyspermy
After about 20 minutes, the egg nucleus
fuses with the sperm nucleus creating a
diploid zygote.
 Fertilization has many common features
among species and many differences.
These differences are mainly with timing
and various stages of meiosis.

travismulthaupt.com
1. Fertilization--Sperm Activation

In mammals, secretions in the female
reproductive tract to help to activate sperm.
 Additionally, the mammalian egg is cloaked
by follicle cells through which the sperm must
penetrate before reaching the zona pellucida.
travismulthaupt.com
1. Fertilization--Sperm Activation

The zona pellucida is the
external matrix of the egg
and functions as a sperm
receptor.
 When sperm binds to
this, it induces many of
the same reactions as
seen in the sea urchin.
 One difference is that
there is no fast block to
polyspermy in mammals.
travismulthaupt.com
1. Fertilization--Sperm Activation
After the egg and sperm membranes
fuse the whole sperm is taken into the
egg.
 The egg lacks a centrosome which the
sperm has.
 The centrosome will duplicate and
assist in the creation of the mitotic
spindle which will be used for the first
cell division.

travismulthaupt.com
1. Fertilization--Sperm Activation
In mammals, the nuclei do not
immediately fuse. Instead, the 2 sets of
c-somes share a common spindle
apparatus during the 1st mitotic division.
 Only after the first division, as diploid
nuclei form in the 2 daughter cells, do
the chromosomes from the 2 parents
come together in a common nucleus.

travismulthaupt.com
1. Fertilization
Fertilization is quite slow in mammals.
 Once fertilization is complete, rapid cell
division ensues and the cells proceed
through the M and S phases of the cell
cycle virtually skipping G1 and G2.

travismulthaupt.com
1. Fertilization

Thus, the embryo doesn’t enlarge
much. Rather, it becomes an aggregate
of blastomeres called a blastocyst
(blastula).
QuickTime™ and a
TIFF (U ncompressed) decompressor
are needed to see this picture.
travismulthaupt.com
Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.
2. Cleavage

The color of the frog
egg cells allows for
easy following of cells
during development.
 Animal hemisphere is
deep gray due to
melanin.
 The vegetal pole is
yellow due to lack of
melanin.
travismulthaupt.com
2. Cleavage

The first 5 to 7 divisions produce what is
called the morula.
 Within the morula a fluid filled cavity called a
blastocoel forms. When fully formed, the
aggregate of cells is now called blastula.
travismulthaupt.com
2. Cleavage

Many animals, with the exception of
possibly mammals, have zygotes with a
definite polarity that determines the
pattern of cleavage.
travismulthaupt.com
2. Cleavage

In classic examples
such as the frog, the
distribution of yolk in the
egg determines
cleavage patterns.
 The vegetal pole
contains a large amount
of yolk, and cleavage
through this is slow.
 The animal pole
contains very little yolk
and cleavage occurs
quickly through it.
travismulthaupt.com
2. Cleavage--3 Body Axes
The 3 body axes are
established early on
in development.
 1. Dorsal/ventral
 2. Right/left
 3. Anterior/posterior

travismulthaupt.com
2. Cleavage
Using the frog as an
example, fusion of
the sperm and egg
initiates a body axis.
 At this point, the
animal pole will
move toward the
vegetal pole passing
through the site of
sperm entry.

QuickTi me™ and a
TIFF (U ncompressed) decompressor
are needed to see this picture.
travismulthaupt.com
Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.
2. Cleavage

As a result of the
cortical rotation, a
light gray region
known as the gray
crescent forms in
some species and
the dorsal/ventral
axis is formed in
the zygote.
QuickTi me™ and a
TIFF (U ncompressed) decompressor
are needed to see this picture.
travismulthaupt.com
Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.
2. Cleavage
This light gray
region forms the
dorsal side of the
embryo.
 In frogs (animals
with a thick vegetal
pole), the
blastocoel, is
located in the
animal pole
because the yolk
isn’t as thick.

QuickTi me™ and a
TIFF (U ncompressed) decompressor
are needed to see this picture.
travismulthaupt.com
Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.
2. Cleavage

Yolk is very plentiful
in many animals
such as a bird.
 In this example, the
yolk is actually an
egg cell swollen
with yolk nutrients.
A small white disk
of actual cytoplasm
is located on the
animal pole.
travismulthaupt.com
2. Cleavage

The entire cell
is surrounded
by a protein
rich solution of
egg white
which provides
additional
nutrients to the
growing
embryo.
Albumen
Chalaza
travismulthaupt.com
Yolk
Disk of Cytoplasm
2. Cleavage
Cleavage is
restricted to the
small disk of
yolk free
cytoplasm.
 Cleavage can’t
proceed through
the dense yolk.

travismulthaupt.com
2. Cleavage
http://scienceblogs.com/pharyngula/2006/07/the_evolution_of_deuterostome.php
Incomplete division of the
yolk rich egg is known as
meroblastic cleavage.
 In contrast, holoblastic
cleavage results in
complete division of the
egg that lacks a lot of
yolk, (or a moderate
amount of yolk).

QuickTime™ and a
TIFF (Uncompress ed) dec ompres sor
are needed to s ee this pic ture.
Quic kTime™ and a
TIFF (Unc ompres sed) dec ompres sor
are needed to see this pic ture.
travismulthaupt.com
http://scienceblogs.com/pharyngula/2006/07/the_evolution_of_deuterostome.php
2. Cleavage
The cleaving cells on the top of the yolk
of a bird egg produce a cap of cells
called the blastoderm.
 The blastoderm then rests on the yolk
and divides into 2 layers called the
epiblast and the hypoblast.

QuickTime™ and a
TIFF (U ncompressed) decompressor
are needed to see this picture.
travismulthaupt.com
Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.
2. Cleavage
The cavity between them is called the
blastocoel.
 This is the equivalent of the blastula
stage of the frog/sea urchin.

QuickTime™ and a
TIFF (U ncompressed) decompressor
are needed to see this picture.
travismulthaupt.com
Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.
3. Gastrulation

Gastrulation is the rearrangement of
cells of the blastula to form a 3 layered
embryo with a primitive gut.
travismulthaupt.com
3. Gastrulation

Gastrulation differs between animal
groups, but it shares some general
characteristics:
 A.
Changes in cell motility.
 B. Changes in cell shape.
 C. Changes in cellular adhesion.
travismulthaupt.com
3. Gastrulation

The result is that some cells
at the surface of the blastula
move inward establishing 3
cell layers.
 The 3 layered embryo is
called the gastrula.
 The 3 layers are given the
term embryonic germ layers
and consist of:
 1. Endoderm--lines digestive


tract.
2. Mesoderm--partially fills the
space between endoderm and
ectoderm.
3. Ectoderm--which forms the
outer layer of the gastrula.
travismulthaupt.com
3. Gastrulation--Sea Urchin




These 3 layers will form all
cell layers and tissues in
the adult animal.
In the sea urchin, the
blastula consists of a single
layer of cells enclosing a
blastocoel.
Gastrulation begins at the
vegetal pole.
Here, migratory cells called
mesenchyme cells enter
the blastocoel.
travismulthaupt.com
3. Gastrulation--Sea Urchin

The cells which remain
form the vegetal plate
which eventually buckles
inward by a process
called invagination.
 Invagination proceeds
forming the archenteron
(primitive gut).
 The open end of the
archenteron will become
the anus, it is called the
blastopore.
travismulthaupt.com
3. Gastrulation--Sea Urchin
The second opening
forms as the archenteron
touches the inside of the
ectoderm on the other
side. This is the mouth
(in deuterostomes).
 A rudimentary digestive
tube is now formed.

Ectoderm
Blastocoel
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Mesenchyme Cells
Vegetal Plate
travismulthaupt.com
http://biology.kenyon.edu/courses/biol114/Chap14/PMC_SEM.gif
3. Gastrulation--Frog

In the frog, gastrulation
also produces a 3
layered embryo with an
archenteron.
 Gastrulation is more
complex due to the yolk
content of the cells, and
the fact that the wall of
the blastula is more than
1 cell-layer thick.
 Gastrulation begins on
the dorsal side of the
blastula in a line along
the region of the gray
travismulthaupt.com
crescent.
3. Gastrulation--Frog
This invagination
forms the dorsal lip
which pinches inward
around the cell.
 The cells on the
surface of the
embryo continue to
roll inward in a
process called
involution forming the
future endoderm and
mesoderm.

travismulthaupt.com
3. Gastrulation--Frog

Eventually, the
blastocoel collapses as
the archenteron replaces
it. The archenteron is
lined with endoderm.
 As gastrulation
completes, the
blastopore encircles the
yolk plug.
 After the archenteron
reaches the opposite
side of the embryo, a
mouth will form.
travismulthaupt.com
3. Gastrulation--Chick

The chick’s gastrulation is similar to that of
a frog except the inward movement of cells
during gastrulation is affected by the yolk.
travismulthaupt.com
3. Gastrulation--Chick

Cleavage in the chick results in a
blastoderm with an epiblast and a
hypoblast.
travismulthaupt.com
3. Gastrulation--Chick

All cells that form the embryo will come from the epiblast.
 During gastrulation, some of the epiblast cells move toward
the blastoderm’s midline producing a thickening called the
primitive streak.
travismulthaupt.com
3. Gastrulation--Chick
The primitive streak will
form the bird’s
anterior/posterior axis.
 The primitive streak is
the functional equivalent
to the lip of the
blastopore in the frog,
but structures are
aligned differently.
 Inward moving cells
displace hypoblast cells
and form endoderm.

travismulthaupt.com
Scott F. Gilbert Developmental Biology
3. Gastrulation--Chick

Other epiblast cells move laterally into the
blastocoel and form mesoderm.
 Epiblast cells remaining on the surface form
ectoderm.
 The hypoblast helps direct the formation of the
primitive streak and forms part of the stalk that
keeps the yolk mass connected to the embryo.
travismulthaupt.com
4. Organogenesis


Organogenesis is the process by which parts of the 3
germ layers develop the rudiments of organs.
Organogenesis involves localized morphogenic
changes in tissue and cell shape versus the large
scale mass movement of cells seen in gastrulation.
travismulthaupt.com
4. Organogenesis

Evidence of organogenesis is seen in the
appearance of folds splits and dense
clustering of cells.
 Easy observations are made in chordates
when the notochord, neural tube, etc. take
form.
travismulthaupt.com
4. Organogenesis

The notochord is formed from dorsal
mesoderm which condenses just above
the archenteron.
travismulthaupt.com
4. Organogenesis

Signals sent from the notochord to the
ectoderm cause the region of the ectoderm to
form the neural plate.
 The neural plate will curve inward forming the
neural tube which runs along the anteriorposterior axis of the embryo eventually
forming the CNS.
travismulthaupt.com
4. Organogenesis
In vertebrates, the
neural crest forms
along the border
where the neural tube
pinches off from the
ectoderm.
 Neural crest cells then
migrate to various
parts of the embryo
giving rise to many
structures such as
peripheral nerves,
travismulthaupt.com
teeth, skull bones,
etc.

4. Organogenesis
Lateral to the
notochord are
condensations of
mesodermal cells
arranged in the block
called somites.
 The somites give rise
to cells that migrate to
new locations forming
vertebrae and
muscles.

travismulthaupt.com
Development
All vertebrates require an aqueous
environment to grow and develop.
 The evolution of structures that allow
this to happen have occurred and are
the shelled egg and uterus. These
structures protect the embryo with an
amnion which is a fluid filled sac.

travismulthaupt.com
Extraembryonic Membranes
The development of mammals, birds,
and reptiles includes the formation of 4
extraembryonic membranes.
 These membranes include:

 Yolk
sac
 Chorion
 Allantois
 Amnion
travismulthaupt.com
Extraembryonic Membranes

The yolk sac covers the yolk and develops
blood vessels that transplant nutrients
from the yolk to the embryo.
travismulthaupt.com
Extraembryonic Membranes

The chorion is involved in gas exchange
with the surroundings.
travismulthaupt.com
Extraembryonic Membranes

The allantois is a disposable sac for metabolic
wastes produced by the developing embryo.
It also functions with the chorion in respiration.
travismulthaupt.com
Extraembryonic Membranes

The amnion protects the embryo in a
fluid filled cavity.
travismulthaupt.com