4.3 development of zygote

8/11/2019 4.3 Development of Zygote

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4.3 THE EARLY DEVELOPMENT OF

ZYGOTE IN HUMANS


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FERTILISATION

Millions of sperm are ejaculated

into the vagina during sexual

intercourse. The sperms swim up pass the

cervix into the uterus towards the

fallopian tubes.

Once a sperm penetrates theovum, it discards its tail, and its

nucleus moves towards the

nucleus of the ovum, fertilisation

occurs.

Fertilisationis the fusion of the

sperm nucleus with the ovum

nucleus to produce a zygote. It

occurs in the Fallopian tube.


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DEVELOPMENT OF ZYGOTE

The development of the zygote begin

after two days,

the zygote divides several times by

mitosis to form a solid mass of cells

called the morulafollowed by a

blastocyst which is a fluid filled sphere

with hundred of cells around it

One end of the blastocyst is made up

of mass of cells which continue to

develop into the embryo, while the remaining cells surrounding

the fluid becometrophoblast which

later develop into placenta.


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Once the blastocyst reaches the uterus,

impantation occurs.

Implantationis the attaching of the blastula

onto the endometrium of the uterus.


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Pregnancy begins at

implantation. Menstrual

does not occur & the femaleis said to be pregnant.

The embryo becomes the

foetus after eight weeks The foetus is enclosed in an

amniotic sac filled with

amniotic fluid to protectfluid to protect it from any

physical shock.


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TWINS

There are three kinds of twins:

Identical twins

Fraternal twins Siamese twins


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FORMATION OF IDENTICAL TWINS FORMATION OF FRATERNAL

TWINS


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IDENTICAL TWINS FRATERNAL TWINS

Formed when sperm fertilises an

ovum to produce a zygote.

The zygote formed divides into twozygotes which develop into two

foetuses in the uterus

Formed when two ova are released at

the same time, and are fertilised by

two different sperms.

The featouses are identical as theywere from the same zygote

The foetuses are not identical as theyare formed from different sperms &

ova

The two foetuses share the same

placenta but have their own umbilical

cord.

The two foetuses do not share the

same placenta. They have their own

placenta & their own umbilical cords.

They are genetically identical.

Therefore they are always the same

sex.

They genetically not identical.

Therefore they can be of the same or

different sex.


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Siamese twins

formed whenthe mitotic

division thatoccurs duringthe formationof identicaltwins is

incomplete. As a result the

twins arejoined at the

parts of thebody where theincompletedivision occurs.


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PLACENTA

During foetal development, the foetusdepends on the mother for food &oxygen, and to get rid its wastes

products. The placenta helps in the exchange of

substances between the foetus & themother.

The foetus is connected to theplacenta by umbilical cord.


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Placenta arises from the chorion(trophoblast atembryo stage) which extends out fingerlikeprojections called villi, into the endometrium to

make contact with the uterine blood. The finger-like villi help too increase the surface

area for the exchange of nutrients.

The membrane of the chorionic villi separates the

foetal blood from the mothers blood.

Diffusion of nutrients occur across the membraneof the chorion


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The umbilical arteries carry deoxygenated

blood, carbon dioxide & waste products from

the foetus to the placenta,

While the umbilical vein carries oxygenated

blood, nutrients, antibodies & hormones from

the mothers artery to the placenta.


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The advantages of the foetus having a

separate circulatory system from that of the

mother are;

1. To protect the foetus from bacteria infection

2. To protect the fine blood vessels of the

foetus from bursting as the mother blood

pressure is too high.

3. To prevent their blood from mixing in case

the foetus has a different blood group from

that of the mother


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However drugs, alcohol, caffeine, nicotine &

some virus like HIV & the Rubella virus are still

able to penetrate this barrier & affect the

development of the foetus.

4.3 development of zygote

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