Transcript Compaction

The Early Development of
Vertebrates:
Birds, and Mammals
Chapter 9
1
This Final Chapter on the Process
• Amniotes
– Those vertebrates whose embryos form an
amnion (water sac)
– Birds and reptiles form similar patterns of
development
– Vertebrate; birds, reptiles, and fish are by
Meroblastic Cleavage
– Mammals however are by Holoblastic
Cleavage
• Modified to make a placenta
2
Cleavage
Patterns
5.3 Summary of
the main patterns of
cleavage
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Early Mammalian
Development
4
Cleavage
• Cleavage in Mammals
– Difficult to study because
• Mammalian eggs are among the smallest in the
world
• Hard to experiment with (manipulate)
– Human zygote 100 nm in diameter
• 1/1000 size of frog embryo
• Not produced in large numbers
– Fertilization/development dependent on
taking place inside another living organism
5
Unique Nature of Mammalian
Cleavage
• Unique nature of mammalian cleavage
starts prior to fertilization
– Mammal egg wrapped in cumulus cells as it
is released from ovary
– Swept by fimbriae into oviduct
– Fertilization at this region or in ampulla
• Region closest to ovary
– Meiosis is completed after sperm entry
– 1st cleavage begins ≈ 1 day later
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Unique Nature of Mammalian
Cleavage
8.15 Development of a human embryo from fertilization to implantation
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Unique Nature of Mammalian
Cleavage
– Cleavage in mammals is among the slowest
of all in animal kingdom
• 12-24 hours apart
– Cilia in oviduct propel the embryo toward
the uterus
• It is during this time cleavage starts and
continues slowly
– Mammals have a unique orientation of
blastomeres in relation to one another
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Unique Nature of Mammalian
Cleavage
– 1st cleavage
• Normal
meridional
division
– 2nd cleavage
• Different
• One divides
meridional
• Other divides
equatorially
– Called Rotational
cleavage
8.16 Comparison of early cleavage in
(A) echinoderms and amphibians and
(B) mammals
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Unique Nature of Mammalian
Cleavage
– Mammalian blastomeres do not divide at
same time
• Asynchrony
• Do not increase exponentially
– (2..4..8..16...) normal synchronous
– Frequently odd numbers
– Mammalian genome is activated during early
cleavage
• As soon as nuclei is formed
• Not maternal cytoplasm
• Mouse, goat  switches from maternal to zygote
control at 2 cell stage
• Humans  switch at 4 to 8 cell stage
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Compaction
– Most studies focus on mouse
• Easily bred
• Large litters
• Compaction
– Mouse blastomeres through 8
cell stage form a loose
arrangement with plenty of
space between them
– Then cell adhesion molecule
• E-cadherin starts to be expressed
– Begin to group together into a
compact ball of cells
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Compaction
9.20 Cleavage of a single mouse embryo in vitro
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Compaction
• Begin to group together into a compact ball of
cells
– This is unique to mammalian cleavage
– Stabilized by tight junctions, sealing off the inside
of the sphere
• Inside cells form gap junctions – ions pass
• Morula
– 16 cell compacted from 8 cell compact group
– Small inner cells surrounded by large outer cells
(external cells)
• Early blastomeres (8-cell stage) can form both
trophoblast cells or embryo precursor cells
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Cleavage and Compaction
14
Compaction
• External cells become Trophoblast
– Also called trophectoderm
– They form chorion tissue – allows O2 and
nourishment (non-embryo forming cells)
– Secretes hormones (-HCG) for uterus to keep
embryo
• First divisions
– To make cell that stick to uterus
– Trophoectoderm is first differentiation in mammals
• Adhere to uterus
• Digest path into lining
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Compaction
• Embryo is derived from inner cells
of 16 cell stage and some divisions
off of the outer cells
– Migrating inside
– Called Inner Cell Mass (ICM) at 32
cell stage
– Give rise to embryo, allantois, yolk
sac, amnion
• 64 cell stage
– Trophoblast separate from inner cell
mass
– Do not contribute cells to the other
now
– Inner cell mass secretes proteins to
make trophoblast divide (Fgf4)
– These cells were totipotent ≡
capable of everything – from early
blastomere
– Inner cells are pluripotent ≡ capable
of many things
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Compaction
• Cells of these 2 regions express different genes
• ICM (Inner cell mass)
– Secrete signals to remain Pluripotency
– Cells divide to become embryonic stem cells
• Morula does not have internal cavity
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Compaction
• Cavitation
– Trophoblast secrete fluid into
morula to create blastocoel
– Has Na+/K+ and Na+/H+
pump
– Pump natural central cavity
– Osmotically drains in water
• As blastocoel expands
– Inner cell mass positions one
side of the trophoblast cell
ring
• Called Blastocyst
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Formation of the Blastocyst Cavity
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Cleavage and Compaction
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Developmental Potential of the Inner
and Outer Cell Masses
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Escape from the Zona Pellucida
• While it moves through oviduct to uterus
– Blastocyst expand within Zona pellucida
– Zona pellucida prevents cells from adhering
to oviduct walls
• If this happens in humans – it is called a ectopic
pregnancy
– Also called a “tubal” pregnancy
– Life-threatening !!!!
22
Escape from the Zona Pellucida
– Later embryo breaks through Zona pellucida
(ZP) and can now adhere to the uterine wall
•
•
•
•
Called Hatching from ZP
Digesting small hole and squeezing through
Using Trypsin like protease
Once they make direct contact with the uterus
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Escape from the Zona Pellucida
9.21 Hatching from the zona and implantation of the mammalian
blastocyst in the uterus
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Mammalian Female Reproductive
Tract and Early Development
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Escape from the Zona Pellucida
– Endometrium (Uterine epithelium)
• “Catches” blastocyst
• Using extracellular matrix of collagen, sugars,
laminin, fibronectin, hyaluronic acid receptors
• First attachment mediated by L-selectin on
trophoblast cells – adhering to sulfated
polysaccharides on uterus cells
• Sulfated polysaccharides synthesized in
response to corpus luteum – secretion of estrogen
and progesterone
– Other adhesion systems then coordinate to
keep blastocyst
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Escape from the Zona Pellucida
– Trophoblast secretes a set of proteases once
it contacts the uterus
– These are protein-digesting enzymes
• Collagenase, Stromelynsin, Plasminogen
activator
– Digest through uterine tissue
• Enabling the blastocyst to bury itself within the
uterine wall
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Mammalian Gastrulation
• Similarities exist between birds and
mammals
– Some believe this is due to being descendants
of reptiles ? (This is believed by evolutionary
theory supporters, only!!)
• Both have parallel developments useful in
studying
• Gastrulation similar even in absence of
large yolk in mammals
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Modification for Development with
Another Organism
• Mammals obtain nutrients directly from
its mother
• Fetus uses organ to obtain/absorb
maternal nutrients
– Chorion
• Derived primarily from embryonic trophoblast
cells with supplemental mesodermal cells
• Forms the fetal portion of the placenta
• It induces maternal portion formation
• Maternal portion called Decidua
• This is rich in O2 and nutrients from mother
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Modification for Development with
Another Organism
•
Origins of mammalian tissue development
summarized:
–
First segregation of cells within Inner cell
mass from trophoblast
•
–
Forms 2 layers
2 layers
1. Hypoblast (lower) layer
–
Sometimes called primitive endoderm
2. Epiblast above the inner mass
– Whether a cell becomes upper or lower does
not depend on its position in ICM
•
Only expression of Nanog or Gata6 transcription
factors
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Cleavage and Compaction
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Modification for Development with
Another Organism
9.24 in 10th edition
Schematic diagram showing the derivation of tissues in human and
rhesus monkey embryos
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Modification for Development with
Another Organism
9.24 Tissue formation in the human embryo between days 7 and 11
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Modification for Development with
Another Organism
9.24 Tissue formation in the human embryo between days 7 and 11
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Modification for Development with
Another Organism
– Epiblast and hypoblast form Bilaminar
germ disc
– Hypoblast ( primitive endoderm) cells
delaminate from inner cell to
• Line the blastocoel cavity
• Form extraembryonic endoderm
– Forms yolk sac
– No part of embryo
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Modification for Development with
Another Organism
– Epiblast splits to form
• Embryonic epiblast
• Other cells of epiblast
line amnionic cavity
And form the Amniotic
ectoderm
• Amnionic fluid fills
cavity
– Shock absorber for
embryo
– Prevents drying out
• Similar to avian
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Modification for Development with
Another Organism
9.26 Amnion structure and cell movements during human gastrulation
37
Modification for Development with
Another Organism
Gastrulation:
– Same as chick epiblast
• Mesoderm and endoderm migrate through
primitive streak
• Migrating cells lose E-cadherin, detach and
migrate through
• Epithelial to mesenchymal transition
• Migrating cells give rise to notochord
– Different from chick
• Notochord becomes integrated into endoderm of
primitive gut
• Converges medially – “buds off” in dorsal
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Modification for Development with
Another Organism
9.26 Amnion structure and cell movements during human gastrulation
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Modification for Development with
Another Organism
– FGF (fibroblast growth factors) coordinate
cell migration and specification
• If there is a loss of Fgf8 gene
– Cell does NOT migrate through primitive streak or
forms the other structures later
– Ectoderm precursors are located anterior to
fully extend primitive streak
– Migrating cells secrete and coat themselves
with hyaluronic acid
• Helps cells separate
40
Modification for Development with
Another Organism
– Human hypoblast replaced by endoderm
• Day 14-15
– Mesoderm starts forming
• Day 16
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Formation of Extraembryonic
Membranes
• Extraembryonic
cells make tissue to
allow fetus to survive
• The trophoblast cells
that divides nuclei
without cytokinesis
– Form a layer of
synctiotrophoblast
• Trophoblast cells that
divide normal
– Form a
cytotrophoblast
42
Formation of Extraembryonic
Membranes
8.24 Human embryo and placenta after 50 days of gestation
Note the sphere to the right is the yolk sac, and chorionic villi
extending outward
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Formation of Extraembryonic
Membranes
– Cytotrophoblast form and
adhere to endometrium
• Contains proteolytic enzymes
• Allows them to enter uterine
wall
• Remodel uterine blood vessels
• Maternal blood bathes fetal
blood vessels
• By secreting paracrine factors
– attract maternal blood vessels
– Syncytiotrophoblast
• Further the progression of the
embryo into uterine wall
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Formation of Extraembryonic
Membranes
– Shortly: mesoderm
extends outward from
gastrulating embryo
• Called extraembryonic
mesoderm
• Links embryo to
trophoblast
• Becomes umbilical cord
– Fully developed
extraembryonic organ
• Trophoblast tissue &
• Mesoderm blood vessels
•  Called the Chorion
45
Formation of Extraembryonic
Membranes
• Fuses with uterine wall to create placenta
– Uterine endometrium (Decidua) – forms
maternal side
– Fetal extraembryonic tissue – forms Chorion
– Deciduous placenta cannot be
separated without damage to both
mother and fetus at this stage
• Villi project from Chorion outer surface
46
Formation of Extraembryonic
Membranes
– Fetal and maternal circulations never merge
(blood cells)
9.27 Relationship of the chorionic villi to the maternal blood supply
in the uterus
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