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Cell Proliferation & Differentiation

in Embryo Development

Department of Histology & Embryology

Ling xiao

• During embryonic development, a fertilized egg gives rise to many different cell types

A Genetic Program for Embryonic Development

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• During embryonic development, a fertilized egg gives rise to many different cell types

• Cell types are organized into tissues, organs, organ systems, and the whole organism

• Gene expression controls this development

• The transformation from zygote to adult results from cell proliferation, cell differentiation, and morphogenesis.

A Genetic Program for Embryonic Development

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What is cell proliferation?

• Cell proliferation is a process of cell

division and regeneration, which

results in an increase in the cell

number with exact passages of

genetic information to their daughter

cells.

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Proliferationof eukaryotic

cell

DNA replication

What is cell cycle?

• Cell cycle is defined as a period from

the end of one division to the

beginning of next division of a

proliferative cell.

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Phases of the cell cycle

G1 phase

S phase

G2 phase

M phase

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Phase of cell cycle

• M-Phase

– Mitosis

– Cytokinesis

• Interphase

– G1 = Gap between M and S

– S = Synthesis (DNA and

centrosomes replicated)

– G2 = Gap between S and M

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Cell Cycle Regulation

• Cyclins

• Cyclin Dependent Kinases (CDKs)

• Cyclin Dependent Kinases Inhibitors (CDKIS)

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The Nobel Prize in Physiology or Medicine 2001 was awarded jointly to Leland H. Hartwell, Tim Hunt and Sir Paul M. Nurse "for their discoveries of key regulators of the cell cycle".

Leland H. Hartwell Tim Hunt Sir Paul M. Nurse

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For adult • Cycling cells with continuous division

Embryo cells, hematopoietic cells

• Differentiated cells

Cardiac cells, nerve cells

• Most cells

Skin fibroblasts, SMCs, VECs, cells of

internal organs

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For Embryo

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For Embryo

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For Embryo

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For Embryo• Cleavage is a series of rapid cell divisions without

cell growth or gene expression which occurs in early embryogenesis.

• Early cleavage divisions in most embryos are reductive. During cleavage, the cytoplasm is divided into smaller and smaller cells, called blastomeres. The total cellular volume of the embryo stays the same, but the number of cells within the embryo increases.

• Cleaving cells have a modified cell cycle, in which the two gap phases, G1 and G2, are completely omitted. The cells cycle rapidly between M and S phases.

For Embryo

• Cleaving cells have

a modified cell cycle,

in which the two gap

phases, G1 and G2,

are completely

omitted. The cells

cycle rapidly

between M and S

phases.

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For Embryo• Morula stage(16-cell stage):

Compaction

Polarity

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Stages of mouse and human preimplantation development.

(TE: trophectoderm; ICM: inner cell mass ; EPI: epiblast;

PE: primitive endoderm)

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Cockburn K, Rossant J. Making the blastocyst: lessons from the mouse. J Clin Invest. 2010, 120(4): 995-1003.

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Abnormal Cleavage

• Delay of cleavage: implantation failure

• Irregular cleavage: abnormal zygote

• Polyspermous fertilization: Polyploid

• Enzygotic twins

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Siamese twins

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Control of early embryo development

• Before genome activation

Maternal-effect genes (miRNAs,

transcriptional factors, DNA

methyltransferase, …)

• After genome activation

Transcriptional factors (Cdx2, Oct4…)

Heat shock proteins

Cytokines…

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Cdx2Oct4

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When a cell divides by mitosis it

produces two “daughter cells”

with identical sets of chromosomes.

SO, HOW DO THESE CELLS

BECOME DIFFERENT FROM ONE

ANOTHER?

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What is cell differentiation?

• Cell differentiation is a cellular

events that gives rise to different

kinds of cells with the specialized

morphology, metabolism and

physiological functions from the cells

of the same origin.

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What is cell differentiation?

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• Determination commits a cell to its final fate.

• Determination precedes differentiation.

• Cell differentiation is marked by the production

of tissue-specific proteins.

Cell Determination &

Differentiation

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Cytoplasmic Determinants and Inductive Signals

• An egg’s cytoplasm contains RNA, proteins, and other substances that are distributed unevenly in the unfertilized egg.

• Cytoplasmic determinants are maternal substances in the egg that influence early development.

• As the zygote divides by mitosis, cells contain different cytoplasmic determinants, which lead to different gene expression.

31(a) Cytoplasmic determinants in the egg

Two differentcytoplasmicdeterminants

Unfertilized egg cell

SpermNucleus

32(a) Cytoplasmic determinants in the egg

Two differentcytoplasmicdeterminants

Unfertilized egg cell

Sperm

Fertilization

Zygote

Nucleus

33(a) Cytoplasmic determinants in the egg

Two differentcytoplasmicdeterminants

Unfertilized egg cell

Sperm

Fertilization

Zygote

Mitoticcell division

Two-celledembryo

Nucleus

34(b) Induction by nearby cells

Signalmolecule(inducer)

Signaltransductionpathway

Early embryo(32 cells)

NUCLEUS

Signalreceptor

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Totipotent Pluripotent Unipotent DifferentiatedTotipotent Pluripotent Unipotent DifferentiatedTotipotent →Pluripotent→ Unipotent →Differentiated

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Control of Cell Differentiation

– Gene control

– Cell interaction

– Hormones

– Environmental factors

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Gene control

• Housekeeping Genes serve a function

required in all the cell types of an organism,

regardless of their specialized role. (Helper)

• Luxury genes perform specialized function

in a definite type of cell, not required by all

cell types of an organism. (Determinant)

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Example

• Myoblasts produce muscle-specific proteins and form

skeletal muscle cells

• MyoD is one of several “master regulatory genes” that

produce proteins that commit the cell to becoming

skeletal muscle

• The MyoD protein is a transcription factor that binds to

enhancers of various target genes

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Embryonicprecursor cell

Nucleus

OFF

DNA

Master regulatory gene myoD Other muscle-specific genes

OFF

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Embryonicprecursor cell

Nucleus

OFF

DNA

Master regulatory gene myoD Other muscle-specific genes

OFF

OFFmRNA

MyoD protein(transcription

factor)Myoblast(determined)

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Embryonicprecursor cell

Nucleus

OFF

DNA

Master regulatory gene myoD Other muscle-specific genes

OFF

OFFmRNA

MyoD protein(transcription

factor)Myoblast(determined)

mRNA mRNA

Part of a muscle fiber(fully differentiated cell)

MyoD

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Embryonicprecursor cell

Nucleus

OFF

DNA

Master regulatory gene myoD Other muscle-specific genes

OFF

OFFmRNA

MyoD protein(transcription

factor)Myoblast(determined)

mRNA mRNA mRNA mRNA

Myosin, othermuscle proteins,and cell cycle–blocking proteinsPart of a muscle fiber

(fully differentiated cell)

MyoD Anothertranscriptionfactor

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Cell interaction

• Cell induction

• Contact inhibition

• Microenvironment-- extracellular

matrix

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Cell induction

• Induction is the process by which one group

of cells produces a signal that determines

the fate of a second group of cells.

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Example

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Extracellular matrix (ECM)

• ECM can control the differentiation by

influencing the intracellular gene

expressions via signal transduction.

Exogenous ECM prevents the inhibition of skeletal muscle differentiation by proteoglycan synthesis inhibitors.

Osses N , and Brandan E Am J Physiol Cell Physiol 2002;282:C383-C394

Chlorate and BDX can inhibit the proteoglycan synthesis.

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Hormones

• Steroids

• Peptides

To influence the gene expressions through

binding with the corresponding receptors

Target cells

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Environmental factors

1) Biological

Viruses:

– rubella virus → cataracts, deafness, cardiac

defects;

– cytomegalovirus → blindness, microcephaly,

hepatosplenomegaly;

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2) Physical

radiation, mechanical pressure, trauma

Radiation: World War II: atomic bomb

explosion over Japan.

Survived pregnant women:

28% aborted;

25% delivered children died <1 year;

25% infants with CNS malformations.

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3) Chemical

chemicals: nitrite, mercury, lead, etc.

drugs: thalidomide (amelia and meromelia)

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4) Others

hypoxia

nutritional deficiencies

…………

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Susceptible period

1) Dependent on the developmental stage.

The embryonic period (3-8 weeks) is

the highest susceptible period.

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2) Different organs have different susceptible period corresponding to their own critical development stage.

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Summary

Embryo development: From one single

zygote to a complex human body

Cell proliferation: Increase of cell numbers

(cleavage)

Cell differentiation: Formation of different

cell types (cell determination)

Control factors: gene, cell communication,

ECM, hormone, environmental factors