CH. 47
ANIMAL DEVELOPMENT
Compiled by…
Micki Lewis
And
Stephanie Langga
IN THE
TH
18
CENTURY…
Preformation
•Idea that an egg
or sperm contains
an embryo, or
“homunculus” that
grows as it
develops.
Epigenesis
•Idea that the form of an animal
gradually comes from an egg
- proposed originally by
Aristotle
…from watching a chick develop in
its egg.
*Displaced Preformation*
MODERN TECHNOLOGY…
…has enabled us to specify the
stages of which human embryos
develop. This one is at about 6-8
weeks after conception.
•Cell Development
- cytoplasmic determinants- proteins and RNA that also put
together the genetic codes
- differences emerge when uneven distribution appears
between early embryonic cells
•Cell Differentiation
- specialization of cells in structure and function
•Morphogenesis
- when the organism takes shape and the cells go to their functional
homes
- Represent
a larger
group
CONCEPT 47.1
After fertilization, embryonic
development proceeds through
cleavage, gastrulation, and
organogenesis.
FERTILIZATION
•Important processes regulating development, a developing stage
in most animals occurs
1. Cleavage
cell division creates a hollow ball of cells, the blastula
2. Gastrulation
rearranges blastula into a three-layered zygote, the
gastrula
3. Organogenesis
interactions with three layers create rudimentary
organs
FERTILIZATION
Main Function
Studied
•Combining haploid sets from 2
organisms into a single diploid cell • SEA URCHINS!!!
(zygote)
- Not vertebrates or
•contact with sperm on the egg’s
chordates, but the characteristics
surface “activates” metabolic
are very similar.
processes for embryonic
~ Deuterostome
development
Development
•Contact, Autosomal reaction,
… Eggs are fertilized
Fusion of sperm and egg
membranes, Cortical reaction, and externally.
Entry of sperm.
FERTILIZATION… IN SEA URCHINS
Acrosomal Reaction
• reaction between the sperm tip
(the acrosome) and egg
•The acrosome discharges hydrolytic
enzymes
•Hydrolytic enzymes digest jelly coat
•Leads to the fusion of the sperm and
egg plasma membranes
*Fast block to polyspermy : depolarization
of the egg’s membrane potential
External
Fertilization
Cortical Reaction
• fusion causes a longer-lasting
polyspermy block
• Cortical granules fuse to the egg
plasma membrane for initiation
• Fertilization envelope is formed from
secretion of hardening enzymes
*Slow block polyspermy : change of egg’s
surface function that block other sperm
This ensures
that only
one sperm
enters the
egg
cytoplasm.
Activation of the Egg
• Egg Activation
•Rise of calcium ions increases rates of
cellular respiration and protein synthesis
•OR, artificially activated by calcium ion
injection (parthenogenesis)
•About 20 minutes later…
•Creation of diploid nucleus
•Timing differs!
•Different species have different times in
meiosis and can stop in a stage until
fertilization
• Faster in sea urchins 90 min
Mammal Fertilization
•Moist environment
•Sperm movement in female
reproductive tract
•Zona Pellucida
• After going through follicle cells, the
sperm reaches the cellular matrix of
the egg (part sperm receptor)
• Binding induces acrosomal reaction
•Afterwards…
• NO fast block to polyspermy
• Whole sperm is taken into egg
• Slower in mammals 12-36 hours
Internal
Fertilization
1. CLEAVAGE
• rapid cell division WITHOUT growth
• goes from the zygote into many smaller cells, or blastomeres
•Follows a specific pattern
•Fluid-filled cavity forms, blastoecoel, in the first five-seven
divisions
•Fully forms into the blastula, a hollow ball of cells
*Polarity of eggs and zygotes established as the egg develops.
Defined by uneven distribution of cytoplasmic substances
1.5 CLEAVAGE
• Holoblastic
Cleavage
Yolk: stored nutrients
- Key factor in influencing the pattern of
cleavage
- Often concentrated toward one pole:
- Vegetal pole
•Division of the entire egg
- Decreases significantly toward the
•Occurs in species with eggs that
have moderate amounts of yolk animal pole, where polar bodies bud
from the cell
• EX. sea urchins, frogs, and mammals
•Meroblastic Cleavage
•Incomplete division of the egg
•Occurs in species with yolk-rich
eggs
• EX. birds and reptiles
• Most animals have a dramatic
rearrangement of cells.
2. GASTRULATION
• Transforms blastula into a gastrula
• Cells near the blastula form three
cell layers, or germ layers :
1. Ectoderm
The outer layer
2. Endoderm
The lining of the embryonic
digestive tract
3. Mesoderm
Space between the ectoderm and
endoderm
3. ORGANOGENESIS
• involves more localized shape changes in
tissues and individual cells
• First evidence of organ building: appearance of tissue folds, splits, and dense clustering of cells
• Notochord
• Skeletal rod characteristic of ALL chordate embryos
• Neural Tube
• The rolled neural plate, caused by cell shape change
• Neural Crest Cells (in vertebrates)
• Pinched off neural crest that migrates to parts of the embryo to form nerves, teeth,
bone, etc.
• The “fourth germ layer”
• Somites
• Mesoderm lateral to the notochord that separate into blocks, arranged along the
notochord
AMNIOTES
Developmental Adaptations
of Amniotes
• embryos of birds, reptiles, and
mammals
• Or amniotes
• Develop within a fluid-filled sac
called the amnion in a shell or the
uterus
• Also forms extraembryonic membranes
1. Chorion – gas exchange
2. Amnion – protective, fluid-filled
cavity
3. Yolk Sac – encloses yolk for
nutrients
4. Allantois – disposes waste
products
Mammalian Development
• Placental Mammals
•Eggs are small and have few nutrients
• Holoblastic cleavage
• No polarity
*similar pattern to birds and reptiles*
Human early embryonic development
goes through 4 stages
MAMMALIAN DEVELOPMENT
• after
fertilization and cleavage…
1. Blastocyst (version of a blastula) implants in the uterus.
 Inner cell mass develops into the embryo and form the extraembryonic
membranes
2. Trophoblast (outer epithelium of the blastocyst) supports the
embryo but DOES NOT actually contribute to the embryo itself.
(formation of the fetal position)
3. The embryo develops from the epiblast and hypoblast within the
blastocyst.
CONCEPT 47.2
Morphogenesis in animals
involves specific changes in cell
shape, position, and adhesion.
MORPHOGENESIS
• major aspect of development in animals AND plants
•Only involves movement in cells of animals
•Can bring changes in shape or migration
•Plants cannot move; rigid cell wall
•Changes in cell shape and cell position are in cleavage,
gastrulation, and organogenesis.
CELLULAR COMPONENTS & BEHAVIOR
Cytoskeleton, Cell Motility, and
Convergent Extension
Rearrangements change both
the shape and position of the
cells.
•Both occur in tissue
invaginations
•Convergent Extension: a type
of morphogenetic movement
where cells of a tissue
rearrange themselves that it
becomes narrower and longer.
Role of Cell Adhesion Molecules and
Extracellular Matrix
1. Cell Adhesion Molecules
• hold cells together in tissues
Ex. Cadherins
2. Extracellular Matrix
• Fibers provide anchorage for cells
•Guides migrating cells to
destinations
*important intracellular molecules:
Fibronectin and glycoproteins
CONCEPT 47.3
The developmental fate of cells
depends on their history and on
inductive signals.
PRINCIPLES OF DIFFERENTIATION
First…
Second…
During early cleavage divisions,
embryonic cells must become different
from one another.
Once initial cell asymmetries are set up,
subsequent interactions among the
embryonic cells influence their fate,
usually by causing changes in gene
expression.
In many animals, the first differences
result from uneven distribution of
cytoplasmic determinants. (transcription
factors like DNA binding proteins that activates one
set of genes)
• Induction:
• mechanism that brings differentiation of the
specialized cell types that make up an animal
FATE MAPPING
-maps of embryos that show specific regions
that develop into specific parts
• development in specific parts of OLDER
embryos
• EX. lineage of C. elegans
ESTABLISHING CELLULAR ASYMMETRIES
In nonamniotes…
•need unevenly distributes cytoplasmic determinants in the egg
•Establish body axes
•Difference between blastomeres from cleavage in the zygote
- Differences matter!
•Undergo different fates with different cytoplasmic determinants
In amniotes…
•Local environmental differences
•Also establish initial differences of cells and body axes
Totipotent:
- Capability of
developing into
all different cell
types of a species
•Development potential
•As embryonic development proceeds, developmental potential of cells become
limited
INDUCTIVE SIGNALS
Cell Fate Determination
Cells in developing embryo receive
and respond to positional
information.
• Dorsal Lip of blastopore in the gastrula
functions as an organizer (or Spemann’s
organizer, gastrula organizer)
• Or AER (apical ectodermal ridge) and ZPA
(zone of polarizing activity) of the
vertebrate limb bud
• Inductions form the notochord, neural tube, and
organs
Pattern Formation
Development of an animal’s
spatial organization
 Arrangement of organs and tissues
in 3D space
• Control Pattern Formation
• Tells a cell where it is relative to the
body axes
•Determines how the cell will respond
to molecular signaling
REFERENCES
AP Biology book
Bozeman Animal Development Video
Crash course Animal Development