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Animal Development
A. P. Biology
Dr. Macomson
FCHS
Figure 47.0 Human embryo
Animals develop throughout their lifetime.
 Development begins with the changes
that form a complete animal from the
zygote and continue as progressive
changes in form and function.

Epigenesis – Gradual change in an
animal’s form

Two early views of how animals developed
from an egg
– Preformation – the embryo contained all of its
descendants as a series of successively
smaller embryos within embryos
– This idea was popular up until the eighteenth
century
Figure 47.1 A “homunculus” inside the head of a human sperm

Epigenesis – the from of an embryo
gradually emerged from a formless egg
– Originally proposed by Aristotle
– Development/improvement of microscope
permitted better study of the development of
embryos

Modern biology has found that
development is determined mostly by two
things:
– The zygote’s genome
– The organization of the egg cell’s cytoplasm
The distribution of mRNA, proteins, and other
cytplasmic material in the unfertilized egg
greatly impacts development
 Cleavage after fertilization divides the cytoplasm
so that newly formed nuclei are exposed to
different environments depending on their
location in the embryo
 The different environments cause different
genes to be expressed in different cells
 This helps guide and control the process of
development

Fertilization
Forms a diploid zygote from haploid
gametes
 Triggers the onset of embryonic
development

1. The acrosomal reaction
Release of hydrolytic enzymes from a
vesicle in the cap (acrosome) a the head
of the sperm cell
 Release is triggered by contact with the
jelly coat surrounding the egg
 Eggs are only fertilized by sperm of the
same species

– Species-specific protein receptors on the egg’s
surface bind with proteins on the acrosome
1. Acrosomal Reaction
When the enzymes digest the vitelline
layer, the tip of the acrosome process
binds and fuses with the egg’s plasma cell
membrane
 The sperm cell’s nucleus then enters the
egg cell
 This triggers depolarization of the egg cell
membrane, which serves to block other
sperm

– Fast block to polyspermy
Figure 47.2 The acrosomal and cortical reactions during sea urchin fertilization
2. The cortical reaction

Fusion of egg and sperm membranes
triggers a series of changes in the egg cell
– Calcium ions are released from the egg cell’s
ER
– The high levels of calcium ions cause cortical
granules to fuse with the plasma cell
membrane
– The contents of the cortical granules are
exocytosed into the perivitelline space just
outside the cell membrane
2. The cortical reaction
Enzymes released cause the vitelline layer
to separate from the plasma membrane
 Fluid rushes into this space by osmosis,
causing swelling of the perivitelline space
 Other enzymes from the cortical granules
casue the elevated vitelline membrane to
harden, forming a fertilization membrane
 The formation of the fertilization
membrane is referred to as slow block to
polyspermy

Figure 47.3 A wave of Ca2+ release during the cortical reaction
3. Activation of the egg

The rise in cytoplasmic calcium ions cause
metabolic changes that activate the egg
cell
– Cell resp and protein synthesis increases
– Cytoplasmic pH changes from acidic to basic
– The sperm nucleus swells and merges with
the egg nucleus to form the zygote
– DNA replication and the first cell division
occur within about 90 minutes
Figure 47.4 Timeline for the fertilization of sea urchin eggs
Figure 47.5 Fertilization in mammals
4. Fertilization in Mammals



Usually internal
Capacitation – secretions in the female reproductive tract
enhance sperm cell motility
Sperm cells must reach the zona pellucida
– Layer of follicular cells surrounding and protecting the egg
– Zona pellucida is an extracellular matrix of protein
surround the egg
Enzymes from the acrosome penetrate the zona
pellucida
 Fast block to polyspermy
 Slow block to polyspermy
 Fusion of two haploid nuclei

Development

The overall body plan of animals is
established during three stages of
development following fertilization:
– Cleavage
– Gastrulation
– Organogenesis
Cleavage
A series of rapid mitotic cell divisions
following fertilization
 Produces a multicellular embryo called the
blastula
 The cytoplasm of the large zygote is
divided into many smaller cells called
blastomeres
 The heterogenous cytoplasm of the zygote
results in each cell produced during
cleavage having a different mix of
cytoplasmic components

Figure 47.6 Cleavage in an echinoderm (sea urchin) embryo
Figure 47.6x Sea urchin development, from single cell to larva
Polarity of the zygote

In most animals (not mammals) the
zygote has definite poles, and cleavage
follows a specific pattern
– Polarity results from the heterogenous nature
of the egg’s cytoplasm
– The concentration of yolk (yolk gradient) has
the greatest effect on polarity
Polarity
Vegetal pole – highest concentration of
yolk
 Animal pole – lowest concentrations of
yolk

– Opposite the vegetal pole
– Site where the most anterior part of the
embryo will form
Polarity

Two hemispheres form in the zygote,
relative to the two poles
– Vegetal hemisphere
– Animal hemisphere

In frogs, each pole has a different color
due to the heterogenous cytoplasm
– Vegetal – yellow
– Animal – light gray – due to melanin granules
Figure 47.7 The establishment of the body axes and the first cleavage plane in an
amphibian
Figure 47.8x Cleavage in a frog embryo
The first two cleavages are vertical and divide
the embryo into four cells extending from animal
pole to vegetal pole
 The third cleavage is horizontal, producing four
cells in the animal pole and four cells in the
vegetal pole

– In deuterostomes (radial cleavage), the animal pole
cells are aligned with the vegetal pole cells
– In protostomes (spiral cleavage), the top layer is
aligned with the grooves in the bottom layer (cells
overlap)

As cleavage proceeds, cells migrate to the outer
surface of the mass of cells (morula), creating a
fluid-filled blastula
Figure 47.8d Cross section of a frog blastula
Gastrulation

Process by which the blastula is
rearranged to form a three-layered
embryo with a primitive gut (gastrula)
Review:
 Zygote  Morula  Blastula  Gastrula

Gastrulation
The time and exact sequence of events
varies with species
 Common features

– Cells become motile
– Cells change shape
– Changes in cellular adhesion and makeup of
extracellular matrix affect where individual
cells migrate
Figure 47.9 Sea urchin gastrulation (Layer 1)
Figure 47.9 Sea urchin gastrulation (Layer 2)
Figure 47.9 Sea urchin gastrulation (Layer 3)
Figure 47.10 Gastrulation in a frog embryo
Embryonic Germ Layers

Gastrulation produces three embryonic layers:
– Ectoderm – outermost layer
 Nervous system, skin
– Endoderm – innermost layer
 Lines the archenteron (gut)
 Forms the lining of the digestive tract and associated organs
– Mesoderm – middle layer
 Kidneys, heart, muscles, inner layers of skin, most other
organs
Gastrulation produces an embryo with
three tissue layers and an archenteron
(gut) that opens through a blastopore
 Review

– Protostomes – blastopore develops into
mouth
– Deuterostomes – blastopore develops into
anus
Table 47.1 Derivatives of the Three Embryonic Germ Layers in Vertebrates
Blastopore formation
A small crease forms on one side of the blastula
Clusters of cells invaginate by burrowing inward
This creates an opening that will eventually be
the blastopore.
 Involution – cells on the surface roll into the
opening and migrate into the embryo’s interior
 Migrating cells organize into layers of mesoderm
and endoderm
 The archenteron forms within the endoderm



– The archenteron will eventually form the digestive
cavity
Organogenesis
Rudimentary organs form from the
embryonic germ layers
 As this process begins, the embryo is seen
to fold and split within the embryonic
layers
 In chordates, the neural tube and the
notochord are the first organs that
develop

Organogenesis
The notochord forms from condensation of
dorsal mesoderm located above the
archenteron
 Ectoderm above the notochord thickens to
form a neural plate
 The neural place sinks below the surface,
rolling inward to form a neural tube
 The neural tube will later develop into
brain and spinal cord

Organogenesis
The notochord elongates, stretching the
embryo lengthwise
 Mesoderm cells clump along its length,
forming blocks of cells called somites
 Somites will eventually form vertebrae and
muscles of the back

Figure 47.11 Organogenesis in a frog embryo
Figure 47.12 Cleavage, gastrulation, and early organogenesis in a chick embryo
Figure 47.13 Organogenesis in a chick embryo
Organogenesis

As organogenesis proceeds, other organs
and tissues develop from the embryonic
tissue layers
– Ectogerm – epidermis, epidermal glands,
inner ear, eye lens
– Mesoderm – notochord, coelom lining
(peritoneum), muscles, skeleton, gonads,
kidneys, circulatory system
– Endoderm – digestive tract linings, liver,
pancreas, lungs
Organogenesis
The neural crest forms from epidermal
cells along the border of the neural tube
 These cells migrate through the embryo,
forming pigment cells of the skin, bones
and muscle of the skull, teeth, adrenal
medulla, and some parts of the peripheral
nervous system

Mammalian Development
Fertilization occurs in the ovarian end of
the oviduct
 Early development proceeds as the zygote
travels to the uterus
 The eggs of placental mammals store little
nutrients (yolk)
 Gastrulation and organogenesis proceed
as previously discussed

Mammalian Development

Cleavage is slow
– 1st division – 36 hours
– 2nd division – 60 hours
– 3rd division – 72 hours

7 days post-fertilization – the embryo
consists of about 100 cells arranged
around the central blastocoel, forming the
blastocyst
Mammalian Development
Inner Cell Mass (ICM) – mass of cells
within the blastula that will form the
embryo proper
 Trophoblast – outer cells forming the wall
of the blastula – forms the placenta

Implantation
Occurs around 7 days post-fertilization
 The trophoblast cells secrete enzymes that
facilitate implantation
 Finger-like projections of cells extend into
the endometrial lining of the uterus

Extra-Embryonic Membranes

Four membranes form in mammals
– Chorion – forms from the trophoblast and surrounds
the embryo and all other membranes
– Amnion – encloses the embryo within a fluid-filled
cavity
– Yolk sac – encloses a fluid-filled cavity, but with no
yolk – membrane is the site of early blood cell
formation – cells later migrate into the embryo
– Allantois – develops from an outpocketing of the
archenteron
 Incorporated into the umbilical cord
 Forms blood vessels for transport of nutrients and waste
between the embryo and placenta
Figure 47.14 The development of extraembryonic membranes in a chick
Figure 47.15 Early development of a human embryo and its extraembryonic
membranes
Organogenesis in Mammals
Rudiments of all major organs have
developed by the end of the first trimester
 Form from the three embryonic tissue
layers

Morphogenesis
The basic body plan is established early on
during embryonic development
 Morphogenesis – specific changes in cell
shape, position, and adhesion
 These changes occur during migration of
cells as cleavage, gastrulation, and
organogenesis occur
 Cell adhesion molecules (CAMs) –
substances on the surface of cells that
contribute to the selective association of
certain cells with each other

Differentiation
The developmental fate of cells depends
on the heterogenous mix of materials in
the cytoplasm of the zygote
 Gene expression in, and therefore the
developmental fate of cells are influenced
by the distribution of cytoplasm as
cleavage proceeds

Fate Maps

Fate maps trace the development of
different parts of the embryo from each
region of the zygote or blastula
Figure 47.20 Fate maps for two chordates
Cytoplasmic Determinants
Substances that determine body axes in
the early embryo
 Bilaterally symmetrical animals have an
anterior-posterior axis, a dorsal-ventral
axis, and left and right sides
 In frogs and many other animals, all three
axes of the embryo are defined before
cleavage even begins

Figure 47.21 Experimental demonstration of the importance of cytoplasmic
determinants in amphibians
Induction
Induction is the ability of one cell group to
influence the development of another
 Cell layers within the morula, blastula, and
gastrula control the development of other
layers based on their positions and
physical contact

Figure 47.22 The “organizer” of Spemann and Mangold
Pattern Formation in the
Vertebrate Limb

Pattern formation is the development of
an animal’s spatial organization with
organs and tissues in their characteristic
places in the three dimensions of the
animal
Vertebrate Limb Formation
Vertebrate limbs all develop from
undifferentiated limb buds
 A specific pattern of tissues emerges as
the limb develops
 Each component has a precise location
and orientation relative to the three axes
of space.

Figure 47.23 Organizer regions in vertebrate limb development