Transcript Chapter 10- Amphibians

Chapter 11- Fish and mammals
• Zebrafish are becoming the sweetheart of developmental
biologists
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Large broods
Breed year-round
Easy and cheap
Transparent embryos
Develop outside mother
Early development complete in 24 hours
A. Cleavage
1st
12 divisions are
sychronous to form
blastoderm
Blastoderm is
perched on a
large yolk cell
6
1
Three cell populations
1. Enveloping
layer (EL)
2. Deep layergives rise to
embryo
proper
Fig. 11.1
3. Yolk syncytial
layer (YSL)
Fig. 11.2
B. Gastrulation
Epiboly
Recall Epiboy
from Ch 9
Deep cells migrate to
outside then encase
entire yolk
Movement not by crawling, but by YSL cells expansion and pulling EL
cells along
1. Enveloping
layer (EL)
Embryonic
shield
epiblast
hypoblast
2. Deep cells
Fig. 11.3
2. YSL cells
YSL
6 hrs post-fertilization
• A hypoblast is formed either by involution of superficial cells or by ingress
• These combine with superficial epiblast cells to form the embryonic shield
(function equivalent of the dorsal lip in amphibians)
B. Gastrulation (cont.)
Animal
Ventral
Head
The hypoblast cells extend in both
directions to form the notochord
precursor
Dorsal
Ectoderm
Trunk
Tail
Vegetal
Fig. 11.3
Mesoderm
Endoderm
Fig. 11.2 -A zebrafish fate map
C. Axis formation
1. Dorsal ventral axisAs with the amphibian dorsal lip (Organizer), the embryonic shield:
1.
Establishes the dorsal-ventral axis
Converts lateral/ventral medoderm to dorsal mesoderm (notochord)
Convert ectoderm to neural rather than epidermal
B-catenin
2. Forms the notochord precursor
3. Secretes proteins to inhibit
Fig. 11.6
BMP from inducing ectoderm
to become epidermis
BMP2
Chordino
•This inhibiting molecule is
called Chordino
Embryonic shield
• If mutate chordino, no
neural tube is formed
4. Acquires its function from B-catenin accumulation in nearby cells
•B-catenin accumulates in YSL cells
•Goosecoid is activated
samois
goosecoid
BMP
inhibitors
e.g.
Chordino
C. Axis formation (cont.)
2. Anterior-posterior axisIn amphibians
, the anterior-posterior axis is formed
during oogenesis
This axis is stabilized during gastrulation by two signaling
centers
Anterior neural inducing signal (from ectoderm cells)
Fig. 11.6
Posterior neural-inducing signal ( from mesoderm cells)
3. Left-right axis Not much known, but involves TGF-b family signaling molecules
Mammalian Development
Tough to study!!
• 100um diamater (1/1000th volume of frog egg!
• Few in number (<10/female)
• Develops within mother
• Cleavage events take 12-24 hours each
• Development occurs en route to uterus
3. Cleavage during migration down oviduct
4. Implant in
uterus
2. fertilization
1. Egg released from ovary
Fig. 11.20
Mammalian Development
A. Cleavage
Distinctions of mammalian cleavage
1. Slow- 12-24 hrs per cleavage
2. 2nd cleavage is rotational
3. Marked asychrony in early cell
division
4. Cleavage at 2nd division requires Amphibians Mammals
newly made proteins from zygote
Fig. 11.21-rotational
5. Compaction (marked cell
huddling) occurs at 8 cell stage
cleavage in mammals
compaction
Fig. 11.23- compaction at 8 cell stage (day 4 in humans)
A. Cleavage (cont.)
16 cell embryo is termed “morula”
•external cells will become trophoblast, which will
become the placenta
•Internal cells will become inner cell mass (ICM), or
the embryo proper
This marks 1st differentiation event in mammalian development
At 64 cell stage, an internal
cavity appears and the embryo
is termed a blastocyst, ready
for implantation onto uterus
wall
The Zona pellucida (recall ch. 7) must be shed in order to implant
• Blastocyst lyses a small hole in zona using the enzyme strypsin
Note- attachment of embryo to oviduct wall is called a tubal pregnancy
B. Gastrulation
Similar to reptiles and birds
•Mammalian embryo relies on mother for nutrients,
not yolk
•Thus, the embryo must have a specialized organ to
accept nutrients- called the chorion
•The chorion induces uterine cells to become a
decidua (rich in blood vessels)
Epiblasts form amnionic cavity
epiblasts
Hypoblasts (from ICM) line
the blastoceol- these give rise
to extraembryonic endoderm
hypoblasts
blastocoel
Fig. 11.28- Day 15 human embryo
B. Gastrulation (cont.)
Mammalian mesoderm and endoderm cells arise from epiblasts that
migrate through primitive streak
E-cadherin attachment is mechanism
Henson’s Node
Primitive streak
Direction of
migration
Fig. 11.28- Day 16 in human
Fig.11.11- Chick gastrulation- similar to mammalian
Those cells that migrate through the Henson’s node will become the
notochord
B. Gastrulation (cont.)
Extraembryonic membrane Formation
Trophoblast cells (originally termed “cytotrophoblast) gives rise
to multinucleated syncytiotrophoblasts
Uterine wall
These syncytiotrophoblasts:
• secrete proteolytic enzyme to invade
uterine wall
• Digest uterine tissue
• Mothers blood vessels contact the
syncytiotrophoblast cells
• Embryo produces its own blood vessels
Embryo’s blood vessels
Chorion
Villi
Embryo chorion
Mother’s Placenta
Mothers blood vessels
Fig. 11.27-Blastocyst
invading uterus
Blood vessels feed
embryo, but blood
cells do not mix
Fig. 11.31
C. Anterior-posterior axis formation
Two signaling centers
1. Anterior visceral endoderm (AVE)
2. Node (Organizer)
These work together
to form forebrain
Fig. 11.34 These are on opposite sides of a
“cup” structure
Node produces Chordin and Noggin
AVE produces Lim-1 and Otx-1
Knock-out of
one of these
results no
forebrain
C. Anterior-posterior axis formation
The Hox genes specify anterior-posterior polarity
These are homologous to homeotic gene complex (Hom-C) of drosophila
Recall that the Hom-C genes are arranged in the same
order as their expression pattern on anterior-posterior axis
Mammalian
counterparts
are clustered
on 4
chromosomes
Equivalent genes
(Hoxb-4 and
hoxd-4) are called
a paralogous
group
C. Anterior-posterior axis formation (cont.)
Fig. 11.36- Hox genes
are organized in a
linear sequences that
concurs with posterior
to anterior structures
This is referred to as
the hox code
Hox gene rules
1. Different sets of Hox genes
are required for specification
of any region of the anteriorposterior axis
Hoxa-2 KO- stapes missing, duplicate incus
Incus
Hoxa-3 KO- thymus, neck cartilage malformed
Stapes
2. Different members of a paralogous group may
specify different organ subsets in a given region
st
Example Hoxd-3 KO deformed atlas (1 vertebra)
Hoxa-3/Hoxd-3 double KO- atlas and neck
cartilage nearly absent
3. A hox gene KO causes defects in the anterior-most region of that gene’s
expression
Retinoic Acid has a profound effect on development
Recall amphibian
development (Ch. 10)
Structure of retinoic acid
(not in textbook)
Fig. 10.41
RA
Retinoic acid activates mammalian hox genes
Lacks all
distal
vertebra
Wild-type mouse
RA-treated
embryo
mouse embryo
Hox gene
Retinoic acid bind a receptor,
then the complex binds
promoter of a hox gene
Retinoic acid is likely
produced in the node, and
perhaps more time spent
in the node dictates more
posterior specification
D. Dorsal-ventral axis formation
Dorsal axis forms from ICM cells near
trophoblast
Inner cell mass
(ICM)
Trophoblast
Blastocoel
Ventral axis forms from ICM cells
near blastcoel
Fig. 11.32
Fig. 11.42
E. Left-right axis formation
Note that mammals are asymmetrical
Two levels of regulation1. Global- a inv gene defect results in all
organs on the wrong side
2. Organ-specific- an iv gene defect
causes the axis of an organ to change
Organs are located in specific locations