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Chapter 47 Animal Development
Fertilization
2005-2006
Embryonic development/fertilization
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Preformation: until 18th century; miniature infant in sperm or egg
At fertilization/conception:
Acrosomal reaction: hydrolytic enzyme action on egg jelly coat….
Fast block to polyspermy: membrane depolarization prevents multiple fertilizations….
Cortical reaction: release of calcium causes hardening of egg outer layer and creates a...
Slow block to polyspermy and...
Egg activation~ increases metabolic activity; protein synthesis
Cleavage
• Repeated mitotic divisions of zygote
– 1st step to becoming multicellular
– unequal divisions establishes body plan
• different cells receive different portions of egg cytoplasm &
therefore different regulatory signals
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Cleavage
• zygote  morula  blastula
– establishes future development
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Gastrulation
• Establish 3 cell layers
– ectoderm
• outer layers
– skin, nails, teeth, nerves
ectoderm
– mesoderm
• blood, bone & muscle
mesoderm
– endoderm
• inner lining
endoderm
– digestive system
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• Gastrulation in a sea urchin
Figure 47.11
– Produces an embryo with a primitive gut and
three germ layers
Key
Future ectoderm
Future mesoderm
Future endoderm
Animal
pole
The blastula consists of a single layer of ciliated cells surrounding the
blastocoel. Gastrulation begins with the migration of mesenchyme cells
from the vegetal pole into the blastocoel.
1
Blastocoel
Mesenchyme
cells
Vegetal
plate
Vegetal
pole
The vegetal plate invaginates (buckles inward). Mesenchyme cells
migrate throughout the blastocoel.
2
Blastocoel
Filopodia
pulling
archenteron
tip
3 Endoderm cells form the archenteron (future digestive tube). New
mesenchyme cells at the tip of the tube begin to send out thin
extensions (filopodia) toward the ectoderm cells of the blastocoel
wall (inset, LM).
Archenteron
Blastopore
Mesenchyme
cells
Blastocoel
50 µm
Archenteron
Ectoderm
Mouth
Mesenchyme:
(mesoderm
forms future
skeleton)
4 Contraction of these filopodia then drags the archenteron across
the blastocoel.
Blastopore
Digestive tube (endoderm)
Anus (from blastopore)
5 Fusion of the archenteron with the blastocoel wall completes
formation of the digestive tube with a mouth and an anus. The
gastrula has three germ layers and is covered with cilia, which
function in swimming and feeding.
• The mechanics of gastrulation in a frog
– Are more complicated than in a sea urchin
SURFACE VIEW
CROSS SECTION
Animal pole
1
Blastocoel
Gastrulation begins when a small indented crease,
the dorsal lip of the blastopore, appears on one
side of the blastula. The crease is formed by cells
changing shape and pushing inward from the
surface (invagination). Additional cells then roll
inward over the dorsal lip (involution) and move into
the interior, where they will form endoderm and
mesoderm. Meanwhile, cells of the animal pole, the
future ectoderm, change shape and begin spreading
over the outer surface.
Vegetal pole
Dorsal lip
of blastopore
Blastocoel
shrinking
2
Blastula
Dorsal lip
of blastopore
Archenteron
The blastopore lip grows on both sides of the
embryo, as more cells invaginate. When the sides
of the lip meet, the blastopore forms a circle that
becomes smaller as ectoderm spreads downward
over the surface. Internally, continued involution
expands the endoderm and mesoderm, and the
archenteron begins to form; as a result, the
blastocoel becomes smaller.
3
Ectoderm
Late in gastrulation, the endoderm-lined archenteron
has completely replaced the blastocoel and the
three germ layers are in place. The circular blastopore
surrounds a plug of yolk-filled cells.
Blastocoel
remnant
Mesoderm
Endoderm
Key
Future ectoderm
Figure 47.12
Future mesoderm
Future endoderm
Yolk plug
Yolk plug
Gastrula
• Gastrulation in the chick
– Is affected by the large amounts of yolk in the egg
Epiblast
Future
ectoderm
Primitive
streak
Migrating
cells
(mesoderm)
Endoderm
Hypoblast
YOLK
Figure 47.13
Organogenesis
• Various regions of the three embryonic germ
layers
– Develop into the rudiments of organs during the
process of organogenesis
• Early in vertebrate organogenesis
– The notochord forms from mesoderm and the
neural plate forms from ectoderm
Neural folds
LM
1 mm
Neural Neural
fold
plate
Notochord
Ectoderm
Mesoderm
Endoderm
Archenteron
Figure 47.14a
(a) Neural plate formation. By the time
shown here, the notochord has
developed from dorsal mesoderm,
and the dorsal ectoderm has
thickened, forming the neural plate,
in response to signals from the
notochord. The neural folds are
the two ridges that form the lateral
edges of the neural plate. These
are visible in the light micrograph
of a whole embryo.
Neurulation
• 1st organ to form is notochord &
nerve chord
– develop into nervous system
Neural groove
Notochord
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Neural tube
Organogenesis
Mammalian embryo
Umbilical blood vessels
Chorion
Bird embryo
Amnion
Yolk
sac
Allantois
Fetal blood vessels
Placenta
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Maternal blood vessels
Placenta
• Materials exchange across membranes
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Human fetal development
• Day 1: 1st cleavage
– 1 cell becomes 2 (2-cell stage)
• Day 2: 2nd cleavage
– 4-cell stage
• Day 3: 6-12 cell stage
– can test at this stage for genetic diseases
if done by IVF
• Day 4: 16-32 cell stage
– solid ball of cells = morula
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Ovulation to implantation
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Human fetal development
• Day 5:
– solid morula develops into hollow, fluid-filled blastula
– embryo will develop from the inner cell mass, or embryonic disc
• Day 6 -7:
– blastocyst attaches to the endometrium (uterine lining) & burrows
in: implantation
– blastocyst starts to secrete HCG = human chorionic gonatotropin
• stimulates estrogen & progesterone to prevent menstrual flow
• causes "morning sickness" in some women...
• pregnancy test measures the amount of this hormone!
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Human fetal developmental
• Days 10 - 14:
– pregnancy becomes established
– fluid filled amniotic cavity starts to form
– yolk sac starts to form
• will make blood cells, germ cells
– embryo starts to form from embryonic disc
– chorion (placenta) starts to form
• At the end of this stage,
a woman will have just
missed her period!
Day 14
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Human fetal developmental
• Days 15 - 21:
– emergence of the vertebrate body plan
– primitive streak starts to form
• the site of gastrulation (formation of 3 tissue layers = ecto, endo,
and mesoderm)
– neural groove begins to form
• future spinal cord & brain
– somites begin to form
• bands of tissue that will become muscles & bones
– pharyngeal arches begin to form
• future face, neck, mouth, nose
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Vertebrate body plan
Day 19
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Human embryonic developmental
• Week 3 - Week 8 = embryo
– development of all organ systems
• Day 22: the heart begins to beat
Day 24
Day 28
Week 4
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Human embryonic development
• embryo showing tail & limb buds
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Human embryonic development
• beginning of the eye can be seen, as well the bulging
heart & the umbilical cord
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Human embryonic development
• lens of the eye can be seen forming, the mass of
the heart bulging from the chest, & the
beginnings of the finger rays
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Human embryonic development
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Human embryonic development
• beginning of the ear is clearly seen
note the bend of
the elbow joint
has begun, the
fingers are
forming and toes
are beginning to
bud off the foot
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Human embryonic development
• Note the formation of the nose, eyelids, ear flap
& well defined toes & fingers
50–60 days (8 weeks)
Both knee & elbow are
visible.
Embryo has formed most
of basic organ systems &
will spend remainder of
development in “fetal”
period.
Organs grow, mature, &
begin to learn their
respective functions
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Human fetal development
• Day 52:
– first brain waves can be detected
• weeks 4-8 is when all major organ systems of body are
formed & when most teratogens have greatest effect
Week 8
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Human fetal development
Week 10
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Human fetal development
• At 15 weeks the embryo begins to take on more
of a typical baby's form & the attachment of
umbilical cord can be clearly seen
Week 15
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Human fetal development
• Week 9 - week 40 = fetus
– after 12 weeks or so, the baby's development is largely
"finished"
• some exceptions: brain & lung development
Week 16
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Human fetal development
• The fetus just spends much of the 2nd & 3rd
trimesters just growing
…and doing various flip-turns & kicks inside
amniotic fluid
Week 20
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Human fetal development
• 24 weeks (6 months)
fetus is covered
with fine, downy
hair called lanugo.
It’s skin is
protected by a
waxy substance
called vernix
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Human fetal development
30 weeks
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Human fetal development
• 32 weeks
The fetus sleeps
90-95% of the
day & sometimes
experiences REM
sleep, an
indication of
dreaming
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The end of the journey!
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