Principles of Embryonic Development

7/27/2019 Principles of Embryonic Development

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EARLY DEVELOPMENT

in MAMMALS


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Overview of Development

Development is the successive process of

systematic gene-directed changes

throughout an organisms life cycle

-Can be divided into four subprocesses:

-Growth (cell division)

-Differentiation-Pattern formation

-Morphogenesis


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Principles of Embryonic

Development Fertilization results in a zygote and triggers

embryonic development

Plasma membrane

Tail

Middlepiece

NeckHead

Mitochondrion(spiral shape)

Nucleus

Acrosome

Mitochondrion(spiral shape)

The shape of a human sperm cell is adapted to its function


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Cleavage starts

Fertilizationof ovum

Oviduct

Secondaryoocyte

Ovulation

Ovary

Blastocyst(implanted)

Endometrium

Uterus

In mammals, fertilization occurs in the oviductwhere sperm encounters and the fuses with the

oocyte, as a result the oocyte nucleus which hadbeen arrested metafase2, completes meiosis and twoparenteral pronuclei fuse to form the diploid zygoticnucleus.


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The process offertilization

Only one of

these spermwill penetratethis humanegg cell toinitiate

fertilization Fertilization is

the union of asperm and anegg to form adiploid zygote

The spermapproachesthe egg

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3

4

The spermsacrosomal enzymesdigest theeggs jellycoat

Proteins on thesperm head bind

to egg receptors

5 The spermnucleusenters

the eggcytoplasm

The plasma membranesof sperm and egg fuse

6 Afertilizationenvelopeforms

SPERM

Nucleus

AcrosomePlasmamembrane

Spermhead

Acrosomalenzymes

Jellycoat

Vitellinelayer

Plasmamembrane

Cytoplasm

EGG CELL

Receptor proteinmolecules

Sperm

nucleus

Eggnucleus

7 The nucleiof spermand egg fuse Zygote

nucleus


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Cell Division

Cleavage is the first major phase of

embryonic development

It is the rapid succession of cell divisions

It creates a multicellular embryo from the

zygote

It partitions the multicellular embryo into

developmental regions


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Cell DivisionAfter fertilization, the diploid

zygote undergoes a periodof rapid mitotic divisions is

called cleavage

-Controlled by cyclinsand cyclin-dependent

kinases (Cdks)

Duringcleavage,the zygote

is divided into smaller &smaller cells called

blastomeres


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Blastomeres are nondifferentiated and can give

rise to any tissue.

Stem cells are set aside and will continue to

divide while remaining undifferentiated.-Tissue-specific: can give rise to only one

tissue

-Pluripotent: can give rise to multiple different

cell types (Blastocyst-inner cell mass)

-Totipotent: can give rise to any cell type

(Morula stage )

Cell Division


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Cleave in mammals continues for5-6 days producing a ball of

cells, the blastocyst

-Consists of:

-Outer layer= Forms theplacenta (Trophoblast cell that

secretes enzymes to enable theblastocyst to implant in theuterine wall)

ENDOMETRIUM

Inner cell mass

Cavity

Trophoblast


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ICM

Trophoblast

cell

-Inner cell mass = Forms the embryo

-Source ofembryonic stem cells (ES cells)

The ICM is marked

here with the antibody

stain against Oct4


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Egg

Sperm Blastocyst Embryo

Embryonic stem-cell

culture

Inner cellmass

Once sperm and egg cell have joined, cell

cleavage produces a blastocyst. The inner cell mass

of the blastocyst develops into the human embryo.

Embryonic stem cells (ES cells) are

isolated from the inner cell mass

Cell Division


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Cell Differentiation

A human body contains more than 210 major

types of differentiated cells

Cell determination commits a cell to a particular

developmental pathway-Can only be seen by experiment

-Cells are moved to a different location in

the embryo-If they develop according to their new

position, they are not determined


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Cell Differentiation

Cells initiate developmental changes byusing transcriptional factors to changepatterns of gene expression

Cells become committed to follow aparticular developmental pathway in oneof two ways:

1) via differential inheritance of cytoplasmicdeterminants

2) via cell-cell interactions


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Gastrulation produces a three-

layered embryo

Gastrulation is the second major phase of

embryonic development and produces a

three-layered embryo

It adds more cells to the embryo

It sorts all cells into three distinct cell layers

The embryo is transformed from the blastulainto the gastrula


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The three layers produced in

gastrulation

Ectoderm, the outerlayer

Endoderm, an

embryonic digestivetract

Mesoderm, whichpartly fills the space

between theectoderm andendoderm

Mesodermcells

Amnioticcavity

Amnion

Chorion

Yolk sac


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Embryonic tissue layers begin to differentiate

into specific tissues and organ systems after

gastrulation.

Organs start to form after gastrulation


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Meanwhile, the four embryonic membranes

develop into

Amnion

Chorion

Yolk sac

Allantois

Mesodermcells

Amniotic

cavity Amnion

Chorion

Yolk sac


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The embryo floats in the fluid-filled amniotic

cavity, while the chorion and embryonic

mesoderm form the embryos part of the

placentaPlacenta

Allantois

Yolk sac

Mothers bloodvessels

Amnioticcavity

Amnion

Embryo

Chorion

Chorionicvilli

Functions of the

chorion:

1. Provide nutrients and O2

to the fetus

2. Secrete hormones into the

mother to help retain the

fetus

3. Repress the mothers

immune response to preventrejection of the fetus

The placentas chorionic villi

absorb food and oxygen from the

mothers blood


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Embryonic induction initiates organ

formation

Induction is the mechanism by which one

group of cells influences the development

of tissues and organs from ectoderm,

endoderm, and mesoderm

Adjacent cells and cell layers use chemical

signals to influence differentiation

Chemical signals turn on a set of geneswhose expression makes the receiving cells

differentiate into a specific tissue


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Pattern formation organizes the

animal body

Pattern formation is the emergence of a

body form with structures in their correct

relative positions

It involves the response of genes to spatial

variations of chemicals in the embryo


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Changes in cell division, cell shape, cell

migration, and programmed cell death give

form to the developing animal

Tissues and organs take

shape in a developing

embryo as a result of

cell division

cell shape changes cell migration

Ectoderm


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programmed

cell death

(apoptosis) Cellsuicide

Dead cellengulfed and

digested byadjacentcell


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Morphogenesis

Cell division

-The orientation of the mitotic spindle

determines the plane of cell division in

eukaryotic cells

-If spindle is centrally located, two

equal-sized daughter cells will result

-If spindle is off to one side, two

unequal daughter cells will result


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Morphogenesis

Cell shape and size

-In animals, cell differentiation is

accomplished by profound changes in cell

size and shape

-Nerve cells develop long processes

called axons

-Skeletal muscles cells are large and

multinucleated


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Morphogenesis

Cell migration

-Cell movement involves both adhesionand loss of adhesion between cells and

substrate-Cell-to-cell interactions are oftenmediated through cadherins

-Cell-to-substrate interactions often involvecomplexes between integrins and theextracellular matrix (ECM)


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Principles of Embryonic Development

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