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CHAPTER 47 ANIMAL DEVELOPMENT
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.
I. The Stages of Early Embryonic Development
A. From egg to organism, an animal's form develops gradually: the concept of
epigenesis
Epigenesis proposes that the form of an embryo gradually emerged from a formless
egg.
- Originally proposed by Aristotle
Modern biology has found that an organism's development is mostly determined by
the
zygote's genome and the organization of the egg cell's cytoplasm.
- The heterogenous distribution of messenger RNA, proteins, and other
components in
the unfertilized egg greatly impacts the development of the embryo in most
animals.
- After fertilization, cell division partitions the heterogenous cytoplasm in such a
way that
nuclei of different embryonic cells are exposed to different cytoplasmic
environments.
- These different cytoplasmic environments result in the expression of different
genes in
different cells.
- This leads to an emergence of inherited traits that is ordered in space and time by
mechanisms controlling gene expression.
B. Fertilization activates the egg and brings together the nuclei of sperm and
egg
Fertilization is important because:
- It forms a diploid zygote from the haploid sets of chromosomes from two
individuals.
- It triggers onset of embryonic development.
1. The acrosomal reaction
The acrosomal reaction is the discharge of hydrolytic enzymes from a vesicle in the
acrosome of a sperm cell (based on studies with sea urchins):
- Upon contacting the egg's jelly coat, the acrosomal vesicle in the head of the
sperm
releases hydrolytic enzymes via exocytosis.
- These enzymes enable an acrosomal process to elongate and penetrate the jelly
coat.
- A protein coating the tip of the process attaches to specific receptors on the egg's
vitelline layer (just external to the plasma membrane).
- This provides species specificity for fertilization.
- Enzymes of the acrosomal process probably digest vitelline layer materials
allowing
the tip of the process to contact the egg's plasma membrane.
- The sperm and egg's plasma membranes fuse, allowing the sperm nucleus to
enter
the egg and causing a depolarization of the plasma membrane that prevents other
sperm cells from also uniting with the egg.
2. The cortical reaction
The fusion of the egg and sperm membranes stimulates a series of changes in the
egg's cortex known as a cortical reaction.
The cortical reaction results in the vitelline layer of the egg to become harden to form
the fertilization membrane.
- The fertilization membrane prevents entry of additional sperm.
3. Activation of the egg
The cortical reaction also incites metabolic changes that activates the egg cell.
- Cellular respiration and protein synthesis rates increase.
- The sperm nucleus within the egg swells and merges with the egg nucleus to
form the
zygote (actual fertilization).
- DNA replication begins and the first division occurs in about 90 minutes. The
events of
fertilization in sea urchins are illustrated in Campbell.
4. Fertilization in mammals
The events of internal fertilization in mammals are similar to those of external
fertilization discussed for sea urchins but include some important differences.
Fertilization in terrestrial animals is generally internal.
Capacitation (enhanced sperm function) results from secretion in the female's
reproductive tract.
- Alters certain molecules on the surface of sperm cells and increases sperm
motility.
The capacitated sperm cell must reach the zona pellucida for the process to continue.
- The secondary oocyte (egg) released at ovulation is surrounded by follicle cells
released at the same time.
- The sperm cell must migrate through this layer of cells.
- The zona pellucida (extracellular matrix of the egg) is a three-dimensional
network of
cross-linked filaments formed by three different glycoproteins.
- The zona pellucida is analogous to the vitelline layer of the sea urchin egg.
- One of the glycoproteins acts as a sperm receptor by binding to a complementary
molecule on the surface of the sperm head.
- The binding of this glycoprotein and its complementary molecule stimulates an
acrosomal reaction similar to that described earlier.
- Protein-digesting enzymes and other hydrolases from the acrosome allow the
sperm
cell to penetrate the zona pellucida and reach the plasma membrane of the egg.
- The acrosomal reaction also exposes a sperm membrane protein that binds
and fuses
with the egg membrane.
Binding of the sperm to the egg depolarizes the egg's plasma membrane (prevents
further
sperm from entering).
- A cortical reaction occurs, this stimulate a hardening of the zona pellucida
(prevents
further sperm from entering).
- Microvilli from the egg pull the whole sperm cell into the egg cell.
- Nuclear envelopes disperse and the chromosomes from the gametes share a
common
spindle apparatus for the first mitotic division of the zygote.
- The chromosomes from the two parents form a common nucleus (offspring's
genome)
as diploid nuclei form in the daughter cells after the first division.
C. Cleavage partitions the zygote into many smaller cells
The basic body plan of an animal is established in three successive stages following
fertilization: cleavage, gastrulation, and organogenesis. The following information is
based upon studies of the sea urchin, frogs, and Drosophila.
Cleavage is a succession of rapid mitotic cell divisions following fertilization that
produce a multicellular embryo, the blastula.
- During cleavage, the cells undergo the S and M phases of the cell cycle but the
G1
and G2 phases are virtually skipped.
- Very little gene transcription occurs during cleavage and the embryo does not
grow.
- The cytoplasm of the zygote is simply divided into many smaller cells called
blastomeres, each of which has a nucleus.
- The heterogenous nature of the zygote's cytoplasm results in blastomeres with
differing cytoplasmic components.
A definite polarity is shown by the eggs of most animals and the planes of division
during cleavage follow a specific pattern relative to the poles of the zygote.
- The polarity results from concentration gradients in the egg of such cellular
components as mRNA, proteins, and yolk (stored nutrients).
- The yolk gradient is a key factor in determining polarity and influencing the
cleavage
pattern in frogs and other animals.
- The vegetal pole of the egg has the highest concentration of yolk.
- The animal pole, opposite the vegetal pole, has the lowest concentration of yolk
and is
the site where polar bodies are budded from the cell.
- The animal pole also marks the area where the most anterior part of the embryo
will
form in most animals.
The zygote is composed of two hemispheres named for the respective poles: vegetal
and animal.
In frogs:
- The hemispheres in the egg of many frogs have different coloration due to the
heterogeneous distribution of cytoplasmic substances.
- The animal hemisphere has a gray hue due to the presence of melanin
granules in the
outer cytoplasm.
- The vegetal hemisphere has a light yellow hue due to the yellow yolk.
- The cytoplasm in amphibian eggs is rearranged at fertilization.
- A narrow gray crescent appears. The gray crescent is the precursor of the
dorsal
lip.
Cleavage in the animal hemisphere of a frog's zygote is more rapid than in the
vegetal
hemisphere.
- Large amounts of yolk impedes cell division.
- This discrepancy in the rate of cleavage divisions results in a frog embryo with
different size cells.
- Animal hemisphere cells are smaller than those in the vegetal pole.
- In sea urchins and many other animals, the blastomeres are about equal in size
due to
small amounts of yolk.
- Animal-vegetal pole axes are present but are due to concentration gradients of
cytoplasmic components other than yolk.
- The absence of yolk permits cleavage divisions to occur at about the same
rate.
The first two cleavage divisions in sea urchins and frogs are vertical and divide the
embryo into four cells that extend from the animal pole to the vegetal pole.
- The third cleavage plane is horizontal and produces an eight cell embryo with two
tiers
(animal and vegetal) of four cells each.
- In deuterostomes, which have radial cleavage, the upper tier of cells is aligned
directly
over the lower tier.
- In protostomes, which have spiral cleavage, the upper tier of cells align with the
grooves between cells of the lower tier.
A continuation of cleavage produces a solid ball of cells called a morula.
- The blastocoel, a fluid-filled cavity, develops within the morula as cleavage
continues
which changes the embryo from the solid morula to a hollow ball of cells, the
blastula.
- In sea urchins, the blastocoel is centrally located in the blastula due to equal cell
divisions.
- Unequal cell divisions in the frog embryo produces a blastocoel in the animal
hemisphere.
The amount of yolk present in an egg greatly effects the cleavage.
- In eggs with little yolk (sea urchins) or moderate amounts of yolk (frogs), a
complete
division of the egg occurs.
- In eggs which contain large amounts of yolk (birds, reptiles), cleavage is
incomplete
and confined to a small disc of yolk-free cytoplasm at the animal pole of the egg.
D. Gastrulation rearranges the blastula to form a three-layered embryo with a
primitive gut
Gastrulation involves an extensive rearrangement of cells which transforms the
blastula,
a hollow ball of cells, into a three-layered embryo called the gastrula.
The three layers produced by gastrulation are embryonic tissues called embryonic
germ
layers. These three cell layers (the primary germ layers) will eventually develop into
all
parts of the adult animal.
- The ectoderm is the outermost layer of the gastrula. The nervous system and
outer
layer of skin in adult animals develop from ectoderm.
- The endoderm lines the archenteron. The lining of the digestive tract and
associated
organs (i.e. liver, pancreas) develop from endoderm.
- The mesoderm partly fills the space between the ectoderm and endoderm. The
kidneys, heart, muscles, inner layer of the skin, and most other organs develop
from
mesoderm.
Gastrulation in sea urchins begins at the vegetal pole.
- The sea urchin blastula consists of a single layer of cells.
- Vegetal pole cells form a flattened plate that buckles inward (invagination).
- Cells near the plate detach and enter the blastocoel as migratory mesenchyme
cells.
- The invaginated plate undergoes rearrangement to form a deep, narrow pouch,
the
archenteron or primitive gut.
- The archenteron opens to the surface through the blastopore which will
become the
anus.
- A second opening forms at the other end of the archenteron, forming the mouth
end of
the rudimentary digestive tube.
- At this point, gastrulation has produced an embryo with a primitive gut and three
germ
layers.
Gastrulation during frog development also results in an embryo with the three
embryonic
germ layers and an archenteron that opens through a blastopore.
- The mechcanics of gastrulation are more complicated than in the sea urchin
because
of the large, yolk-laden cells in the vegetal hemisphere and the presence of more
than
one cell layer in the balstula wall.
E. In organogenesis, the organs of the animal body form from the three
embryonic layers
The three germ layers that develop during gastrulation will give rise to rudimentary
organs through the process of organogenesis.
- The first evidence of organ development is morphogenetic changes (folds, splits,
condensation of cells) that occur in the layered embryonic tissues.
The neural tube and notochord are the first organs to develop in frogs and other
chordates.
- The dorsal mesoderm above the archenteron condenses to form the notochord in
chordates.
- Ectoderm above the rudimentary notochord thickens to form a neural plate that
sinks
below the embryo's surface and rolls itself into a neural tube, which will become
the
brain and spinal cord.
- The notochord elongates and stretches the embryo lengthwise; it functions as the
core
around which mesoderm cells that form the vertebrae gather.
- Strips of mesoderm lateral to the notochord condense into blocks of mesodermal
cells
called somites from which will develop the vertebrae and muscles associated with
the
axial skeleton.
- As organogenesis continues, other organs and tissues develop from the
embryonic
germ layers.
- Ectoderm also gives rise to epidermis, epidermal glands, inner ear, and eye
lens.
- Mesoderm also gives rise to the notochord, coelom lining, muscles, skeleton,
gonads,
kidneys and most of the circulatory system.
- Endoderm forms the digestive tract linings, liver, pancreas and lungs.
- The neural crest forms from ectodermal cells which develop along the border
where
the neural tube breaks off from the ectoderm.
- These cells migrate to other parts of the body and form pigment cells in the
skin, some
bones and muscles of the skull, teeth, adrenal medulla, and parts of the
peripheral
nervous system.
F. Amniote embryos develop in a fluid-filled sac within a shell or uterus
All vertebrate embryos require an aqueous environment for development.
- Fish and amphibians lay their eggs in water.
- Terrestrial animals live in dry environments and have evolved two solutions to this
problem: the shelled egg in birds and reptiles and the uterus in placental
mammals.
- The embryos of reptiles, birds, and mammals develop in a fluid-filled sac, the
amnion.
These three classes of animals are referred to as amniotes due to the presence of
the
amnion around the embryo.
Important differences between cleavage and gastrulation occur in the development of
birds and mammals, as well as differences between these amniotes and
nonamniotes,
such as the frog previously discussed.
1. Avian development
The larger yellow "yolk" of a bird egg is actually the ovum containing a large food
reserve properly called yolk.
- Surrounding this large cell is a protein-rich solution (the egg white) that provides
additional nutrients during development.
- After fertilization, cleavage will be restricted to a small disc of yolk-free cytoplasm
at
the animal pole.
- Cell division partitions the yolk-free cytoplasm into a cap of cells called the
blastodisc
which rests on large undivided yolk mass.
- The blastomeres sort into an upper layer (epiblast) and lower layer (hypoblast)
with a
cavity (blastocoel) forming between them.
- Although different in appearance, this blastula is equivalent to the hollow ball
stage in
the frog.
Subsequent organogenesis occurs as in the frog, except that primary germ layers
also
form four extraembryonic membranes: the yolk sac, amnion, chorion and allantois.
- the yolk sac digests the yolk.
- the amnion encloses the amniotic cavity (cushions the developing embryo).
- the chorion is the outer membrane that functions in gas exchange.
- the allantois is below the chorion and first functions to store wastes and later
in gas exchange.
2. Mammalian development
Fertilization occurs in the oviducts of most mammals and the early development
occurs
while the embryo travels down the oviduct to the uterus.
- The egg of placental mammals stores little nutrients and there is complete
division of
the zygote .
- Gastrulation and organogenesis follow a similar pattern to that in birds.
Cleavage is relatively slow in mammals and the zygote has no apparent polarity.
- Cleavage planes appear randomly oriented and blastomeres are of equal size.
The development of a human embryo can represent mammalian development.
- Cleavage is relatively slow with the first, second and third divisions being
completed at
36, 60, and 72 hours, respectively.
- At 7 days post-fertilization the embryo consists of about 100 cells arranged
around a
central cavity forming the blastocyst.
- The inner cell mass protrudes into one end of the cavity and will develop into the
embryo and some of its extraembryonic membranes.
- The trophoblast is the outer epithelium surrounding the cavity which will, along
with
mesodermal tissue, form the fetal part of the placenta.
- During implantation, the inner cell mass forms a flat disc similar to those in birds;
the
embryo develops from epiblast cells and the yolk sac from hypoblast cells.
The blastocyst stage reaches the uterus and begins to implant.
- The trophoblast layer:
- Secretes enzymes that enable blastocyst implantation in the uterus
- Thickens and extends fingerlike projections into the endometrium
- Gastrulation occurs by inward movement of mesoderm and endoderm.
Four extraembryonic membranes homologous to those in birds and reptiles form in
mammals.
- The chorion forms from the trophoblast and surrounds the embryo and all
extraembryonic membranes. It unites with maternal tissue to form the placenta.
- The amnion forms as a dome above the epiblast and encloses the embryo in a
fluid-filled cavity. It cushions the developing embryo.
- The yolk sac encloses a fluid-filled cavity but no yolk; its membrane is the site of
early
blood cell formation.
- These cells later migrate into the embryo.
- The allantois develops from an outpocketing of the rudimentary gut and is
incorporated
into the umbilical cord where it forms blood vessels that transport oxygen and
nutrients
from the placenta to the embryo and waste products from the embryo to the
placenta.
Organogenesis begins with formation of the neural tube, notochord, and somites.
- Rudiments of all major organs have developed from the three germ layers by the
end
of the first trimester in humans.
II. The Cellular and Molecular Basis of Morphogenesis and Differentiation
Early in the embryonic development of an animal, a sequence of changes takes place
that establishes the basic body plan of that animal.
- These include not only morphogenetic changes, which result in characteristic
shapes,
but also differentiation of many kinds of cells in specific locations.
A. Morphogenesis in animals involves specific changes in cell shape, position,
and
adhesion
The changes in cell shape and the cell migrations during cleavage, gastrulation and
organogenesis are morphogenetic movements. These various morphogenetic
movements help shape an embryo.
- cell extensions, contractions and adhesions are involved in these movements.
- chages in shape usually involve reorganization of the cytoskeleton.
B. The developmental fate of cells depends on cytoplasmic determinants and
cell-cell induction: a review
Two general principles appear to integrate the genetic and cellular mechanisms
underlying differentiation during embryonic development.
1. The heterogeneous distribution of cytoplasmic determinants in the unfertilized egg
leads to regional differences in the early embryos of many animal species.
- Different blastomeres receive different substances (mRNA, proteins, etc.) during
cleavage due to the partitioning of the heterogenous cytoplasm of the ovum.
- Gene expression in, and the developmental fate of, cells in the early embryo are
influenced by these local differences in the distribution cytoplasmic determinants.
2. In embryonic induction, interactions among the embryonic cells themselves induce
changes in
gene expression.
- Interactions among embryonic cells induce development of many specialized cell
types.
- Induction may be mediated by diffusible chemical signals.
- Membrane interaction between cells that are in contact may also induce
cytoplasmic
changes.
C. The eggs of vertebrates contain cytoplasmic determinants that help establish
the body axes and differences among cells of the early embryo
1. Polarity and the basic body plan
Bilaterally symmetrical animals have an anterior-posterior axis, a dorsal-ventral axis,
and left and right sides.
- The first step in morphogenesis is establishment of this body plan, which is
prerequisite to tissue and organ development.
- In humans and other mammals, the basic polarities do not appear to be
established
until after cleavage.
- In most animals polarity of the embryo is established in the unfertilized egg or
during
early cleavage.
2. Restriction of cellular potency
The developmental fate of different regions of the embryos of some animals are
affected by the distribution of cytoplasmic determinants and the zygote's
characteristic
pattern of cleavage.
- Cytoplasmic determinants are substances localized within specific regions of the
egg's
cytoplasm, which leads them to be included in specific blastomeres.
- May control gene expression
Only the zygote is totipotent in many species, while in others, there is a progressive
restriction in potency of the cells.
- Only the zygote is totipotent in those where the first cleavage plane divides the
cytoplasmic determinants in a way that each blastomere will give rise to only
certain
parts of the embryo.
- In mammals, cells of the embryo remain totipotent until they become arranged
into the
trophoblast and inner cell mass of the blastocyst.
- The cells of the early gastrula of some species can still give rise to more than one
type
of cell even though they have lost their totipotency.
- By the late gastrula stage, the developmental fate of all cells is fixed.
Determination is the progressive restriction of a cell's developmental potential.
- A determined cell is one whose developmental fate can not be changed by
moving it to
a different location in the embryo.
- Daughter cells receive a developmental commitment from the original cell.
- Involves the cytoplasmic environment's control of the genome of the cell.
- The partitioning of heterogeneous cytoplasm of an egg during cleavage exposes
the
nuclei of the cells to cytoplasmic determinants that will affect which genes are
expressed as the cells begin to differentiate.
E. Inductive signals drive differentiation and pattern formation in vertebrates
Induction = The ability of one cell group to influence the development of another
1. The "organizer" of Spemann and Mangold
In the 1920s, Spemann and Mangold performed a series of transplantation
experiments
in which they discovered that the dorsal lip of the blastopore acts as a primary
organizer, setting up the interaction between chordamesoderm and the overlying
ectoderm.
- Transplanting the chordamesoderm, which forms the notochord, to an abnormal
site in
the embryo will cause the neural plate to develop in an abnormal location.
- The rudimentary notochord induces the dorsal ectoderm of a gastrula to form the
neural plate.
2. 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.
- Occurs in addition to the determination and differentiation of cells.
Pattern formation is controlled by positional information, which is a set of molecular
cues that indicate a cell's location relative to other cells in an embryonic structure and
that help to determine how the cell and its descendants respond to future molecular
signals.
- Vertebrate limbs all develop from undifferentiated limb buds.
- A specific pattern of tissues (e.g., bone, muscle) emerges as the limb develops.
- Every component has a precise location and orientation relative to three axes.
- For proper development to occur, embryonic cells in the limb bud must receive
positional information indicating location along all three axes.
Experiments have led to the conclusion that pattern formation requires cells to
receive
and interpret environmental cues that vary between locations.
- Certain polypeptides are believed to be the cues which function as positional
signals
for vertebrate limb development.
- Regional gradients of these polypeptides along the three orientation axes would
provide a cell positional information needed to determine its position in a
three-dimensional organ.
- The regional variation in production of these polypeptides result from differential
gene
expression in different locations of the embryo.