Early Development Review
• Zygote = 1 celled stage
• Cleavage=early cell division (no cell growth)
• Morula = solid multicelled embryo
• Blastula = “hollow” multicelled embryo
• Blastocoel = space/cavity in blastula
• Gastrulation - formation of germ layers
• Gastrula - embryo gastrulating
• Germ Layers - ecto-, endo-, & mesoderm
Early Development Review
• Neurulation - formation of nervous
system from ectoderm
• Neurula - embryo neurulating
Cleavage and Blastula
Zygote & Cleavage
• Yolk (lecithin) = food storage
• Animal pole = less yolky end of zygote
• Vegetal pole = yolky end of zygote
• Holoblastic cleavage = zygote divides
completely (plesiomorphic for vertebrates)
• Meroblastic cleavage = yolky region
does not divide (apomorphic, evolved at least 3
times in vertebrates)
Cleavage
Blastula
• Cavity is flat and “above”
yolk in species with
meroblastic cleavage.
• Blastodisc = small cellular
area “on” large yolk
• In most therian mammals
a blastocyst forms.
Blastocyst = a blastula
containing a distinct inner
cells mass (stem cells)
ds
Amniota
meroblastic
cleavage
&
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s
cleavage
C
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Ac dric
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Craniate Phylogeny
meroblastic
cleavage
Class Actinopterygii
meroblastic
cleavage
single, dorsal
lung
*
holoblastic
cleavage
*blastocyst
(with inner
cell mass)
Gastrulation
• Hollow blastula “pushes” in.
• Cells invaginate (pouch in), involute
(fold in),
or ingress (move in as individual
to form a multi-layered embryo.
• Forms germ layers.
• Method of gastrulation varies among
vertebrates.
• Amount of yolk affects gastrulation.
cells)
Lancelet Gastrulation
Frog Gastrulation
Similar in lampreys, nonteleost actinopterygiians,
lungfishes, & other
amphibians
Amniote Gastrulation
• Primitive Streak = elongate region of
ingressing cells in an amniote
gastrula
• Primitive streak gastrulation modified
gastrulation for a large volume of yolk.
Small disk (blastodisc) of cells on top
of a large yolk.
• Therian Mammals = little yolk but still
retain primitive streak gastrulation.
Chicken Blastula
animal
pole
Primitive Streak Gastrulation
Chicken Gastrulation
Primitive
Streak
Posterior
Chicken Gastrulation
Node
Primitive
Streak
Posterior
Chicken Gastrulation
Node
Primitive
Streak
Posterior
yx
i
n
la i (h
a
m
pr gfis
ey he
s)
s
C
ho
n
Ac dric
tin ht
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gi
ac
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an
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fis s
am he
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M
ns
am
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M
Chicken Gastrulation
Node
Primitive
Streak
Posterior
Fate Maps
Craniate Phylogeny
primitive
streak
gastrulation
Neurulation
• Mesodermal notochord signals overlying
ectodermal epithelium to form hollow
neural tube.
• Neural tube J central nervous system.
• Primary neurulation = ectodermal
epithelium infolds anteriorly.
• Secondary neurulation = occurs in the
post-anal tail of (all) vertebrates;
mesenchymal mesoderm cells form a
rod-shaped mass that cavitates.
Neurulation
Primary Neurulation
Primitive Streak Gastrulation
Amniote Gastrulation
Node
Primitive
Streak
Posterior
Amniote Neurulation
Node
Primitive
Streak
Posterior
Neural
Fold
Amniote Neurulation
Node
Primitive
Streak
Neural
Tube
(brain)
Neural
Fold
Posterior
Vertebrate Ectoderm
• Generalized Ectoderm = covers embryo.
• Neurectoderm = cells that fold in to form
neural tube (brain, spinal cord, retina, some
nerves).
• Neural Crest = migratory ectodermal
tissue; forms in neurulation (pigment cells,
“face” skeleton, ganglionic nerves, meninges, &
chromaffin cells of adrenal glands).
• Ectodermal Placodes = ectodermal
epithelium thickens to form most of the
sense organs (and some head nerve ganglia).
Neural Crest Migration
M
yx
i
n
la i (h
a
m
pr gfis
ey he
s)
s
C
ho
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Ac dric
tin ht
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ry
el
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ac
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an
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th
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fis s
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M
ns
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m
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R
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Craniate Phylogeny
secondary neurulation posteriorly
neural crest; ectodermal placodes
Vertebrate Mesoderm
• Chordamesoderm = notochord.
• Somites (Paraxial) = pouched mesoderm
segments formed lateral to notochord.
dermatome, sclerotome, & myotome
• Intermediate Mesoderm = lateral &
posterior pouched mesodermal segments.
• Lateral Plate Mesoderm
= ventral
unsegmented mesoderm; around coelom.
splanchnic – deep; along endoderm
somatic – superficial; along body wall
Vertebrate Mesoderm
Somitomeres
Somites
Lateral Plate Mesoderm
Intermediate
Mesoderm
Cephalochordate Mesoderm
Formation During Neurulation
Vertebrate Mesoderm
Somitomeres & Somites
• Somitomeres = small, less defined
head somites.
Vertebrate Neurula (section)
somite
notochord
coelom
gut
intermediate
mesoderm
lateral
plate mesoderm
Vertebrate Mesoderm
Vertebrate Neurula (section)
somite
notochord
coelom
gut
intermediate
mesoderm
lateral
plate mesoderm
Vertebrate Embryo (section)
neural
crest
cells
sclerotome
dermatome
myotome
splanchnic
mesoderm
gut
intermediate
mesoderm
coelom
lateral
plate mesoderm
somatic
mesoderm
Vertebrate Embryo (section)
neural
crest
cells
sclerotome
dermatome
myotome
gut
epidermal
ectoderm
coelom
Vertebrate Embryo (section)
pigment cell
precursors
sclerotome
(neural crest)
ganglionic
neuron
precursors
dermatome
myotome
developing
skin
epidermis
neural crest
dermis
(neural crest)
gut
coelom
somatic
(lateral plate)
mesoderm
Vertebrate Embryo (section)
sclerotome
horizontal
septum
dermatome
myotome
developing
skin
gut
epidermis
neural crest
dermis
coelom
somatic
(lateral plate)
mesoderm
Vertebrate Embryo (section)
vertebra
horizontal
septum
dermatome
myotome
developing
skin
gut
epidermis
neural crest
dermis
coelom
somatic
(lateral plate)
mesoderm
Vertebrate Embryo (section)
vertebra
horizontal
septum
dermatome
myotome
developing
skin
epidermis
neural crest
dermis
gut
coelom
somatic
(lateral plate)
mesoderm
Salmon Section Diagram
Lamprey Section
Human Thorax Section
Mesentaries & “Membranes”
• Septum = a “wall”-like division
• Membrane = relatively thin sheet of
tissue
• Mesentary = a relatively thin twolayered sheet of mesothelium with
connective tissue between within the
coelomic cavity. (General usage)
• Mesothelium = simple squamous
coelomic epithelium derived from lateral
plate mesoderm.
Visceral Mesenteries
• Dorsal Mesentery – (splancnic mesoderm)
supports digestive tract (and blood
vessel pathway) from dorsal body wall.
[greater omentum/mesogaster, mesentery
(proper), mesocolon]
• Ventral Mesentery – (splancnic mesoderm)
supports digestive tract from ventral
body wall. – developmentally lost along
most of body except in region of liver
[lesser omentum (hepatogastric & hepatoduodenal
“ligaments”) and coronal & falciform “ligaments”]
Vertebrate Neurula (section)
somite
notochord
gut
intermediate
mesoderm
lateral
plate mesoderm
Vertebrate Embryo (section)
splanchnic
mesoderm
gut
somatic
mesoderm
lateral
plate mesoderm
Vertebrate Embryo (section)
splanchnic
mesoderm
gut
somatic
mesoderm
lateral
plate mesoderm
Vertebrate Embryo (section)
coelom
dorsal &
ventral
mesenteries
(splanchnic
lat. pl. mesoderm)
gut
somatic
lateral
plate
mesoderm
splanchnic
lat. pl. mesoderm
Vertebrate Embryo (section)
coelom
dorsal &
ventral
mesenteries
gut
somatic
lateral
plate
mesoderm
splanchnic
lat. pl. mesoderm
(splanchnic
lat. pl. mesoderm)
Vertebrate Embryo (section)
coelom
dorsal
mesentery
(splanchnic
lat. pl. mesoderm)
gut
somatic
lateral
plate
mesoderm
splanchnic
lat. pl. mesoderm
Visceral Mesenteries
Extraembryonic Membranes
• Extraembryonic membranes =
membranes that do NOT contribute to
the adult body. Shed at birth/hatching.
• Chorion in non-amniotes = acellular
layer covering developing embryo.
• Yolk sac in non-amniotes =
membrane continuous with midgut,
surrounds yolk mass; composed of
endoderm, mesoderm, and ectoderm.
ch
or
io
n
Non-Amniote Membranes
Yolk
Sac
Yolk
endoderm
splanchnic
mesoderm
Amniotes
• 4 extraembryonic membranes.
• Yolk sac = endoderm and splanchnic
mesoderm; contains yolk
• Allantois = endoderm and splanchnic
mesoderm; extension of posterior gut
• Chorion = ectoderm and somatic
mesoderm; surrounds “everything”
• Amnion = ectoderm and somatic
mesoderm; surrounds embryo
Reptile Development
embryo
somatic
mesoderm
coelom
endoderm
splanchnic
mesoderm
ectoderm
Yolk
Reptile Development
somatic
mesoderm
allantois
splanchnic
mesoderm
coelom
Reptile Development
amnion
wa
st
e
yolk sac
O2
CO2
nutrients
chorion
allantois
Human Development
• Humans exhibit typical eutherian
mammal development.
• Holoblastic cleavage leads to blastocyst
• Implantation of blastocyst in uterus
using a special tissue called trophoblast
• Early formation of amnion.
• Primitive streak gastrulation
• Primary neurulation with secondary
neurulation in post-anal tail
Human Cleavage
Human Early Development
Human Blastocyst/Implantation
Human Blastocyst/Implantation
Human Gastrulation
Human Neurulation
Human Development
Human Extraembryonic Memb.
=
Human Extraembryonic Memb.
Human Extraembryonic Memb.
Mesoderm Differentiation
• Chordamesoderm – notochord
• Lateral Plate Mesoderm
somatic mesoderm
splanchnic mesoderm
• Intermediate Mesoderm
• Somites & Somitomeres (Paraxial)
dermatome
sclerotome
myotome
Ectoderm Differentiation
• Generalized Ectoderm
• Neurectoderm (neural tube)
• Neural Crest
• Epidermal Placodes
Endoderm