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Animal Embryonic Development
From Fertilization to Organogenesis
Early Stages of Development
•Fertilization•Cleavage•Gastrulation•Neurulation
Figure 20.1
Fertilization•unequal gamete contributions–egg contributes•nutrients•proteins, mRNAs•mitochondria•essential developmental genes (imprinted)
–sperm contributes•centriole•tubulin•essential developmental genes (imprinted)
Fertilization•rearrangements of egg cytoplasm–egg contents are distributed heterogeneously
–frog model system•animal hemisphere–contains nucleus–heavily pigmented cortical cytoplasm–lightly pigmented inner cytoplasm
•vegetal hemisphere–contains nutrients–unpigmented
formation of the gray crescent
Figure 20.2
Fertilization•rearrangements of egg cytoplasm–imposes bilateral symmetry on egg•site of sperm entry–head (anterior) end–ventral region
•gray crescent–tail end–dorsal region
•(hence) left-right axis
GSK-3
-catenin
molecular events during
rearrangementFigure 20.3
Cleavage - blastulation•rapid cell divisions
–divisions oriented in specific directions
•little gene expression•little cell growth•packaging of cytoplasmic heterogeneity
•final product is a hollow ball of cells = blastula–cells = blastomeres–hollow cavity = blastocoel
Cleavage - blastulation•yolky eggs alter pattern of
divisions–animal hemisphere divides normally
–vegetal (yolky) hemisphere•divides less often•produces larger cells
yolk affects the cleavage pattern
Figure 20.4
Cleavage - blastulation•amount of yolk affects cleavage
pattern–if yolk is divided into cells•complete cleavage
–if yolk is not divided•incomplete cleavage•embryo is a blastodisc atop the intact yolk
formation of blastodiscFigure 20.4
Mammalian Cleavage
•in oviduct•slow cell divisions•asynchronous cell divisions
mammalian cleavage
is rotational
Figure 20.5
Mammalian Cleavage
•in oviduct•slow cell divisions•asynchronous cell divisions•accompanied by gene expression•produces a blastocyst–inner cell mass - primordial embryo
–trophoblast - primordial placenta component
formation of mammalian blastocystFigure 20.5
frog blastula fate map
Figure 20.6
Fate Maps•undifferentiated cells of blastula have distinct fates–determination fixes fates of blastomeres•early determination yields mosaic development–a lost blastomere causes a lost body part
•later determination yields regulative development–a lost blastomere is compensated during development
humans exhibit regulative developmentFigure 20.7
Gastrulation-organizing the body plan•undifferentiated cells produce germ
layers–ectoderm - prospective epidermis, nervous system
–endoderm - prospective gut tissues–mesoderm - prospective organs, etc.
•germ layers migrate to new positions•contact between layers allows inductive interactions to direct differentiation
sea urchin involutionFigure 20.8
vegetal pole flattens
vegetal cells form 1˚ mesenchyme
involution of a tube of cells
primitive gut
(archenteron) is formed prospective ectoderm, endoderm
& mesoderm are formed
Gastrulation-organizing the body plan•blastopore becomes mouth or
anus–mouth in protostomes–anus in deuterostomes
Gastrulation-organizing the body plan•frog model system
–gastrulation begins at gray crescent
–“bottle cells” bulge into blastocoel & pull neighbors along
–initial involution forms the dorsal lip of the blastopore (d.l.b.)
–epiboly •surface cell layers migrate to blastopore•migrating cells form endoderm, mesoderm
frog gastrulati
onFigure 20.9
Figure 20.12
gray crescent
is necessary
for normal
development
Figure 20.10
Gastrulation-organizing the body plan•frog model system
–ß-catenin activates genes to produce proteins that cause bottle cells to initiate involution
–cells of the gastrula are determined during migration over the d.l.b.•dlb is necessary for normal development•dlb is sufficient for normal development
role of dlb in development in frog
Figure 20.11
Gastrulation-organizing the body plan•reptile/bird model
–two-layered blastodisc + large yolk mass•upper layer–epiblast–becomes embryo
•lower layer–hypoblast–becomes extra-embryonic membranes
chick gastrulationFigure 20.13
earlymammalia
n gastrula
tionFigure 20.14
Neurulation•organogenesis –formation of organs and organ systems
–caused by inductive interactions among germ layers
frog neurulationFigure 20.15
Neurulation•vertebrate body segmentation–alongside neural tube•segments of mesoderm = somites•somites direct development of vertebrae, ribs, trunk muscles, limbs, outgrowth of nerves, blood vessels, etc•repeated segments are modified along the anterior/posterior axis
somites contribute to
vertebrae, ribs & muscles
neural crest cells give rise to peripheral
nerves
Figure 20.16
HOX genes control anterior-posterior differentiation
•families of ~10 HOX genes are on different chromosomes
•HOX genes are expressed “in order”
•HOX genes guide differentiation from anterior to posterior
mouse HOX gene clustersFigure 20.17
vertebrate extraembryonic membranes
•reptiles, birds and mammals produce membranes that–surround the embryo–originate in the embryo–are not part of the embryo–provide nutrition, gas exchange and waste removal
chick extraembry
onic membranesFigure 20.18
shell lining
embryo compartmen
t
waste storage
pantry
placenta: chorion+
uterine tissuesFigure 20.19