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Morphogenesis process by which an animal takes shape
determined by genome and differences between embryonic cells
The Acrosomal Reaction
Triggered when sperm reaches egg
releases hydrolytic enzymes that digest material surrounding the egg
Sperm-bindingreceptors
Jelly coat
Acrosome
Actin
Spermhead
Basal body(centriole)
Sperm plasmamembrane
SpermnucleusContact
Acrosomalreaction
Acrosomalprocess
Contact and fusionof sperm and eggmembranes Entry of sperm
nucleus
Cortical reaction
Fertilizationenvelope
Egg plasmamembrane
Vitelline layer
Hydrolytic enzymes
Corticalgranule
Fused plasmamembranes
Perivitellinespace
Cortical granulemembrane
EGG CYTOPLASM
The Cortical Reaction
Initiated by the fusion of egg and sperminduces a rise in Ca2+
stimulates cortical granules to release their contents outside the egg
cause formation of a fertilization envelope block to polyspermy
1 sec beforefertilization
Point ofspermentry
10 sec afterfertilization
Spreading waveof calcium ions
20 sec 30 sec
500 µm
Activation of the Egg
sharp rise in Ca2+ increases the rates of cellular respiration and
protein synthesis by the egg cell = activation of egg cell
Binding of sperm to egg
Acrosomal reaction: plasma membranedepolarization (fast block to polyspermy)
Increased intracellular calcium level
Cortical reaction begins (slow block to polyspermy)
Formation of fertilization envelope complete
Increased intracellular pH
Fusion of egg and sperm nuclei complete
Increased protein synthesis
Onset of DNA synthesis
First cell division
1
Se
co
nd
s
2
3
68
10
4
20
30
501
2
40
34
10
5
20
3040
9060
Min
ute
s
Fertilization in Mammals
the cortical reaction modifies the zona pellucida as a slow block to polyspermy
Folliclecell
Acrosomalvesicle
Egg plasmamembrane
Zonapellucida Sperm
nucleus
Corticalganules
Spermbasalbody
EGG CYTOPLASM
Cleavage
a period of rapid cell division without growth
partitions the cytoplasm of one large cell into many smaller cells blastomeres
Fertilized egg Four-cell stage Morula Blastula
Polarityeggs and zygotes of many animals (except
mammals)Animal poleVegetal pole
Most of yolk
Anterior
Right
Animal pole
Graycrescent
DorsalVentral
Left
Posterior
Body axes Establishing the axes
Futuredorsalside oftadpole
Point ofspermentry
Firstcleavage
Vegetalhemisphere Vegetal pole
Point of sperm entry
Animalhemisphere
Cleavage planes follow a pattern relative to the zygote’s animal and vegetal
poles
Zygote
2-cellstageforming
8-cellstage
4-cellstageforming
Animal pole Blasto-coel
Blastula(crosssection)
Vegetal poleBlastula (at least 128 cells)
0.25 mm
Eight-cell stage (viewedfrom the animal pole)
0.25 mm
Meroblastic cleavageincomplete division of the eggoccurs in species with yolk-rich eggs
Ie. reptiles and birds
Blastocoel
Fertilized egg
BLASTODERM
HypoblastEpiblastYOLK MASS
Cutaway view ofthe blastoderm
Blastoderm
Four-cell stage
Zygote
Disk ofcytoplasm
Holoblastic cleavagecomplete division of the eggoccurs in species whose eggs have little or
moderate amounts of yolk Ie. sea urchins and frogs
Gastrulationrearranges the cells of a blastula into a three-
layered embryoa gastrula
has a primitive gut
The three layers produced by gastrulation are called embryonic germ layersThe ectoderm forms the outer layer The endoderm lines the digestive tractThe mesoderm partly fills the space
between the endoderm and ectoderm
Video: Sea Urchin Embryonic Development
Animalpole
Blastopore
Filopodiapullingarchenterontip
Archenteron
Mesenchymecells
Blastocoel
Future ectoderm
Vegetalpole
Key
Future mesoderm
Future endoderm
Vegetalplate
Blastocoel
Mesenchymecells
Archenteron
Blastocoel
Mesenchume(mesodermforms futureskeleton)
50 µm
Mouth
Ectoderm
Blastopore
Digestive tube (endoderm)
Anus (from blastopore)
Future ectoderm
Key
Future mesoderm
Future endoderm
Archenteron
Blastocoelremnant
Ectoderm
MesodermEndoderm
Yolk plugYolk plugGastrula
Blastocoelshrinking
Blastocoel
Dorsal tip of blastopore
CROSS SECTION
Animal pole
Dorsal lipof blastopore
Vegetal pole Blastula
SURFACE VIEW
Organogenesisregions of the germ layers develop into rudimentary organs
the notochord forms from mesodermthe neural plate forms from ectoderm
curves inward, forming the neural tube
Video: Frog Embryo Development
Neural folds
Neuralplate
LM1 mm
Neuralfold
Notochord
Archenteron
Neural plate formation
Endoderm
Mesoderm
Ectoderm
Neuralfold
Neural plate
Neural tube
Formation of the neural tube
Neural crest
Outer layerof ectoderm
Neural crest
SomitesblocksFormed from mesoderm lateral to the notochord
CoelomLateral to the somitesmesoderm splits to form the coelom
1 mm
Notochord
Archenteron(digestive cavity)
Neural tube
Neural crest
Eye Somites Tail bud
SEM
Coelom Somite
Somites
Notochord
ArchenteronEndoderm
Mesoderm
Ectoderm
Neural tube
Eye
Coelom
Somite
Somites
Neural tube
Lateral fold
Yolk stalkYOLK
Form extraembryonicmembranes
Yolk sac
Early organogenesis
Forebrain
Heart
Bloodvessels
Late organogenesis
Adaptations of Amniotesdevelop in a fluid-filled sac in a shell or the
uterusBirdsReptilesmammals
Germ layers give rise to membranes surrounding embryos
Embryo
Amnioticcavitywithamnioticfluid
AllantoisAmnion
Albumen
Yolk(nutrients)
Yolk sacChorion
Shell
Mammalian Development
eggs of placental mammalsSmallstore few nutrientsExhibit holoblastic cleavageShow no obvious polarity
Gastrulation and organogenesis similar to birds and reptiles
Early cleavage is relatively slow
Cleavage forms the blastocyst formstrophoblast, the outer epithelium of the blastocyst,
initiates implantation in the uterus, and the blastocyst forms a flat disk of cells
As implantation is completed, gastrulation begins
The extraembryonic membranes begin to form
By the end of gastrulation, the embryonic germ layers have formed
Blastocystreaches uterus.
Endometrium(uterine lining)
Maternalbloodvessel
Blastocystimplants.
Inner cell mass
Trophoblast
Blastocoel
Hypoblast
Trophoblast
Epiblast
Expandingregion oftrophoblast
Hypoblast
Chorion (fromtrophoblast
Epiblast
Amnioticcavity
Amnion
Yolk sac (fromhypoblast)
Extraembryonic mesoderm cells(from epiblast)
Extraembryonicmembranes startto form andgastrulationbegins.
Amnion
Chorion
Endoderm
Mesoderm
Ectoderm
Yolk sac
Extraembryonicmesoderm
Gastrulation has produced a three-layered embryo with fourextraembryonic membranes.
Allantois
Expandingregion oftrophoblast
Fate Maps
general territorial diagrams of embryonic development
Fate map of a frog embryo
Epidermis Centralnervoussystem
Blastula
Epidermis
Neural tube stage(transverse section)
Endoderm
Mesoderm
Notochord
DevelopmentDifferentiationSignal molecules
Influence gene
Anterior
Organizer regions
Limb bud
PosteriorZPA
AER
50 µm
Apicalectodermalridge
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