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GENERAL FEATURES OF CHORDATEDEVELOPMENT
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EMBRYOLOGY
the science dealing with the embryo and
its development
EMBRYOGENY/EMBRYOGENESIS - the
development of the embryo
EMBRYO - the juvenile stage of an animal while
it is contained in the egg (within the egg
membranes) or in the maternal body
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Types of eggs based on the amount of yolk they
contain:
1. microlecithal (oligolecithal) - little yolk present
(amphioxus, marsupial and placental mammals)
2. mesolecithal - moderate amounts of yolk present(lamprey, sturgeon, lungfish, and amphibians)
3. macrolecithal (polylecithal) - large amounts of yolk
present (hagfish, sharks, rays, teleost fish, reptiles,birds, and monotreme mammals)
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Types of eggs based on how the yolk is distributed in it:
1. isolecithal (homolecithal) - the yolk is diffuse throughoutthe egg (true for microlecithal eggs)
microscopic eggs of
mammals,amphioxus
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2. telolecithal - the yolk is concentrated towardsone side of the egg (true for mesolecithal andmacrolecithal eggs) : fishes, reptiles amphibians,
birds
The pole where the yolk is concentrated is called vegetalpole, the opposite end (with nucleus) is the animal pole.
- aphibians, fishes, lamprey 5
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Embryogenesis
Fertilization zygote (cleavage)
Organogenesis
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Entry of sperm cell to the egg
cell
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Embryogenesis development of
primary germ layers and their locations
Organogenesis formation of the
different organs
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Stages of Embryogenesis
1.Cleavage
2. Gastrulation
3. Neurulation
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CLEAVAGE* during this stage, a number of cell divisions
subdivides the originally single cell of the
zygote into a ball of smaller cells - morula
* the hollow sphere of cells is called
the blastula
* each single cell is a blastomere, and the central
cavity is the blastocoel
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Types of Cleavage
1. Holoblastic (=complete, equal) Cleavage
in oligolecithal eggs, where yolk is uniformly distributed andmeager in amount
daughter cells are equal but smaller than the parent cell
e.g. Amph ioxus
Holoblastic unequal cleavage-
in mesolecithal eggs (moderate amount of yolk)
cells of different sizes result from different amounts of yolk in each pole
* amphibians 14
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2.Meroblastic Cleavage in telolecithal eggs, where there is sufficient concentration of yolk at
vegetal pole and active cytoplasm at opposite pole
asymmetrically distributed yolk
Only the small germinal disk undergoes cleavage
two effects:
1. displaces nucleus toward animal pole
2. retarded cleavage
yolk impedes division at vegetal pole and cells of this hemisphereremains much larger than at the animal pole
fish, birds, reptiles
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3. Discoidal Cleavage incomplete or meroblastic
relatively enormous mass of yolk in eggs
mass of inert yolk cause the yolk not to divide
partial separation of cells
segmentation is confined to animal pole and results inblastoderm (=blastodisc) perched on massive yolk(embryo develops from this blastoderm)
most of vegetal pole with most of the yolk does notdivide and becomes the periblast, a layer of cytoplasmadhering to the undivided yolk mass
e.g. chicken
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Cleavage in Amphibians
rapid within 24 hours along with blastula formation
begins at the animal pole and bisects the graycrescent
elongate at a rate of 1mm/minute at the animal pole
slows to 0.02 to 0.03 mm/minute as it nears thevegetal pole
second cleavage still at animal pole but plane isperpendicular to first cleavage
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third cleavage is horizontal and passesnear to animal pole dividing the embryo
into:
- four small blastomeres,the micromeres
- four large blastomeres,the macromeres
embryo between the 16 to64 cell stages is commonlycalled a morula
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Cleavage pattern in a frog egg
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Cleavage in Birds
occurs as the egg is passing down the oviduct
first cleavage furrow appears near the center of
the blastodisc (=blastoderm)
sequence of cleavage furrows is not regular
after the third cleavage, the sequence becomessynchronous
fourth cleavage is circumferential and cuts off a central row
from a peripheral row ofblastomeres
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Cleavage in a bird embryo
Outer layer of
blastula that
gives rise to
ectoderm after
gastrulation
Beneath the
epiblast and
gives rise to
endoderm
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Cleavage in Mammals
much slower than other vertebrates
first cleavage is not completed for 24 hours
Rotational typewhere it involves normal first divisionalong the meridional axis, giving rise to 2 daughter cells
then one will divide meridionally while the other dividesequatorially.
subsequent divisions take 12 hours each
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after this time, cell division is asynchronous, producingat times odd number of cells (rather than typical pattern2-4-8-16-32-64)
by 16-cell stage, inner cell mass is present that willform body of embryo; it forms embryonic disc
at 32-cell stage, outer layer is expanded around
blastocoel and formstrophoblast
- absorb nutrients from uterine fluids
- site of contact with uterus when implantationoccurs
Trophoblast - contributes to formation of placenta and
chorion) --- called blastocyst in mammals
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There are several differences between the cleavage in
mammals and the cleavage in other animals.
1. Mammals have a slow rate of division that is between 12and 24 hours. These cellular divisions are asynchronous.
2. Zygotic transcription starts at the two, four, or eight-cell
stage.
3. Cleavage is holoblastic and rotational.
At the eight-cell stage, the embryo goes through some
changes. Most of the blastomeres in this stage becomepolarized and develop tight junctions with the other
blastomeres. This process leads to the development of two
different populations of cells: polar cells on the outside and
apolar cells on the inside. 25
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4. The outer cells, called the trophoblast cells, pump
sodium in from the outside which automatically brings
water in with it to the basal (inner) surface to form a
blastocoel cavity in a process called compaction. Theembryo is now called a blastula or early blastocyst.
5. The trophoblast cells will eventually give rise to the
embryonic contribution to the placenta called thechorion.
6. The inner cells are pushed to one side of the cavity
(because the embryo isn't getting any bigger) to form theinner cell mass (ICM) and will give rise to the embryo
and some extraembryonic membranes. At this stage, the
embryo is called a blastocyst.
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http://en.wikipedia.org/wiki/Trophoblasthttp://en.wikipedia.org/wiki/Inner_cell_masshttp://en.wikipedia.org/wiki/Inner_cell_masshttp://en.wikipedia.org/wiki/Trophoblast -
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Human embryogenesis
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http://upload.wikimedia.org/wikipedia/commons/0/06/HumanEmbryogenesis.svghttp://upload.wikimedia.org/wikipedia/commons/0/06/HumanEmbryogenesis.svg -
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Gastrulation
Process by which the blastula becomes athree-layered embryo or gastrula
Zygote Morula Blastula Gastrula
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Gastrulation:
a dynamic process of cellular movementswhere:
a. endoderm
b. mesoderm
c. notochord cells of blastula flow to the interior,forming the 3 germ layers from which allfuture tissues and organs form
changes the radial symmetry into bilateral symmetry
these cellular migrations are referred to as formative ormorphogenetic movements
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Gastrula:
converts the blastula into a gastrula
(little stomach)
gastrula consists of3germ layers
1. Endodermmoves inwards to form gut lining
2. Ectodermon the outside gives rise to
epidermis and nervous system
3. Mesodermproduces the skeleton, muscles,circulatory system
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Cell Movement Patterns1. EPIBOLY = movement of epithelial sheets(usually ectoderm cells) which spread to
enclose the deeper layers of the embryo
2. INVAGINATION = infolding of a region of cells;epithelial cells fold inwards
3. INVOLUTION = outer layer expands and movesinwards covering the internal surface of the
remaining external cells (rolling in of cells overa rim)
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4. INGRESSION = migration of individual
cells from the surface layers into theinterior of the embryo
5. DELAMINATION = splitting of onecellular sheet into two more or less
parallel sheets
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Changes in epithelial organization that areimportant during gastrulation
Delamination - splitting of one cellular
sheet or layer into two parallel layers , an
outer layer (epiblast) and an inner layer
(hypoblast)
Involution - tissue sheet rolls inward to
form an underlying layer through bulk
movement of tissue
Epiboly- sheet of cells spreads by
thinning
Invagination - epithelial sheet bendsinward to form an inpocketing
Ingression - cells leave an epithellial
sheet by transforming from well-
behaved epithelial cells into freely
migrating mesenchyme cells
G t l ti i A hibi
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Gastrulation in Amphibians
(Moderate telolecithal eggs)
Invagination at margin of grey crescent (point of sperm entry at animal pole)
Cleft forms (beginning of archenteron)
Gives rise to dorsal lip ofblastopore
Sheet of cells at animal pole expand
Moves towards dorsal lip, rolls over and turns inward - Involution
Inward turning cells obliterate the blastocoel
Form chordamesoderm
Li f bl t lk ll
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Lip of blastopore overgrows yolk cellsEpiboly
Dorsal lip spreads laterally forming completeblastopore
Blastopore surrounds yolk plug
As yolk is used, blastopore closes
Anus invaginates near closed blastopore
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-modified by the presence of large yolk
laden cells in the vegetal pole (VP) (fig.A)
-begins at the dorsal blastopore lip, just
dorsal to the yolk
Fig.A
blastopore
bottle cells
dorsal lip
blastopore
involution occurs at all points around the
dorsal lip
Amphibian Gastrulation
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This involution creates anddestroys the blastocoel.
So, what is achieved?
-the dorsal lip forms a circle,
with cells moving to the
interior, all around theblastopore
the yolk plug is visible
through the blastopore.
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Gastrulation in Reptiles, Birdsand Mammals
All germ layers develop from epiblast (upper surface of blastocoel)
Epiblast cells move to middorsal longitudinal axis
Forms a thickened region called Primitive streak
Cells move inwards and spread laterally into the blastocoel(manifested on surface of blastoderm as a longitudinal depression in the primitivestreak)
No opening (blastopore) for cells to move through
Cells move individually into the blastocoel: Ingression
Primitive streak the equivalent of blastopore
First cells into blastocoel form endoderm
Later cells form mesoderm
Cells remaining on surface from ectoderm
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Gastrulation in birds
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Fate of germ layers formed at gastrulation
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Ectodermnervous system, epidermis, nails, hair
and other skin derivatives
Mesodermnotochord, skeleton (bone and cartilage),
muscles, circulatory system, excretory system,respiratory system, dermis, outer layer of digestive
tube and other structures, part of the respiratory
system
Endodermlining of the digestive tube & otherstructures that develop from it, lining of respiratory
system
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Gastrulation
It is not birth, marriage, or death,but gastrulation which is truly themost important time in your life.
Lewis Wolpert (1986)
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Neurulation nervous tissue is formed
mesoderm finally forms the notochord, and
the coelom is created
nervous tissue is derived from ectoderm(neurectoderm)
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Neurulation
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neurulation starts with ectodermal folds growing over theneural plate, which is folding upwards at the same time
the result is a neural tube enclosing the neurocoel
the neurocoel has two openings, the neurenteric canaland the neuropore.
Temporary duct connecting
the neural tube and the
primitive gutOpening at each end of the
neural tube formed at the cranial
and caudal regions which closes
as the embryo develops
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Neurulation II
in the roof of the gastrocoel orarchenteron,themesoderm layer called chordamesoderm -gives rise to the notochord.
mesoderm stimulates the neural ectoderm toform the central nervous system
the flanking regions of the mesoderm foldinwards and pinch off parts of the gastrocoel
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Neural Tube Stage Embryo
Has a head and tail region
Has a dorsal surface and a ventral surface
Has a primitive nervous system (neural tube)
Has a skeletal support (notochord)
Has a primitive gut (archenteron)
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this occurs as a series of pouchesalong the length of the embryo; each
pouch is called somite
the single pouch, i.e. the pocket ofmesoderm, grow downward betweenectoderm and endoderm
meet at the bottom and fuse to asingle cavity, the coelom.
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Neurulation
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Amphioxus coelom formationis calledenterocoely
if the mesoderm is a solid mass of cells
(vertebrates) - form of coelom development
is known as schizocoely
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Schizocoely vertebratecoelom formation
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Development
of the human
nervoussystem
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In terrestrial vertebrates the three germ
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In terrestrial vertebrates, the three germlayers give rise to four extraembryonicmembranes:
1. Chorion - used in gas exchange
2. Amnion - a fluid-filled sac
3. Allantois - which stores wastes
4. Yolk sac - which makes food available to theembryo
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Extraembryonic membranes
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Organogenesis
Gastrulation organogenesis(organ formation)
Cells of 3 germ layers continue patternformation
formation of specific structures* ectoderm skin and nervous system
* endoderm lining of the gut and internal organs
* mesoderm muscles, bones , heart
Early human development
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Early human development
Fertilization occurs in the oviduct
Cleavage takes place as the embryo movesdown the oviduct toward the uterus
The blastocyst undergoes implantation inthe endometrium
The umbilical cord connects the embryo tothe placenta
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Implantation
and early
developmentin the uterus
Prenatal and postnatal life
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Prenatal and postnatal life
Human prenatal development requires 266 days
Organogenesis begins during the firsttrimesterand continues throughout
After the first two months, the embryo is referred to asa fetus
The neonate undergoes rapid adaptations, especiallyin the respiratory and digestive systems
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Developmental
events in the
human embryo
Days 7 10
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Days 7 10
Gastrulation
Ectoderm skin, nervous system
Endoderm lining of gut and internal organs
Mesoderm muscles, bones, heart
All cells have the
same DNA, however,
different cells begin to
express different genes
to become different organs.
Week 2 Pregnancy becomes
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Week 2 Pregnancy becomes
established
Formation of:1. Fluid-filled amniotic cavity
2. Yolk sac
3. Embryo from embryonic disc
Chorion (placenta)
* at the end of this period,
a woman will just have
missed her period!
Days 15 21: Emergence of the body
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Days 15 21: Emergence of the body
plan
Formation of:a. primitive streak site of the formation of 3
tissue layers: ectoderm, endoderm, mesoderm
b. Neural groove - future spinal cord
and brain
c. Somites - bands of
tissue that will become
muscles and bones
d. pharyngeal arches future face,
neck, mouth, nose
Weeks 3 8: (Embryo)
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Weeks 3 8: (Embryo)
Development of all organs
Day 22 heart begins to beat
Week 4: How you looked at 1
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Week 4: How you looked at 1
month past conception
Week 7/Day 48: First brain
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Week 7/Day 48: First brain
waves can be detected
f
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Human fetus
at ten weeks
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Week 9 Week 40 (Fetus)
After 12 weeks or so, the babysdevelopment is largely finished
except brain and lung development.
The fetus spends much
of the 2nd and 3rd trimester
just growing ( and doingvarious flip-turns and kicks
inside the amniotic sac.
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Week 11
I have two hands the left and the right.
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4 months week 16
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6 months
The unborn child is covered with a fine , downy hair calledlanugo.
Ako ang tatalo sa
yo Manny.
8 th
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8 months
The fetus sleeps 90 95% of the day.
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9 months/ week 40
The baby, now approximately 71/2 pounds, isready for life outside its mothers womb.
Kaulaw bayaani mogawas
daghan
nagtan-aw.
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P t iti
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Parturition
Di na gyud ko
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Di na gyud ko
muusab. . .
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Aging
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Aging
Marked by a decrease in homeostaticresponse to stress
All body systems decline with age, but not atthe same rateExamples: loss of taste buds and axons in spinal
nerves
Breakdown of collagen and elastic fibers
Decrease in subcutaneous fats
Decrease in sebum production by sebaceous glands
Decrease in melanin production (gray hair and dark pattern ofpigmentation on the skin)
Decrease in hair replacement
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END
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The End