CHAPTER 8
Principles of
Development
8-1
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Organizing cells during development
8-2
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8-3
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Development

Development
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8-4
Series of progressive changes in an
individual from its beginning to maturity
Begins when a fertilized egg divides
mitotically
Specialization occurs as a hierarchy of
developmental “decisions”
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8-5
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Fertilization

Fertilization and Activation
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Contact and Recognition Between Egg
and Sperm
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8-6
A century of research has been conducted
on marine invertebrates
 Especially sea urchins
Marine organisms
 release enormous numbers of sperm in
the ocean to fertilize eggs
 Many eggs release a chemical molecule
 Attract sperm of the same species
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Fertilization

Sea urchin sperm
 Penetrate a jelly layer surrounding egg
 Next, contacts the vitelline envelope


Egg-recognition proteins bind to species-specific
sperm receptors on the vitelline envelope
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Ensures an egg recognizes only sperm of the same species
In the marine environment
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8-7
Thin membrane above the egg plasma membrane
Many species may be spawning at the same time
Similar recognition proteins are found on sperm of
vertebrate species
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Fertilization

Prevention of Polyspermy
Fertilization cone forms where the sperm
contacts the vitelline membrane
 Sperm head drawn in and fuses with egg
plasma membrane
 Important changes in the egg surface block
entrance to any additional sperm
 Polyspermy, the entry of more than one
sperm
 In the sea urchin, an electrical potential
rapidly spreads across the membrane
 “fast block”

8-8
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8-9
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Fertilization

8-10
The cortical reaction follows
 Fusion of thousands of enzyme-rich
cortical granules with the egg membrane
 Cortical granules release contents
between the membrane and vitelline
envelope
 Creates an osmotic gradient
 Water rushes into space
 Elevates the envelope
 Lifts away all bound sperm except the
one sperm that has successfully fused
with the egg plasma membrane
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8-11
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Fertilization
 One
cortical granule enzyme
 Causes the vitelline envelope to
harden
 Now called the fertilization
membrane
 Block to polyspermy is now complete
 Similar process occurs in mammals
8-12
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Binding Sperm to Sea Urchin Egg
8-13
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8-14
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Fertilization
After sperm and egg membranes fuse
 Sperm
loses its flagellum
 Enlarged sperm nucleus migrates inward to
contact the female nucleus
 Fusion of male and female nuclei forms a
diploid zygote nucleus
8-15
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Fertilization

Fertilization
 Sets in motion important changes in the egg
cytoplasm


8-16
Fertilized egg called a zygote
Zygote now enters cleavage
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Cleavage and Early Development

Cleavage
 Embryo divides repeatedly
 Large cytoplasmic mass converted into small
maneuverable cells: blastomeres
 No cell growth occurs, only subdivision until cells
reach regular somatic cell size
 At the end of cleavage
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
8-17
Zygote has been divided into many hundreds or
thousands of cells
Blastula is formed
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Types of
Cleavage is
Determined
by Yolk
8-18
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Cleavage Types

Holoblastic
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Cleavage extends entire length of egg
Egg does not contain a lot of yolk, so cleavage
occurs throughout egg
Example: mammals, sea stars, worms
Meroblastic



Cells divide sitting on top of yolk
Too much yolk and yolk can’t divide
Examples: birds, reptiles, fish
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Egg Types and Cleavage

Isolecithal
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Mesolecithal
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Very little yolk, evenly distibuted
Use Holoblastic cleavage- full cleavage
Moderate yolk
Use Holoblastic - full cleavage
Telolecithal


Have an abundance of yolk
Use Meroblastic cleavage - partial cleavage
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Development of Sea Urchin
8-21
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An Overview of Development Following Cleavage

Blastulation
 Cleavage creates a cluster of cells called the
blastula



Blastula stage typically consists of a few hundred to
several thousand cells
During blastula stage, first germ layer forms
In most animals

Cells are arranged around a fluid-filled cavity called the
blastocoel (blas-to-seal)
8-22
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An Overview of Development Following Cleavage

Gastrulation and Formation of Two Germ Layers
 Gastrulation
 Results in the formation of a second germ
layer
 Involves an invagination of one side of
blastula

Forms a new internal cavity
 gastrocoel
 Opening into the cavity: Blastopore
 Gastrula
has an outer layer of ectoderm
and an inner layer of endoderm
8-23
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Generalized Development showing germ layers
Incomplete/
Blind Gut
Blastopore
(Opening)
8-24
Complete
Gut
Gastrocoel
(Cavity)
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An Overview of Development Following Cleavage
 The
only opening into embryonic gut is
the blastopore

Blind or incomplete gut
 Some
animals retain the blind gut - the
opening does not fully extend to other
side (flatworms, sea anemones)
 Most develop a complete gut - in which
the opening extends and produces a
second opening, the anus
8-25
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Generalized Development showing germ layers
Incomplete/
Blind Gut
8-26
Complete
Gut
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An Overview of Development Following Cleavage

Formation of Mesoderm
 Animals with two germ layers
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
Most animals add a 3rd germ layer
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Diploblastic (Endoderm and Ectoderm)
Triploblastic
Mesoderm
3rd germ layer
 Forms between the endoderm and the
ectoderm
 Mesoderm arises from endoderm

8-27
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Developmental Characteristics
Germ Layer Outcomes:

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
Ectoderm
 Epithelium and nervous system
Endoderm
 Epithelial lining of the digestive and respiratory
tract, liver, pancreas,
Mesoderm
 Muscular system, reproductive system, bone,
kidneys, blood
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Germ Layer Outcome in mammals
8-29
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An Overview of Development Following Cleavage

Formation of the Coelom (see-lom)
 Coelom
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
The method by which the coelom forms is
an inherited character
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8-30
Important in grouping organisms based on
developmental characters
Upon completion of coelom formation
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
Body cavity surrounded by mesoderm
Body has 3 tissue layers and 2 cavities
Animals Without a Coelom are called
Acoelomates (Ex. flatworms)
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Developmental Characteristics


8-32
Two major groups of triploblastic animals
(animals with 3 germ layers)
 Protostomes and deuterostomes
The groups are identified by four
developmental characters
 Cleavage Patterns (radial or spiral)
 Fate of Blastopore (mouth or anus)
 Coelom Formation (split mesoderm or
outpocketing mesoderm)
 Embryo Type (Regulative or Mosaic)
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8-33
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Protostomes and Deuterostomes Blastopore Fate

Fate of Blastopore
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Deuterostome embryos
Develop a complete gut
 Blastopore becomes the anus
 Second opening becomes the mouth


Protosome embryos
Blastopore becomes the mouth
 Anus forms from a second opening

8-34
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Coelom Formation - mesoderm movement
Enterocoely
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
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8-35
Mesoderm sides push
outward and expand into
a pouch-like coelomic
compartment
Pouch-like compartment
pinches off and forms a
mesoderm bound space
surrounding the gut
Occurs in
Deuterostomes ( Sea
stars, fish, frogs, etc.)
Schizocoely
 Coelom forms from
 Endodermal cells move
to blastopore and
develop into mesoderm
 Mesoderm seperates or
splits to form cavity
(coelom)
 Occurs in Protostome
(Earthworms, snails)
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Blastula and Gastrula
Of Embryos
8-37
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Vertebrate Development

The Common Vertebrate Heritage
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8-38
All vertebrate embryos share chordate
hallmarks
 Dorsal neural tube
 Notochord
 Pharyngeal gill pouches with aortic
arches
 Ventral heart
 Postanal tail
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8-39
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Vertebrate Development

Amniotes and the Amniotic Egg
Reptiles, birds, and mammals
 Embryos develop within the amnion
 Fluid-filled sac that encloses the embryo

Provides an aqueous environment in which
the embryo floats
 Protection from mechanical shock

 Amniotic
egg contains 4 extraembryonic
membranes including the amnion
8-40
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Vertebrate Development

In the shelled amniotic egg:
Yolk sac
 Stores yolk
 Allantois
 Storage of metabolic wastes during
development
 Respiratory surface for gas exchange

8-41
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Vertebrate Development

8-42
Chorion
 Lies beneath the eggshell
 Encloses the embryo and other
extraembryonic membrane
 As embryo grows
 Need for oxygen increases
 Allantois and chorion fuse to form a
respiratory surface, the chorioallantoic
membrane
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Chick Embryo
8-43
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A. Fish Larvae - 1 day old, has large yolk sac
B. 10 day old fish larva, developed mouth, yolk sac smaller
8-44
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Vertebrate Development

The Mammalian Placenta and Early
Mammalian Development

Most mammalian embryos do not develop
within an egg shell
 Develop within the mother’s body
 Most retained in the mother’s body
 Monotremes
 Primitive
mammals that lay eggs
 Large yolky eggs resembling bird eggs
 Duck-billed platypus and spiny anteater
8-45
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Vertebrate Development

Marsupials
Embryos born at an early stage of
development and
 Continue development in abdominal pouch
of mother


Placental Mammals
Represent 94% of the class Mammalia
 Evolution of the placenta
 Required reconstruction of
extraembryonic membranes
 Modification of oviduct
 Expanded region formed a uterus

8-46
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Extraembryonic membranes of a mammal
8-47
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Vertebrate Development

Early Stages of Mammalian Development (Human)
Germinal Period (1st two weeks)

Blastocyst transported by oviduct to the uterus
 Propelled by ciliary action
Around 6th day
 Blastocyst = 100 cells
 Contacts uterus

By the twelfth day
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8-48
Implantation is complete
Embryo surrounded by pool of maternal blood
Chorion thickens, sends out tiny fingerlike projections
 Chorionic villi
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Early Development of the human embryo
8-49
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Vertebrate Development

Amnion

Remains unchanged

Surrounds embryo
Secretes fluid in which embryo floats

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Yolk sac

Contains no yolk
Source of stem cells that give rise to blood and
lymphoid cells

Stem cells migrate to into the developing embryo

8-50
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Vertebrate Development

Allantois
Not needed to store wastes
 Contributes to the formation of the
umbilical cord


Chorion

8-51
Forms most of the placenta
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Human embryo showing somites - They will
differentiate into skeletal muscle and the axial skeleton
8-52