3 - embryology

7/30/2019 3 - Embryology

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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

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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

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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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The End

3 - embryology

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