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Early Development Review Zygote = 1 celled stage Cleavage=early cell division (no cell growth)
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)
Craniate Phylogeny
Myx
ini (
hagf
ishes
)
lam
preys
Chondrichth
ys
Actin
optery
gii
coel
acan
ths
lungfis
hes
amphib
ians
Mam
mal
ia
Reptil
iam
ero
bla
stic
clea
vag
e
mero-blasticcleavage
mer
o. c
leav
age
Class Actinopterygii
meroblasticcleavage
sturg
eons
& pad
dlefis
h
gars bowfin
Amia
cal
va
TELEOSTS
reed
fishes
single, dorsallung
Amniota
montre
mes
mar
supia
ls
euth
eria
turtl
es
lizar
ds & s
nakes
croco
dilian
s
birds
meroblastic cleavage
holoblasticcleavage
*blastocyst (with inner cell mass)
*
Gastrulation Hollow blastula “pushes” in. Cells invaginate (pouch in), involute
(fold in), or ingress (move in as individual
cells) to form a multi-layered embryo. Forms germ layers. Method of gastrulation varies among
vertebrates. Amount of yolk affects gastrulation.
Lancelet Gastrulation
Frog Gastrulation
Similar in lampreys, non-teleost 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.
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)
notochordsomite
inter-mediatemeso-derm
lateralplate mesoderm
gut
Vertebrate Embryo (section)
splanchnicmesoderm
somaticmesoderm
gut
lateralplate mesoderm
Vertebrate Embryo (section)
gut
lateralplate mesoderm
splanchnicmesoderm
somaticmesoderm
Vertebrate Embryo (section)
gut
coelom
dorsal & ventralmesenteries(splanchnic lat. pl. mesoderm)
splanchniclat. pl. mesoderm
somaticlateralplate
mesoderm
Vertebrate Embryo (section)
gutdorsal & ventralmesenteries(splanchnic lat. pl. mesoderm)
coelom
splanchniclat. pl. mesoderm
somaticlateralplate
mesoderm
Vertebrate Embryo (section)
gutdorsalmesentery(splanchnic lat. pl. mesoderm)
coelom
splanchniclat. pl. mesoderm
somaticlateralplate
mesoderm
Visceral Mesenteries
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.
Extraembryonic Membranes
Non-Amniote Membranes
YolkYolkSac endoderm
splanchnicmesoderm
chor
ion
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
Amniotes
Reptile Development
coelom
endoderm
ecto-derm
somaticmesoderm
splanchnicmesoderm
embryo
Yolk
Reptile Development
coelom
allantois
somaticmesoderm
splanchnicmesoderm
Reptile Development
allantois
amnionyolk sac
chorion
nutrients
was
te
O2
CO2
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