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EMBRYOLOGY

BY Dr.

THAAER MOHAMMED DAHER ALSAAD

SENIOR LECTURER MSU-IMS

M.B.Ch.B. (MBBS)

F.I.B.M.S. (Ph.D.)SPEICIALIST IN GENERAL SURGERY


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TYPES:

Causal (experimental)

embryology

Comparative embryology

Descriptive embryology


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EMBRYOLOGY/em br-ol-je/

The science of the development

of the individual during the

embryonic stage and,

by extension, in several or even

all preceding and subsequentstages of life cycle.


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General definitions of embryology

The branch of biology that deals with

the formation, early growth, and

development of living organisms

Study of theformation and

developmentof an embryo andfetus


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Why medical students should study Embryology?

Know how a single cell develops into a newborn,

containing numerous tissues and organs.

Help us understand many complicated facts of adult

anatomy.

Explain why some children are born with organs that

abnormal.

Appreciate the factors responsible for maldevelopment to

assist us in preventing or treating such abnormalities.


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Some preliminary consideration

Embryo = developing individual during 1st two months.

Fetus = developing individual from 3rd month until birth.

Gametes : are produced by testes and ovaries.

Testis : produces gametes ( spermatozoa ).

Spermatogenesis : process of producing spermatozoa.

Ovary : produces gametes ( ova ).

Oogenesis : process of producing ova.

Fertilization : takes place when one spermatozoon enters anovum.

Zygote : the fused ovum and sperm.


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What is Embryology?

Embryology is the study of the development of an individual before birth.

Every individual spends 266 days

38 weeks ( to be exact )

One celled structure pass stages to become organism havingbillions of cells, numerous tissues and organs are formed and

come to function in perfect harmony.

The most spectacular of these changes occur in the 1st two months ( the unborn

baby acquires its main organs and just begins to be recognized as human ).

1st two months = Embryo.

3rd month until birth = fetus.

1st 28 days post birth = Newborn baby .


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Chromosome A structure in the nucleus containing a linear thread of DNA,

which transmits genetic information and associated withRNA and histones; during cell division.

Chromatid =

one of the paired chromosome strands, joined at thecentromere, which make up a metaphase chromosome,resulting from chromosome reduplication during the DNA

synthetic phase of interphase.


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Chromatin The more readily stainable protein of the cell nucleus, forming a network

of nuclear fibrils.

It is a DNA attached to a protein primarily (histone) structure base and is thecarrier of the genes in inheritance.

It coils to form the metaphase chromosome.

GeneA segment of DNA molecule that contains all the information required for

synthesis of a product ( polypeptide chain RNA molecule ).

It is biologic unit of hereditary, self reproducing and transmittedfrom parent to progeny.

Each gene has a specific position (locus) on the chromosome.


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DNA

THE NUCLEIC ACID IN WHICH THE SUGAR DEOXYRIBOSE, constituting theprimary genetic material of all cellular organism and the DNA viruses;

and occurring predominantly in the nucleus.

DNA is duplicated by replication and it serves as atemplate for synthesis of

ribonucleic acid (RNA ) (transcription).


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Some facts about chromosomes

Number of chromosomes in each cell is fixed for a given

species and in man it is (46).

This is referred to as diploid(or double number ).

In spermatozoa and ova the number of chromosomes in onlyhalf the diploid number (23), this is called haploid or half

number .

There are (46) chromosomes in human cell (44) autosomal

(2) are sex chromosomes.

(44) autosomal chromosomes = (22) pairs.

The chromosomes forming a pair being exactly alike.


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Other facts Characters of parents are transmitted to offspring through

codes borne on strands of DNA.

Genes are made up of such strands of DNA.

Genes are located on chromosomes.

A typical cell contains (46) chromosomes = diploid number.

A gamete contains (23) chromosomes = haploid number.

The diploid number of chromosomes is restored as a result

of fertilization.


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Human chromosome with coiled and uncoiled


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Significance of chromosomes(continue)

The nature and functions of a cell depend on the

proteins synthesized by it

So the genes control the development and

functioning of cells by determining what types of

proteins will be synthesized within them.Genes play an important role in the development of tissues and organs of

the individual.

Characters ( traits) of an individual are determined

by genes carried on his (or her) chromosomes.

Half of these characters are inherited from the

father and halffrom the mother.


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Significance of chromosomes CONTINUE

Each chromosome bears on itself a very large number of

genes.

Genes are made up of a nucleic acid called DNA and all

information is stored in the molecules of this substance.

Genes are involved in synthesis of proteins.

Proteins are the most important constituents of our body,

they make up the greater part of each cell and intercellularsubstance. Enzymes, hormones and antibodies are also

proteins.


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Chromosome structure

In resting cell, the chromosomes arent visible under light microscope.

During cell division the chromatin network becomes condensed into anumber of chromosomes. (visible under light microscope)

Chromosome is made up of two

( rod shaped structure) called chromatids.Each chromatid has two arms.

Chromatids unite at an area called

centromere ( kinetochore ).


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Chromosome structure (continue)

Individual chromosome differs from one another

in totallength, relative length of the two armsand in various other characteristics and these

differences enable us to identify each

chromosome individually .

Classification of chromosomes in this way is

called KARYOTYPING.

Karyotyping makes it possible for us to detect abnormalities inchromosome number or in the individual chromosome.


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Cell

Division

MITOSIS /maitosis/ MEIOSIS /maiosis

Cell

Division

Cell

Division


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MITOSIS/maitosis/

This term is used interchangeably with cell

division,

but strictly speaking it refers to nuclear division,

whereas cytokinesis refers to division of

cytoplasm.


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Cell Division

multiplication is an essential feature for embryonic development

and is necessary after birth for growth and for replacement of

dead cells.

Chromosomes with the nuclei of cells carry genetic information that

controls the DEVELOPMENT and FUNCTIONING of various cells andtissues.

When cell divides the daughter cells must have

chromosomes identical in number ( and in

genetic content ) to those in the mother cell.

This type of division is called mitosis


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Cell Division (continue)

A different kind of cell division is called MEIOSIS occurs during

the formation of gametes.

MEIOSIS consist of two successive divisions.

The cells resulting from the first and second MEIOTICdivisions(gametes) differ from other cells of the body in that:

1. The number of chromosomes is reduced to half the normal

number.

2. The genetic information in various gametes produced isnt

identical.


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MitosisMultiplication of cells takes place by cell division.

The usual method of cell division, seen in most tissues.

Daughter cells resulting from mitotic division are similar to

the parent cells and have the same number of chromosomes

(46).

Many cells of the body have limited span of functional

activity, At the end of functional activity, cells undergo

division into two daughter cells.

The daughter cells in turn have their own span of activity;followed by another division.

The period between two successive division is called

interphase.


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Stages of MITOSIS

MITOSIS IS DIVIDED INTO FOUR STAGES

1. Prophase

2. Metaphase3. Anaphase

4. Telophase

The best phase for understanding isTELOPHASE


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Interphase

During a specific period of the interphase, the DNAcontent of the chromosome is duplicated.So that

Another chromatid is

formed.(identical to the originalone)

Chromosome now is made up of twochromatids.

In Early interphase = chromosomes are in theform of extended threads.

In Late interphase DNA of each chromosomehas undergone duplication.

Interphase

refers to all

stages of the

cell cycleother than

mitosis.


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Prophase The chromatin of the chromosome

becomes gradually more and morecoiled,

That means the chromosome becomesrecognizable as a thread-like then as arod like appearance.

Toward the and of prophase, the two

chromatids become distinct and thechromosome now has its typicalstructure.

The two centerioles separate andmove to opposite poles of the cell.

The two centerioles produce a numberof microtubules that pass from onecenteriole to the other and form aspindle.

The nuclear membrane breaks down.

The nucleoli disappear.


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Metaphase

With the formation of spindle,

chromosomes move to aposition midway between thetwo centerioles. ( at the

equator of the cell)

Each chromosomebecomes attached tomicrotubules of thespindle by itscenrtomere.


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Anaphase

The centromere splitslongitudinally into two,

The chromatids nowbecome independentchromosomes.

The cell contains (46)

pairs of chromosomes. One chromosome moves along

the spindle to either pole of thecell.


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Telophase Two daughter nuclei are

formed by appearance ofnuclear membranes. Chromosomes gradually elongate

and become indistinct.

Nucleoli reappear. Centeriole is duplicated at

this stage or in the earlyinterphase.

The division of the nucleus is

accompanied by the division of thecytoplasm = (CYTOKINESIS). In thisprocess

THE ORGANELLES are presumablyduplicated .

Each daughter cell comes to have afull complement of them.


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Meiosis

special kind of cell division takes place in the

testis and ovary for formation of gametes.

The gametes resulting from meiosis have thehaploid number of chromosomes (23).

The various gametes formed dont have thesame genetic content.


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Meiosis (continue)

Consists of two successive divisions.

They are called the 1st and 2nd meiotic divisions.

During the interphase preceding the 1st division;

DUPLICATION OF DNA as in mitosis.

Another chromatid identical to the original one is formed.

Each chromosome is made up of two chromatids.


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Meiosis

Prophase Leptotene

Zygotene

Pachytene

Diplotene

Metaphase

Anaphase

Telophase


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First MeioticDivision

The Prophase is prolonged and isusually divided into a number of stages asfollow:

1. Leptotene2. Zygotene

3. Pachytene4. Diplotene


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Leptotene

Chromosomes become visible ( as in mitosis).

Chromatids arent distinguished.


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Pachytene Chromatids become distinct.

The bivalent has (4)chromatids is called aTETRAD.

There are (2) central and (2)

peripheral chromatids onefrom each chromosome.

The (2) central chromatidsbecome coiled over eachother at a number of points

= CROSSING OVER. Chromatids become

adherent at crossing points,these points are calledchiasmata.


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Diplotene

Exchange of genetic

material between chromatids.

The two chromosomes of

bivalent move apart

This result in crossing over BREAK at the points of crossing over.

Loose pieces become attached to the opposite chromatid.


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Metaphase

As in mitosis

the (46) chromosomes

become attached to the

spindle at the equator.

The two chromosomes

of pair being close to

each other.


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Anaphase ( differs from that in mitosis).

There is no splittingof the centromeres.

One entire chromosome

of each pair moves toeach pole of the spindle.

Daughter cells have (23)

chromosomes (eachmade up of twochromatids.

T l h


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Telophase

Two daughter

nuclei are formed. The division of the

nucleus is followed

by division of the

cytoplasm Telophase- the finalstage of meiosis

Telophase 1


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Significance of Meiosis

WHY ARE NO TWO PERSONS ALIKE?

EXCEPT IDENTICAL TWINS NO TWO PERSONS ARE ALIKE!

1. There is a reduction of the number of chromosomes from

diploid to haploid. At time of fertilization the diploid number

is restored. This provides consistency of chromosome

number from generation to generation.

2. Chromosomes from mother and father are distributed

between the daughter entirely.

3. Crossing over results in thorough SHUFFLING of genetic

material (ova and spermatozoa all have a distinctive genetic

content).

4. A THIRD STEP OF SHUFFLING TAKES PLACE AT FERTILIZATION.


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Teratogen The development of defects in an embryo

Abnormalities in development can result due to exposing the

embryo to certain agents (chemical or physical).

The list of teratogens keeps increasing.

Some particular organs are most sensitive to teratogens when

they are passing through critical phase in their development. This period of great susceptibility to teratogens differs from organ

to organ.

In early stages of development , the age reckoned in days.

Later when the events are less dramatic, age can be expressed inweeks or months. However , the exact age of embryo isnt always

known.


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Teratogen (continue)

An estimate can be made by observing the size of the embryo

(expressed as C.R. length).

Some other feature like the number of somites.

There are numerous references to the timing of embryonicevents (most commonly in terms of C.R. length).

The disadvantage of doing so is that it adds yet one more

complication to understanding of an already intricate

subject.


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CRL

Drown-rump length =

the length of any embryo, fetus, or infant

from the crown of the head to the breech;

used in estimating the age of the embryos

from thefourth to the eighth week .

It is the equivalent of sitting vertex height in

older individuals.

S it


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Somite

/somt/

One of the paired, block like masses of

mesoderm, arranged segmentally alongside

the neural tube of the embryo, forming the

vertebral column and segmental musculature;called also mesodermic, mesoblastic,

primitative, primordial, or protovertebral

segment.


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This shows prophase, prometaphase,

metaphase, anaphase and telophase


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Aster short fibers produced by cells during


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Aster short fibers produced by cells during

mitosis and meiosis.

The end

Thank you

next lecture = spermatogenesis and oogenesis

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