Transcript Figure 47.9 Sea urchin gastrulation (Layer 3)

Chap 47: Animal Development
Brain
Heart
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Figure 47.2 The acrosomal and cortical reactions during sea urchin fertilization
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Sea Urchin as Our Model
Acrosomal Reaction
1) Acrosome releases hydrolytic enzymes allows actin
filaments to extend and bind with receptors on the
vitelline layer
2) This is one prezygotic barrier: no match between protein
and receptors, no sperm penetration
3) Sperm and egg cell membranes fuse
4) Sperm nucleus enters the egg
5) Fusion causes sodium ions to enter the egg cell which acts
as a fast block to polyspermy
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Cortical Reaction
6) Calcium ions are released upon fusion of sperm with egg
7) This triggers the cortical granules to fuse with plasma
membrane and release enzymes that separate the
vitelline layer from the plasma membrane. Water
enters into this space, separating these two layers
8) Vitelline layer becomes the fertilization envelope which
acts as the slow block to polyspermy.
Sea Urchin Time Lapse
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Activation of the Egg
9) Rate of metabolism increases greatly in the egg; protein
synthesis increases rapidly.
10) Sperm contributes nothing to activation. Eggs can be
activated by calciuim ions alone.
11) After about 20 minutes the sperm nucleus merges with
egg nucleus producing 2n zygote.
12) DNA synthesis occurs along with cell division in about 90
minutes.
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Figure 47.3 A wave of Ca2+ release during the cortical reaction
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Figure 47.4 Timeline for the fertilization of sea urchin eggs
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Figure 47.6 Cleavage in an echinoderm (sea urchin) embryo
Occurs at about 45-90
minutes after fertilization
Cleavage
4 cell stage
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Figure 47.6x Sea urchin development, from single cell to larva
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Figure 47.5 Fertilization in mammals
Capacitation
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Capacitation occurs to the sperm cells
Vaginal secretions alter molecules on the sperm’s surface
Sperm motility also increases
Sperm migrates through follicle cells and penetrates zona
pellucida
Within the ZP is a glycoprotein that functions as a sperm
receptor
Sperm then goes through its acrosomal reaction, releasing its
hydrolytic enzymes and sperm reaches plasma
membrane
A depolarization or charge change occurs across the egg’s
membrane (fast block)
Cortical granules release contents
Sperm enters the egg.
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Cleavage Partitions The Zygote Into Many Smaller Cells
Cleavage:
a bunch of quick divisions after fertilization. All
the cells resulting from cleavage are called blastomeres.
Blastomeres: each can contain different cytoplasmic
components because the cytoplasm itself is not made up of
evenly distributed substances. This type of division creates
different cells with different components or polarity which
affect development.
•Vegetal Pole: area where stored nutrients or yolk is present
•Animal Pole: lower concentration of yolk
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Figure 32.1 Early embryonic development (Layer 3)
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Figure 32.7 A comparison of early development in protostomes and deuterostomes
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An Amphibian’s Polarity
1. Start with the zygote and it has an animal pole and vegetal
pole.
2. The animal pole contains pigment granules, melanin, giving it
a gray color and the vegetal pole with yolk is yellowish.
3. At fertilization things change.
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Cytoplasm is rearranged
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Plasma membrane rotates towards where the sperm cell
entered and this exposes cytoplasm (light gray) which is
called the gray crescent
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Opposite to where the sperm entered is a region which
will become the dorsal or back side of the embryo.
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4. Yolk blocks cell division so most cellular division occurs at
the animal pole. Cells are larger here than near the vegetal
pole.
5. Cleavage occurs to produce a solid ball of cells-the
MORULA.
6. A BLASTOCOEL or fluid filled cavity forms within the
morula.
7. This fluid filled hollow ball of cells is the BLASTULA.
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The Impact of Yolk on Cleavage
Birds, reptiles, insects and many fishes have lots of yolk in
their egg cells
•Meroblastic Cleavage: when the yolk takes up so much of
the fertilized egg and subsequent cell division occurs in a
small area of the animal pole.
Sea Urchins, frogs, mammals demonstrate Holoblastic
Cleavage where here is little yolk and division of the egg is
relatively complete.
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Figure 47.7 The establishment of the body axes and the first cleavage plane in an
amphibian
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Figure 47.8x Cleavage in a frog embryo
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Figure 47.8d Cross section of a frog blastula
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Figure 47.9 Sea urchin gastrulation (Layer 1)
Gastrulation: rearrangement of the
blastula cells into 3 germ layers, ectoderm,
mesoderm and endoderm
•Cells undergo changes in motility,
adhesion and shape
•The three layered end-product is
called a gastrula from which tissues
and organs will develop.
The invagination continues
inward and forms a pocket called
the archenteron which will
develop into the gut or digestive
tube. Depending on the
organism, one end becomes the
mouth, the other the anus.
The opening is called the
blastopore.
Endoderm forms the archenteron
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Figure 47.9 Sea urchin gastrulation (Layer 2)
Continued invagination of
archenteron
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Figure 47.9 Sea urchin gastrulation (Layer 3)
Digestive tube is formed from endoderm
Ectoderm forms outer surface
Mesoderm forms some of internal skeletal
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Table 47.1 Derivatives of the Three Embryonic Germ Layers in Vertebrates
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Figure 47.10 Gastrulation in a frog embryo
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Figure 47.11 Organogenesis in a frog embryo
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Figure 47.12 Cleavage, gastrulation, and early organogenesis in a chick embryo
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Figure 47.13 Organogenesis in a chick embryo
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Figure 47.14 The development of extraembryonic membranes in a chick
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Mammalian Development
Egg of mammals is quite small
Divisions of the fertilized egg
•1st Within about 36 hours
•2nd at about 60 hours
•3rd at about 72 hours
Blastocyst: 100 cells arranged around the blastocoel
•There is an inner cell mass that will become the embryo
•Trophoblastic layer will form the fetal contribution to the
placenta
•It is the blastocyst that implants itself into the uterus (7
30
days)
Inner cell mass becomes a flattened disc with two layers.
•Epiblast: which will form embryo
•Hypoblast: which will form the yolk sac but contains no yolk
Yolk sac will make blood cells for the embryo
Trophoblast
•Expands into endometrium
•Develops the chorion: contributes to placenta; surrounds
all other (3) extraembryonic membranes
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Epiblast: forms the amnion which contains the amniotic fluid
that cushions the developing fetus
•Gastrulation occurs in the epiblast
Allantois: this extraembryonic membrane becomes part of the
umbilical cord and forms blood vessels to transfer oxygen and
nutrients from placenta to embryo; also carries carbon dioxide
and wastes to placenta.
So the four extraembryonic membranes are:
•Chorion
Yolk Sac
Amnion
Allantois
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Figure 47.15 Early development of a human embryo and its extraembryonic
membranes
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Figure 47.16 Change in cellular shape during morphogenesis
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Figure 47.17 Convergent extension of a sheet of cells
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Figure 47.18 The extracellular matrix and cell migration
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Figure 47.19 The role of a cadherin in frog blastula formation
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Figure 47.20 Fate maps for two chordates
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Figure 47.21 Experimental demonstration of the importance of cytoplasmic
determinants in amphibians
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Figure 47.22 The “organizer” of Spemann and Mangold
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Figure 47.23 Organizer regions in vertebrate limb development
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Figure 47.24 The experimental manipulation of positional information
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HHMI Holiday Lecture Series
2001: Lecture 1 of 4
How is Sex Determined?
How is the sex of the human embryo determined?
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At about 6 weeks after fertilization, there is no anatomical difference.
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At the 7th week, the gonads are bipotential, that is, that can either
differentiate into testes or ovaries.
• Hormonal secretions determine the sexual fate of the reproductive
structures; the masculinization or feminization of other structures
also occurs, including the brain.
In humans, the presence of the Y chromosome is sex determining but . . .
There are XX males)
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XX Males
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1 out of every 20,000 males is XX
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They have a penis, scrotum, testes, but will not produce sperms or
eggs
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The reason why they are XX males is because they have a portion of
the Y chromosome that was crossed over during meiosis in making
sperm cells. And the X chromosome from the father that fertilized
the egg of the mother had this small portion of the Y chromosome.
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This portion is the testes determining portion so this has the critical
gene or genes.
• The gene is called the SRY gene: This codes for a DNA binding
protein that affects many other genes.
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SRY causes development of the testes so if you are +SRY you
follow the testicular path and if you are – SRY you follow the
ovarian path.
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XY Females
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Well, if you’ve been able to follow my description of the loss of this
SRY portion from the Y chromosome, then you probably have
realized there is a sperm cell with a Y chromosome without the SRY
gene because of this crossing over.
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1 out of every 20,000 females is an XY female.
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She has a clitoris, labia, ovaries, fallopian tubes, uterus but no sperm
or egg production.
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Yes, she has a Y chromosome but not the SRY portion that would
make her a guy. So “Dude looks like a lady?” (Aerosmith, 19_ _?)
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XY females do not produce testosterone but do have female hormone
levels.
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Transgenic mice research to prove the role of SRY gene
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In vitro, researchers took an XX fertilized mouse egg and injected
the SRY gene
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The SRY integrated into the host genome
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Then the implanted into the uterus of the female mouse.
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Development for 20 days
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Produced an XX +SRY transgenic mouse that had testes and is male
but no sperm production.
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So: SRY is a sex determining gene
• Two X chromosomes are incompatible with producing sperm even
in the presence of SRY gene. Another portion of the Y chromosome
is responsible for this also.
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