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Vertebrate Development
Chapter 51
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Material to be covered (for Tri-C)
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Describe the events of each of the three stages
of fertilization in an advanced vertebrate.
Compare the cleavage patterns, describe the
appearance of the blastula and indicate how
gastrulation proceeds in primitive chordates,
aquatic vertebrates and reptiles/birds/mammals.
State the tissues produced by the three germ
layers: endo-, meso- and ectoderm.
Understand the developmental processes that
occur during neuralation.
Explain Haeckel’s “biogenic law”, ontogeny
recapitulates phylogeny.
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Material to be covered (for Tri-C)
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Understand the importance of extra-embryonic
membranes in terrestrial vertebrate development.
Describe the characteristic events of each trimester of
human pregnancy and of postnatal development.
Use vertebrate models to understand embryonic
development.
Discussion and evaluation of bioethical issues related to
embryology: cloning, stem cells and in vitro fertilization.
Describe the ways that cells can signal each other.
Differentiate between intracellular receptors and cell
surfaces receptors.
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Outline
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Stages of Development
Cell Cleavage Patterns
Gastrulation
Developmental Process During Neurulation
How Cells Communicate During Development
Embryonic Development-Vertebrate Evolution
Extraembryonic Membranes
Human Trimesters
Birth and Postnatal Development
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Stages of Development
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Fertilization
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combination of gametes
Cleavage
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series of extremely rapid mitotic divisions
Gastrulation
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series of extensive cell rearrangement
Neuralation
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the process where tissue forms a neural tube
Organogenesis
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the process where cells interact with one another and rearrange
themselves to produce tissues and organs
Gametogenesis
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the development of gametes
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often not complete until the organism matures
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varies greatly within animal kingdom
Maturity
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Larvae – pupae – adult
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metamorphosis
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Fertilization
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Penetration
– glycoprotein-digesting enzymes in
acrosome of sperm head
Activation
– events initiated by sperm penetration
 chromosomes in egg nucleus complete
second meiotic division
 triggers movement of egg cytoplasm
 sharp increase in metabolic activity
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Stages of Development
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Nuclei fusion
– The third stage of fertilization is fusion of
the entering sperm nucleus with the
haploid egg nucleus to form the diploid
nucleus.
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Mammalian Reproductive Cells
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Vertebrate Development Review
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Formation of blastula
– water drawn into cell mass forming a hollow ball
of cells - blastula or blastocyst
Gastrulation
– some cells of blastula push inward, forming a
invaginated gastrula
 invagination creates main axis of vertebrate
body
 Has an animal pole and a vegetal pole
 Animal pole end forms external tissues
 Vegetal pole form internal tissues
 embryo now has three germ layers
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Vertebrate Development Review
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Neurulation
– zone of ectoderm thickens on dorsal surface
of embryo
– neural tissue rolls and forms neural tube
cell migration
– variety of cells migrate to form distant
tissues
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Vertebrate Development Review
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Organogenesis
– basic body plan established
– tissues develop into organs
– embryo will grow to be a hundred times
larger
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Vertebrate Development
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Vertebrate Development
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Cell Cleavage Patterns
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Initial cell division, cleavage, is not
accompanied by an increase in the overall
size of the embryo.
– morula - mass of 32 cells
 Each cell is a blastomere.
 eventually a blastula is formed
The pattern of cleavage is influenced by the
presence of yolk
Animal Pole – small amount of yolk
Vegetal pole – large amount of yolk
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Cell Cleavage Patterns
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Primitive chordates
– holoblastic cleavage - egg contains little or
no yolk, and cleavage occurs throughout
the whole egg
Amphibians and advanced fish
– Eggs contain much more cytoplasmic yolk
in one hemisphere than the other.
 large cells containing a lot of yolk at one
pole, and a concentrated mass of small
cells with very little yolk at the other pole.
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Holoblastic Cleavage
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Cell Cleavage Patterns
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Reptiles and birds
– eggs composed almost entirely of yolk
– cleavage only occurs in polar cytoplasm
 meroblastic cleavage
Mammals
– contain very little yolk
– holoblastic cleavage
– inner cell mass forms developing embryo
– outer sphere, trophoblast, enters
endometrium
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Meroblastic Cleavage
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Cell Cleavage Patterns
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Blastula
– Each cell is in contact with a different set
of neighboring cells.
 Interactions are a major factor
influencing developmental fate.
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Gastrulation
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Certain groups of cells invaginate and
involute from the surface of the blastula
during gastrulation.
– By the end of gastrulation, embryonic cells
have rearranged into three primary germ
layers:
 ectoderm
 mesoderm
 endoderm
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Gastrulation
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Gastrulation in primitive chordates
– surface of blastula invaginates into the
blastocoel
 eventually inward-moving wall pushes up
against the opposite side of the blastula
 produces embryo with two cell layers:
 outer ectoderm
 inner endoderm
 mesoderm forms later between the
ectoderm and endoderm
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Gastrulation in a Lancet
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Gastrulation
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Gastrulation in most aquatic vertebrates
– Yolk-laden cells of the vegetal pole are
fewer and much larger than the yolk-free
cells of the animal pole.
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Frog Gastrulation
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Gastrulation
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Gastrulation in reptiles, birds, mammals
– no yolk separates two sides of embryo
 lower cell layer differentiates into
endoderm and upper layer into ectoderm
without cell movement
 primitive streak
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Mammalian Gastrulation
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Developmental Processes During Neurulation
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Tissue differentiation begins with the
formation of the notochord and the hollow
dorsal nerve cord.
– neurulation
After the notochord has been laid down,
ectodermal cells above the notochord
invaginate, forming the neural groove down
the long axis of the embryo.
– edges move toward each other and fuse
creating neural tube
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Mammalian Neural Tube Formation
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Developmental Processes During Neurulation
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On either side of the developing notochord,
segmented blocks of mesoderm tissue
called somites form.
– Ultimately, somites give rise to muscles,
vertebrae, and connective tissues.
 Mesoderm in the head region remains
connected as somitomeres and form
striated muscles of the face, jaws, and
throat.
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Developmental Processes During Neurulation
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Neural crest
– Edges of neural groove pinch off and form
the neural crest.
 Nearby clusters of ectodermal cells
thicken into placodes.
Gill chamber
– Some of the neural crest cells form
cartilaginous bars between the embryonic
pharyngeal slits.
 forms efficient pump
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Developmental Processes During Neurulation
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Elaboration of the nervous system
– Some neural crest cells migrate ventrally
toward the notochord and form sensory
neurons in the dorsal root ganglia.
 others become specialized Schwann
cells
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How Cells Communicate During Development
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Inductions between the three primary tissue
types are referred to as primary inductions.
Inductions between tissues that have
already been differentiated are called
secondary inductions.
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How Cells Communicate During Development
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Nature of development decisions
– Some cells become determined early in
development.
– At some stage, every cell’s fate becomes
fixed (commitment).
 not irreversible, but rarely reverses
under normal conditions
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Embryonic Development - Vertebrate Evolution
• Ontogeny recapitulates phylogeny
– Embryological development (ontogeny)
involves the same progression of changes
that have occurred during evolution
(phylogeny).
– Homework (due by wednesday) find out if
this makes any sense. (turnitin.com)
 Do you agree or disagree with this
statement? Why?
 Does science agree with this
statement? Why?
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Vertebrate Embryonic Development
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Extraembryonic Membranes
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Fluid-filled amniotic membrane: an
adaptation to terrestrial life
– amniotic membrane an extraembryonic
membrane
 Extraembryonic membranes, later to
become fetal membranes, include the
amnion, chorion, yolk sac, and allantois.
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Extraembryonic Membranes
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First Trimester
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First trimester
– fourth week - organ development
 organogenesis
 most women not yet aware of
pregnancy
 Fetal Alcohol Syndrome
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First Trimester
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Second month - morphogenesis
– limbs assume adult shape
– major organs become evident
– embryo is about one inch in length
Third month - completion of development
– now referred to as fetus
 nervous system and sense organs
develop
 all major organs established
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Second and Third Trimesters
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Second trimester - growth
– bone formation occurs
– covered with fine hair (lanugo)
– by the end of the sixth month, baby is one
foot in length
Third trimester - pace of growth accelerates
– weight of fetus more than doubles
– most major nerve tracts formed within brain
– by end, fetus is able to survive on own
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Birth and Postnatal Development
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Uterus releases prostaglandins
– begin uterine contractions, but then
sensory feedback from the uterus
stimulates the release of oxytocin from the
mother’s pituitary gland
 rate of contraction increases to one
contraction every two or three minutes
 strong contractions, aided by the
mother’s pushing, expels the fetus
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Birth and Postnatal Development
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Nursing
– Milk production, lactation, occurs in the
alveoli of mammary glands when they are
stimulated by prolactin.
– milk secreted in alveolar ducts which are
surrounded by smooth muscle and lead to
the nipple
 first milk produced after birth called
colostrum - rich in maternal antibodies
 Milk synthesis begins about three days
following birth.
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Birth and Postnatal Development
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Postnatal development
– Babies typically double their birth weight
within a few months.
– Neuron production occurs for six months.
– allometric growth
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