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
-catenin
GSK-3
molecular events
during
rearrangement
Figure 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 blastodisc
Figure 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 blastocyst
Figure 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 development
Figure 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
vegetal cells form 1˚ mesenchyme
sea urchin involution
Figure 20.8
vegetal pole flattens
prospective ectoderm, endoderm
& mesoderm are formed
involution of a tube of cells
primitive
gut
(archenteron)
is
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
gastrulation
Figure 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 gastrulation
Figure 20.13
early
mammalian
gastrulation
Figure 20.14
Neurulation
• organogenesis
– formation of organs and organ systems
– caused by inductive interactions among
germ layers
frog neurulation
Figure 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 clusters
Figure 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
extraembryonic
membranes
Figure 20.18
shell lining
embryo
compartment
pantry
waste storage
placenta: chorion
+
uterine tissues
Figure 20.19