Transcript Chapter 47: Animal Development

Chapter 47: Animal Development
Theories of embryonic development
Preformation
Belief
that egg or sperm contain a miniature
embryo/adult form
Epigenesis
Embryo
develops gradually from an egg
Developmental plan influenced by
Zygote
genome
Maternal mRNA
Cytoplasmic determinants
Cell
signaling influences gene expression
Fertilization
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Acrosomal Reaction
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Activates the egg
brings nuclei of egg & sperm together begins
embryo metabolic reactions
Acrosome at sperm tip releases hydrolytic enzymes
which digest egg jelly coat/vitelline membrane
Ion channels open in egg membraneNa+ flows in
depolarizing membrane prevents other sperm
from fusing with egg (fast block polyspermy)
Cortical Reaction
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Fusion of egg & sperm stimulates a series of
changes in egg cortex
Formation of fertilization envelope to function as a
slow-block to polyspermy
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Activation of the egg
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Sharp rise in Ca2+ in egg causes:
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Increased cell respiration & protein synthesis
Sperm & egg nuclei to fuse
DNA replication & first cell division
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90 minutes in sea urchins & frogs
12-36 hours in mammals
Can be stimulated to occur without sperm
Fertilization in mammals
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Fertilization is internal (fallopian tubes)
Secretions of female tract enhance sperm motility
Sperm migrates into zona pellucida (3D matrix of egg)
Sperm binds to & depolarizes egg to prevent polyspermy
Diploid nuclei form after 1st cell division (12-36 hours after
sperm bind)
Cleavage
 Partitions the zygote into smaller cells
 Rapid succession of mitotic cell divisions without G1 & G2
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First 5-7 divisions form cluster of cells= morula
Fluid cavity blastocoel forms in morula
Hollow ball of cells with fluid within= blastula
Polarity & planes
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Vegetal pole
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Animal pole
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low concentration of yolk
forms the anterior end of embryo
Gray crescent
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high yolk concentration
forms at posterior end of embryo
near equator
Types of cleavage
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Meroblastic- incomplete division; found in yolk-rich eggs
Holoblastic- complete division; found in eggs with little or moderate
amounts of yolk
Gastrulation
 Rearranges the cells in the blastula
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Invagination distributes the cells into layers
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forms a 3 layered embryo with a primitive gut (archenteron)
open end of archenteron= blastopore (future anus)
Germ layers
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Ectoderm
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Endoderm
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outermost layer
forms nervous system, skin, hair, nails, eye lens
inner layer
gives rise to digestive lining, pancreas, liver, thyroid, lungs, bladder
Mesoderm
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middle layer
gives rise to muscles, skeleton, gonads, excretory & circulatory systems
3 layer embryo= gastrula
Organogenesis
 Formation of organs from germ layers
 Localized morphological changes in tissue & cell shape
 Neural tube & notochord first to appear & elongate in chordate embryo
 Blocks of somites from which vertebrae & muscles will form appear along
the notochord
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Amniotes
 Embryos develop in a fluid-filled sac within a shell or
uterus
 Formation of extraembryonic membranes (life support
system for embryo); occurs simultaneously with
gastrulation
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Amnion
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Chorion
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gas exchange
Allantois
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fluid protecting the embryo from temperature & movement shocks
gas exchange & metabolic waste storage
incorporated into umbilical cord
Yolk sac
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blood vessels
carry food to the embryo
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Avian development
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Yolk contains food supply
Blastodisc= embryo forming portion of egg on
upper surface
Primitive streak- longitudinally thickens along
blastodisc
Extraembryonic membranes
Mammalian development
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Fertilization & cleavage occurs in oviducts
Blastocyst reaches uterus & implants at day 7
Extra-embryonic membranes
Organogenesis begins with formation of neural
tube, notochord, & somites
Morphogenesis
 Involves specific changes in cell shape,
position, & adhesion; reorganization of the
cytoskeleton
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Morphogenetic movements partially guided by
extra-cellular matrix
Cell Adhesion Molecules (CAM) contribute to
selective association of cells with one another
Cytoplasmic determinants help establish body axes
& differences among cells of the early embryo
Inductive signals drive differentiation & pattern
formation in vertebrates
The “organizer” influences & induces the development of
other cells
 Inducers turn some genes on & other off
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