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Allows gamete interaction in the tubalvastness...with one oocyte
Allows for procreation with isolated act ofcoitus
Allows fertilization by the fittestsperm
Blocks multiple fertilization
Allows for genetic recombination
Fertilization is a process that:
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sexualactivity
gamete
transport
Fertilization
Male Female
gametogenesisspermatogenesis ovulation
patent vas
and urethra
patent fallopiantubes / normalcervix and uterus
erection andejaculation
coitus appropriate
of ovarian cycle
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Adequate # of healthysperms produced bysem. tubules
deposited at theupper vagina
penetrate cervicalmucous
travel throughuterus
Reach ampullary region
of fallopian tubes
FERTILIZATION
Fertilization
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The oocyte is viable for 12 to 24 hours
Sperm is viable 24 to 72 hours
For fertilization to occur, coitus mustoccur no more than:
Three days before ovulation 24 hours after ovulation
Fertilization
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Fertilization
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Gametogenisis (Oogenesis, Spermatogenesis)
Sperm and oocyte transport though the femalereproductive tract
Sperm capacitation, chemotaxis. Sperm penetration through the cumulus
Acrosome reaction, zona binding
Penetration of the zona
Sperm-oocyte membrane fusion
Cortical + zona reactions (polyspermy block)
Pronuclei formation/polar body formation
Steps in Human Fertilization
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Fate of ejaculated sperm Leak out of the vagina immediately after
deposition
Destroyed by the acidic vaginalenvironment Fail to make it through the cervix Dispersed in the uterine cavity or
destroyed by phagocytic leukocytes Reach the uterine tubes
Sperm Transport
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Sperm attrition in the femalereproductive tract
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Sperm ascend in cervical mucousmicrostructure - filtering action?
Sperm found in tube/peritoneal cavitywithin 5 minutes
Non-capacitated sperm stored in cervicalcrypts
Fertilizable lifespan ~ 72 hours
Sperm transport in the femalereproductive tract starts process ofcapacitation
Sperm Transport
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Freshly ejaculated sperm are incapable offertilizing an egg
Sperm capacitation acquisition of
- the ability to undergo the acrosomereaction
- the ability to bind the zona pellucida
- hypermotility
Sperm Capacitance
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The sperm membrane becomes lessstable
The sperm loses epididymally-derivedcoating factors The sperm gains the ability to respond to
oocyte chemotactic factors
The sperm develops a finite andshortened lifespan
With sperm capacitation;
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Oocyte enters fallopian tube within 15minutes of ovulation
Fimbriae find oocyte-granulosa complexadherent to ovarian surface
Oocyte fertilizable lifespan 24 hrs max
Oocyte Transport
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An ovulated oocyte is encapsulated by:
The corona radiata and zona pellucida Extracellular matrix
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Sperm binds to the zona pellucida and
undergoes the acrosomal reaction
-Enzymes are released near the oocyte- Hundreds of acrosomes release their
enzymes to digest the zona pellucida
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Acrosome of sperm contain hyaluronidase
an enzyme that digest a channel through thezona pellucida spermatozoa fuses with theova cell membrane
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Fusion of sperm and oocyteplasma membranes
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If more than one sperm is to penetrate theova the genetic complement would be 3n.
To prevent multiple sperm penetraion two
responses have evolved in the egg.
Fast block to polyspermy membranedepolarization prevents sperm from fusingwith the oocyte membrane
Slow block to polyspermy zonal inhibiting
proteins
Blocks to Polyspermy
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when the first sperm head penetrates the egg
massive influx of Na+is triggered
depolarization of the egg
Makes it positive inside
Repels positively charged sperms
Inhibit penetration of more sperms
Fast block to Polyspermy
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Depolarization of the egg
will repel positivelycharged sperms
triggers influxof Ca2+
exocytosis ofcortical vesicles
contents of vesicles,surround egg, swell with
water and gel
block entry ofother sperms
Fast block to Polyspermy
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Zonal inhibiting proteins (ZIPs):
- Destroy sperm receptors
- Cause sperm already bound toreceptors to detach
Slow block to Polyspermy
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Completion of Meiosis II and Fertilization
Upon entry of sperm, the secondaryoocyte:- completes meiosis II- casts out the second polar body
The ovum nucleus swells, andthe two nuclei approach eachother
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Completion of Meiosis II and Fertilization
When fully swollen, the two nuclei arecalled pronuclei
Fertilization when
the pronuclei fusetogether
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3 hours: meiosis II completed-second polarbody extruded
10 hours: protein synthesis begins
Maternal legacy:maternal mRNA involved in
early embryo protein synthesis(to blastocyst stage)
Post-fertilization events
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Preembryonic Development
The first cleavage produces two daughtercells called blastomeres
Morula the 16 or more cell stage(72 hours old)
By the fourth or fifth day the preembryo
consists of 100 or so cells
blastocyst
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IMPLANTATION
The blastocyst floats freely for abouta day in the uterine cavity before it implants( 5-7 days after ovulation)
3 day period of uterine receptivity(high P4/E2 ratio)
implantation leads to full-term pregnancy
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Inner cell mass will developinto the fetus
Trophoblasts will develop
into the placenta and externalmembranes.
Blastocyst
Implantation the floatingblastocyst has to becomeattached to theendometrium.
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Primitive Trophoblast differentiates into:
Cytotrophoblast
cells of the inner layer thatretain their cell boundariesSyncytiotrophoblast cells in the outer layerthat lose their plasma membranes and invade theendometrium
IMPLANTATION
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embryo
attach tomaternal tissue
fetal membranes
IMPLANTATION
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Embryonic/fetal cells are separated frommaternal tissues and blood by the layer ofcytotrophoblasts and syncytio-
trophoblasts.
Critical for nutrient exchange and toprotect the developing fetus frommaternal immunologic attack.
IMPLANTATION