5th Lecture 1st Week of Fertilization

7/27/2019 5th Lecture 1st Week of Fertilization

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1st week of development

ovulation to implantation

Dr. Eman khammas Al-sadiEmbryology lecturer


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1st week of developmentovulation to implantation

Ovarian cycle

Monthly cycle starts at puberty Controlled by the hypothalamus----release GnRH acts on the cells of Ant.

Pituitary gland,

Which secrete GT(FSH, LH)they stimulate, control cyclic changes in the ovary

Gonadotropin-releasing hormone


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15-20 primordial follicles growswith each ovarian cycle under theinfluence of FSH

Normally one follicle reaches maturity

Only one oocyte is discharged, theothers degenerates /&become atretic

Corpus atreticum:Synonym for atretic ovarian follicle ... Afollicle that degenerates before coming

to maturity.When a follicle becomesatretic( the oocyte &the surroundingfollicular cells) degenerate & arereplaced by connective tissue, formingcorpus atreticum.


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few days before ovulation

Under influence of FSH&LH

1. the graffian follicle grows rapidly to a diameter of 25mm

2. The primary oocyte(in diplotin stage)resumes & finishes it is 1stmeiotic division

3. The surface of the ovary begins to bulge locally, & at the apex a vascularspot(stigma) appears.

ovulation


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A- FSH stimulates Maturation & proliferation

of the granulosa cellssurrounding the oocyte

B- Granulosa & thecacells produceestrogen

What are the functions

of estrogen? 1-cause the uterine

endometrium to enterthe follicular orproliferative phase.

2-causing thinning of thecervical mucus to allowpassage of the sperm.

3-stimulate the pituitarygland to secrete LH


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At mid cycle :There is an LH surge that

1-elevates concentration of (maturation promoting factor,) causing oocyte

to complete meiosis 1 & initiate meiosis 2

2-stimulates production ofprogesterone by follicular stromal which

stimulates local muscle contraction in the ovarian wall to extrude theoocyte as (comulus oophorus )

LH stimulate( lutinization)which causes follicular rupture & ovulation by

increasing the level of collagenase activity which digest the

collagen fibers surrounding the follicle


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From this region (C.O) float out of the ovary,=(ovulation)

at this moment starts it is 2nd meiotic division

some of the (c.o) cells arranges themselves around the Zunapellucida to form the (corona radiata).


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after ovulation ,immediatelyoocyte

showing the spindle of 2nd meiotic

division

Ovulationoccurs when the Grafian follicle

ruptures, releasing the oocyte. At the time

of ovulation the oocyte

- is a secondary oocyte

- is surrounded by a zona pellucida

- is surrounded by the corona radiata

- has a polar body within the zona

pellucida


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Clinical correlate

during ovulation: 1-Middle pain :some women have slight pain

normally near the middle of menstrual cycle.

2-rise in body temperature

3- some women fail to ovulate:

because. Of low level of gonadotropin

.s.t medication with these drugs can cause

ovulation but increases incidence of multiplepregnancy.

Up to 10 times as high as women in generalpopulation


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Following ovulation:1. Granulosa cells +theca

interna cells , becomepolyhedral

2. Under influence of LHpolyhedral cells change in toluteal cells forming thecorpus luteum & secrete the

hormone progesterone

3. Progesterone &estrogenichormone, causes the uterinemucosa to enter the secretary

stage in preparation forimplantation of theembryo


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oocyte transport Shortly before ovulation:

1-fimbria of the oviduct begin to cover the surface of theovary.

2-the tube begins to contracts rhythmically.

3-start sweeping movement of the fimbria.

4-The tube & the cumulus cellswith draw their cytoplasmicprocesses from the zuna pellucida & loss contact with theoocyte

5-Contraction of the uterine tube push the oocyte towardsthe lumen.

6- endocrine status affects the rate of transport during&after ovulation

7-In human fertilized oocyte reaches uterine tube lumen inabout(3-4 days)

C


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Corpus a can when fertilization dose not occur

amass of fibrotic scar tissue due to corpus luteum

degeneration when fertilization dose not occur 9 days afterovulation as it reaches maximum development.

Progesterone decreases -precipitating menstrual bleeding


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FSH LH

0 7 14 21 28 days

proliferative phasesecretory phase

Menstrual & recovery phase

0 7 14 21 28 days

ovulation

estrogen

progesterone


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If the oocyte is fertilized:trophoblast of the developing embryo will secrete (hCG) which prevent the

degeneration of the C.L The corpus luteum grows &form (corpus luteumgraviditatus)By the 3rd month this structure=1/3 0r of the total size of

the ovary

The yellow luteal cells secrete progesterone until the end of the 4 thmonth then regress slowly As secretion of progesterone by the

trophoblastic component of the placenta becomes adequate for

maintenance of pregnancy

degeneration of the C.L is prevented by human chorionic gonadotropin(hCG)i.e. secreted by the PLACENTA

f f l


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If fertilization occurs:

- the oocyte completes the second meiotic division - the zygote develops into a conceptus

implantation occurs 5-6 days after fertilization

Following implantation, the conceptus secretes Human ChorionicGonadotrophin (HCG), which is very similar to LH and has similar effects. Thiscauses the corpus luteum to continue to proliferate and to secrete increasinglevels of progesterone so that the secretory endometrium is maintained.


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fertilizationProcess by which male& female gametes fusesin the ampullary region of the uterine tube

Spermatozoa pass rapidly from the vagina---uterus---isthmus

Spermatozoa to fertilize oocyte in the female

genital tract undergoes:1- capacitation2-acrosome reaction


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Capacitation:

Isa period of conditioning in the female genital tract that

in human last 7hrs.mainly in uterine tube

During this time

a glycoprotein coat &a seminal plasma proteins are

removed from the plasma membrane that overlies theacrosomal region of the spermatozoa to pass throughthe corona cells & undergo the acrosome reaction


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The acrosome reactionWhich occurs after binding to zuna pellucida is

induced byzuna proteins, this reaction causes the release of

enzymes acrosin & trypsin-like substances needed topenetrate zuna pellucida,
http://en.wikipedia.org/wiki/File:Acrosome_reaction_diagram_en.svg


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Phases of fertilizationPhase 1:Penetration of the corona radiata

Pgase2:Penetration of the zuna pellucida.

Phase 3:Fusion of the oocyte & the sperm cellmembrane


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Phase1

penetration of the corona radiata200million-300million spermatozoa deposited in

the female genital tract, only300-500 reaches thesite of fertilization

Only one of these fertilize the egg(capacitatedsperm pass freely through corona cells.


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Phase 2:penetration of the zuna pellucidaThe zuna is a glycoprotein shell surrounding the egg that

1-facilitate sperm binding&induce acrosome reaction :Release of the acrosomal enzymes (acrosin ) Allows one sperm to

penetrate the zuna coming in contact with (plasma membrane of theoocyte) which stimulate the release of lysosomal enzymes from corticalgranules lining the membrane& penetrate the oocyte while other sperms

been found embedded in pellucida.
http://en.wikipedia.org/wiki/File:Acrosome_reaction_diagram_en.svg


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Fusion of the oocyte & sperm cell

membraneInitial adhesion is mediated in part by:

Interaction of integrins on the oocyte & theirligands, disintegrins on sperm


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Thank you

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5th Lecture 1st Week of Fertilization

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