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\di-ve-ləpmənt\
By James Kwan, Kai Orans,
and Amy WAAHN
Different Forms of Development
Tadpoles and caterpillars are larval forms
of adult forms
 Larval form of sea urchin drifts in ocean
surface waters until it develops into an
adult
 The zygotes of some animals (humans)
develop into an infant that is much like the
adult form.
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Stages of Development
Fertilization
 Cleavage
 Gastrulation
 Organogenesis
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Fertilization
Internal and external fertilization
 Gonads of the parents produces sperm
and eggs
 Contact of sperm with egg surface initiates
metabolic reactions, activating egg
 Main function: combining haploid sets of
chromosomes into a single diploid zygote
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The Acrosomal Reaction
Acrosomal reaction: specialized vesicle at
tip of sperm (acrosome) discharges
enzymes that digest the jelly coat
surrounding the egg
 Contact of the tip of acrosomal process
(sperm structure) with egg membrane
leads to fusion of plasma membranes
 Fast block to polyspermy
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The Corsical Reaction and
Activation of Egg
Vesicules fuse with egg plasma membrane
 Formation of fertilization envelope,
resisting entry of additional sperm
 Slow block to polyspermy
 Activation of egg: substantial increase in
the rates of cell respiration and protein
synthesis
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Acrosomal and Cortical Reactions
Fertilization shares many features in
common with that of sea urchins.
 Differences in timing among species
 Example: in sea urchins, eggs have
completed meiosis when released from
female
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Human eggs are arrested at metaphase of
meiosis II
Mammalian Fertilization
Zona pellucida: extracellular matrix of the
egg
 Ensures moist environment for sperm
 No fast block to polyspermy
 Functions as sperm receptor: binding
initiates acrosomal reaction and cortical
reaction
 Example: increase in sperm motility
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Fertilization in Mammals
Cleavage
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Succession of rapid cell
division
Cells carry out S and M
phases
Blastomeres: smaller cells that
result from the partition of the
zygote
Zygotes of sea urchins and
other animals have definite
polarity
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Planes of division follow a
pattern relative to poles of
zygote
Establishment of three body
axes
Cleavage cont’d
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Polarity defined by uneven distribution of
substances in cytoplasm
Yolk: stored nutrients
Vegetal pole (pole of egg where yolk is
concentrated) and animal pole (pole where yolk
concentration decreases)
Animal-vegetal axis of egg determines head-tail
axis of embryo (ex: sea urchin eggs have a-v
axis due to uneven distribution of substances
Cleavage cont’d
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Holoblastic cleavage:
blastocoel (fluid-filled cavity)
centrally located, cleavage
furrow passes all the way
through the cells
Examples: animals whose
eggs contain relatively little
yolk, such as frogs, sea
urchins, echinoderms, most
chordates and deuterostomes
Meroblastic cleavage:
incomplete division of egg
Example: bird eggs
Gastrulation
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Morphogenetic process:
groups of cells take up
new locations that allow
the formation of tissues
and organs
For most animals, it is a
rearrangement of blastula
cells, producing 3-layered
embryo with primitive
digestive tube
Positioning allows cells to
interact with each other,
generating organs
Gastrulation cont’d
Three layers produced collectively called
embryonic germ layers:
 Ectoderm: outer layer
 Endoderm: lines embryonic digestive tract
 Mesoderm: fills space between ectoderm
and endoderm
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Gastrulation cont’d
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Shallow invagination
(cells buckle inward)
transforms into
archenteron
Blastopore: open end of
archenteron
Gastrulation in sea
urchins and frogs
produces three-layered
embryo (characteristic of
most animal phyla,
established early in
development)
Gastrulation (chicken)
Organogenesis
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Regions of the three embryonic
germ layers develop into
rudimentary organs during
organogenesis
Cell migration, cell condensations,
cell signaling, cell shape changes
(similar mechanisms for
vertebrates and invertebrates, but
still differences because of different
body plans)
Condensation of dorsal mesoderm
forms notochord
Infolding of ectodermal neural plate
forms neural tube, becomes
central nervous system
Neural Crest formed, a collection
of cells that disperse throughout
the body giving rise to teeth,
bones, cartilage
Cell Differentiation
Cells
become specialized in structure and function
Organized into tissues and organs in a 3-D arrangement
Cytoplasm contains both RNA and proteins encoded by mother’s
DNA
Maternal substances in the egg that influence the courses of
development are called cytoplasmic determinants
Combination of these helps determine developmental fate by
regulating expression of cell’s genes
Another source of developmental info is the environment around
a cell (induction)-signaling pathways
Transcription regulates gene expression
Morphogenesis
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Major aspect of development,
involve movement of cells
Changes in cell shape involve
reorganization of the cytoskeleton
(ex. cells of neural plate into
neural tube)
Also drives cell migration
Convergent extension, cells of a
tissue become narrower while
becoming longer, changes
spherical shape of gastrula to
rectangular shape of embryo
Extracellular Matrix (ECM)
ECM= glycoproteins and macromolecules
outside cell plasma membrane
 Cell membrane receptors bind to ECM
 Substances to inhibit migration
 Dialogue between cells as move on ECM
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Fate Mapping
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Territorial diagrams of
embryonic development
Combined with
manipulation of test subject
development
Discoveries:
1) specific tissue of older
embryo are of founder cells
with unique factors b/c of
asymmetrical division
2) older cells have less
development potential
Pattern Formation
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Cells influence fates through induction, switch
on special genes to differentiate cell tissue
Dorsal lip of blastopore = “organizer”
Pattern formation
Positional information (molecular cues)
Apical ectodermal ridge (AER) – promote
limb-bud outgrowth
Zone of polarizing activity (ZPR) – limb bud
organizer
Axes of the Body Plan
Nonamniote vertebrates instructions for forming
the body axes are established early (oogenesis
or fertilization)
 Amniote body axes aren’t established until later
 Positional information is provided by
cytoplasmic determinants and inductive signals
 Axes genes are encoded by genes from the
mother, fittingly called maternal effect genes
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Cell Movement
Cell adhesion molecules (CAMs) –proteins
that guide cell migration and stabilize
tissue by binding to CAMs of other cells
 Cadherins (important group of CAMs)
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Body Plan Overview
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Body plan is a set of morphological and developmental traits
integrated into a whole
Animals can be classified based on symmetry (radial, top and
bottom, or bilateral)
Bilateral animals have four sides and a central nervous system
(brain) in the anterior end
Symmetry fits lifestyle
Number of germ layers can differentiate as well
Cnidarians and comb jellies have only two: ectoderm and
endoderm (diploblastic) while bilaterally symmetrical animals
have three (triploblastic)
Most triploblastic animals possess a body cavity, called coelom,
which forms from tissue derived from mesoderm
Coelom contains fluid that cushions suspended organs and
enables organs to grow independently of outer body wall
Body Plan Illustrated
Hox Genes
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Specify anterior-posterior segment
identity during embryonic
development
Play a role in limb pattern formation
Mutations in regulatory sequences
cause major changes in body form
(Drosiphila growing legs in place of
antennae)
Differing patterns in insects and
crustaceans can explain variation in
number of leg bearing segments
In arthropods changes in the
sequence of existing Hox genes
influenced increased body segment
diversity, a hard exoskeleton, and
jointed appendages
Control development of the major
regions of the vertebrate brain
(similarities between anterior-posterior
order in lancelets and vertebrates)
Protostome vs. Deuterostome
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Three differences: cleavage,
coelom formation, fate of
blastopore
Protostomes undergo spiral
determinate cleavage,
mesoderm splits and forms
coelom
Deuterostomes have radial
indeterminate cleavage, folds
of archenteron become
coelom
In protostomes, blastopore
becomes head while in
deuterostomes it becomes
anus
Invertebrate Adaptations
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Cnidarians have diploblastic
radial body, gastrovascular
cavity, polyp and medusa form
Cephalopods (octopus) feet
evolved into a muscular siphon
and part of the tenticle, well
developed sense organs and
complex brain, lost shell from
mollusk ancestors
Arthropods have a rigid
exoskeleton, a variety of gas
exchange organs, special
appendages for specific tasks
Crustaceans and Echinoderms
(starfish) can regrow lost arms
and legs
Chordate-Vertebrate Adaptations
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Simple chordates Lancelets and
tunicates show that ancestral
chordates had genes related to
the mammalian heart, thyroid
gland, and brain
Aquatic vertebrates developed
fins, vertebrae, and a more
extensive skull
Mineralization allowed animals to
become predators (started in the
mouth and skull)
Gnathostomes developed gills for
gas exchange and jaws also
underwent duplication of Hox
genes
Tetrapods developed legs in
place of fins to walk on land
Amniote Adaptations
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Vertebrate embryos require an aqueous environment to
develop
Movement of vertebrates onto land required shell and
uteral adaptations, increased embryonic contact with fluid
Amniotes include birds and mammals (amnions)
Extraembryonic membranes include (chorion, amnion,
yolk sac, allantois) provide life support system
Mammalian Development
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Small eggs, store few nutrients, holoblastic cleavage
Fertilization takes place in the oviduct
Despite lack of yolk, mammalian gastrulation and organogenesis are
similar to that of birds and other reptiles
At completion of cleavage, embryo is in blastocyst stage
Trophoblast initiates implantation and provides support
Inner cell mass forms epiblast and hypoblast, homologous to those of
birds
When implantation is completed, gastrulation begins, invading
trophoblast, mesodermal cells, and endometrial tissue contribute to
forming a placenta
By the end of gastrulation, embryonic germ layers have formed, as
well as extraembryonic mesoderm and membranes
Extra-embryonic membranes in mammals are homologous to those
of birds and other reptiles
Developmental Disorders
Mental Retardation
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Top 3 causes: Down syndrome, fetal alcohol syndrome
and Fragile X syndrome
Include: epilepsy, autism, cerebral palsy and other
disorders
Fetal Alcohol Syndrome
When the female ingests alcohol, it
crosses the placental barrier and can stunt
or modify growth, damage neurons and
brain structure, and cause other physical
or mental disorders
 Amount, frequency, and timing of ingestion
is currently not known. So when you get
pregnant, do not drink alcohol.
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Down Syndrome
Caused by an extra whole or part of the
21st chromosome
 Inhibits cognitive ability, physical growth,
and can cause facial anomalies
 Lower than average mental capability
 Family environment and vocal lessons can
lesson the disadvantages, but currently, a
cure is unknown
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Cerebral Palsy
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cerebral palsy often includes limitations of
sensation, perception, cognition,
communication, and behaviour, by epilepsy, and
by secondary musculoskeletal problems
Occurs during pregnancy, birth, or rarely, after
birth
Disturbance of the cerebrum is the area of
cause, but specific cause is still debated
There is currently no known cure
Congenital Physical Anomalies
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Did you know, most people have a congenital physical
anomaly if examined enough?
Some Examples of these include…
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Curved pinky (clinodactyly )
Triple nipples (supernumerary nipples)
Short fourth toe
Sacral Dimples (dimple over the spine)
Extra fingers (polydactyly)
Indentations near the ear (preauricular pits )
Webbed toes
Extremely long fingers (Arachnodactyly ) and can bend back 180
degrees
Genu Valgum (knees bend inwards and hit each other)
Flat feet
Genu Varum (knees bend outwards – bowlegged people)
Pectus Excavatum (caved in or sunken chest)
What Causes these Physical
Anomalies?
Well, in general, we don’t know. They’re
called “Sporadic” disorders
 What can be some causes?
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Some are genetic (polydactyly)
 Some are environmental (mom drinks alcohol
or mercury or other harmful substance during
pregnancy)
 Many are multifactorial (combination of
genetic and environmental disorders)
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What are some cures?
We don’t cure most of the Congenital
Physical Anomalies.
 For the ones that make us stick out in
society, we sometimes chop them off
(amputation) to hide them
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References
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"Fertilization." Gale Encyclopedia of Science. Ed. K.
Lee Lerner and Brenda Wilmoth Lerner. 4th ed.
Detroit: Gale Group, 2008. Student Resource Center Bronze. Gale. PIEDMONT HIGH SCHOOL. 10 Apr.
2009
<http://find.galegroup.com/srcx/infomark.do?&co>.
Cobb, Bryan H., PhD. "Embryo and embryonic
development." Gale Encyclopedia of Science. Ed. K.
Lee Lerner and Brenda Wilmoth Lerner. 4th ed.
Detroit: Gale Group, 2008. Student Resource Center Bronze. Gale. PIEDMONT HIGH SCHOOL. 10 Apr.
2009 <http://find.galegroup.com/srcx/infomark.do>.
Campbell, Neil A., and Jane B. Reece. Biology. San
Francisco: Pearson Education, Inc., 2008.