2-fertilization & implantation

8/13/2019 2-Fertilization & Implantation

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FERTILIZATION & IMPLANTATION


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FERTILIZATION It is the process during

which a male gamete(sperm ) unites with afemale gamete ( oocyte ) toform a single cell ( ZYGOTE).


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Fertilization

Begins with acontact betweenthe sperm & theovum.

Ends up withintermingling of

the maternal andpaternalchromosomes.


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ite Usually in the ampulla of

the uterine tube. Ampulla is the longest

and widest part.

Fertilization may occur inother parts of tubes. Does not occur in the

uterine cavity. Chemical signals from

oocyte attract thesperms.


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Phases of Fertilization

1- Passage of sperm throughcorona radiata, under theeffect of : hyaluronidaseenzyme from sperms, andmovement of tail of sperm.

2- Penetration of the zonapellucida by head of spermsthrough acrosine E. fromacrosome of one sperm.

3- Fusion of the plasmamembrane of the oocyte andthat of the sperm. sosperms plasma membraneremains behind .


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Phases of Fertilization

4- Completion of thesecond meiotic division &formation of the femalepronucleus.

5- Formation of the male

pronucleus :It is a swollen nucleus of thesperm.

Its tail is detached and

degenerated.Zona reaction : it is a change inproperties of zona pellucida thatmakes it impermeable to othersperms.

zygote


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Chromosomes in zygote

Zygote is genetically unique. Half of its chromosomes come from the father and the other

half comes from the mother. zygote contains 46 chromosomes (diploid). New combination is formed which is different from either of the

parents. This mechanism forms biparental inheritance and leads to

variation of the human species.


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Sex of the Embryo

Embryo's chromosomalsex is determined at thetime of fertilization.

Sex is determined by thetype of sperm (Xor Y) that fertilizes theoocyte.

So, it is the father whosegamete decides the sex.


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Results of Fertilization

Stimulates thepenetrated oocyte tocomplete its 2 nd meiotic division.

Restores the normaldiploid number ofchromosomes in thezygote (46).

Determines the

chromosomal sex of theembryo. Initiates cleavage (cell

division) of the zygote.


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Cleavage of Zygote

Consists of repeatedmitotic divisions of thezygote.

Rapid increase in thenumber of the cells.

These smallerembryonic cells are

called Blastomeres. Normally occurs in the

uterine tube.


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Cleavage of Zygote It begins about 30 hours

after fertilization. Zygote divides into 2,

then 4, then 8, then 16cells. Zygote lies within the

thick zona pellucida during cleavage.

Zygote migrates in theuterine tube from itslateral end to its medialend.

Zona pellucida istranslucent under lightmicroscope.


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Morula When there are 32 blastomeres

the developing human is calledMORULA.

The Morula reaches the uterinecavity at this stage. Spherical Morula is formed

about 3 days after fertilization.


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Formation of blastocyst : The Morula reaches the uterine cavity by the 4 th day

after fertilization, & remains free for one or two days.Fluid passes from uterine cavity to the Morula.

Now the Morula is called Blastocyst , its cavity is calledblastocystic cavity, its cells divided into Embryoblast &Trophoblast.


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BLASTOCYST

It is formed of : (1) Trophoblast . (2) Inner cell mass. (3) Blastocyst cavity.


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Definition : It is the process by which the

Blastocyst penetrates thesuperficial (Compact) layer of theendometrium of the uterus.

Site: The normal site of implantation is

the posterior wall of theuterus near the fundus .

Time: It begins about the 6 th day after

fertilization. It is completed by the 11th or 12th day .

IMPLANTATION

6 th day


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By the 5 th day the Zona pellucida degenerates.

Blastocyst begins implantation by the 6 th day.

Trophoblast cells penetrate the epithelium of the endometrium.Penetration results from proteolytic enzymes (eg.COX-2)produced by the trophoblast .

6 th day7 th day


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Zona pellucida degenerates & disappears by the5 th day to allows the blastocyst to increase in size andpenetrates the endometrium.

The embryoblast projects into the blastocysticcavity, while the trophoblast forms the wall of theblastocyst.

By 6 th day the blastocyst adheres to the endometrialepithelium.

By 7 th day, Trophoblast differentiated into 2 layers:Cytotrophblast, inner layer, mononucleated mitotically

active cells.

Syncytiotrophoblast (outer multinucleated mass, withindistinct cell boundary.

By 8 th day the blastocyst is superficially embeddedin the compact layer of the endometrium (by the endof 1 st week,the blastocyst is superficially implanted inendometrium).

7 th day


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Blood-filled Lacunae appear inthe Syncytiotrophoblast

which communicate forming anetwork by the 10th

day or 11th

day. Syncytiotrophoblast

erodes the endothelial liningof the maternal capillarieswhich known as sinusoids.

Now blood of maternalcapillaries reaches the lacunaeso

Uteroplacental circulationis established by 11 th or 12 th day.

8 th day


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Endometrial cells undergoapoptosis (programmed celldeath) to facilitates invasion ofendometrium by theSyncytiotrophoblast.

Syncytiotrophoblast engulfthese degenerated cells fornutrition of the embryo.

Implantationcan be detected by:

1- Ultrasonography.

2- hCG (human chorionicgonadotrophin which issecreted by theSyncytiotrophoblast) about theend of 2 nd week


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2-fertilization & implantation

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