Fertilisation & Contraception

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  • Created by: Becca
  • Created on: 29-12-13 23:50
What is fertilisation?
A sequence of co-ordinated events that begins with contact between a sperm and an oocyte and ends with intermingling of maternal and paternal chromosomes
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What pre-ovulatory changes happen in the oocyte?
LH surge, first meiotic division completed, unequal cell division: secondary oocyte (half the chromosomes, almost all the cytoplasm, 2nd meiotic division) & first polar body (remaining chromosomes, small bag of cytoplasm, degenerates subsequently)
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What is the structure of the oocyte?
Corona radiata: granulosa cells suspended in matrix produce progesterone & chemo-attractants. Haploid nucleus gets nutrients from cytoplasm, the first polar body degenerates. Zona pellucida remains as a protective shell
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What is the structure of the spermatozoa?
Spermatozoal plasma membrane contains receptors for chemo-attractants, ADAM3, sperm adhesion molecule 1 & hyaluronidase enzyme. Minimal cytoplasm in sperm head where the haploid nucleus lies. Acrosome contains enzyme acrosin
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How do spermatozoa move from the ****** to oviduct?
Undergo capacitation. Starts in vaginal environment just prior to ovulation & 2/3 days after (less acidic). Full capacity reached by time the spermatozoa travels through isthmus to ampulla region of oviduct (fertilisation occurs in ampulla)
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What does capacitation allow?
Changes in movement capability (hyperactivated motility pattern, wave-like beats, release of chemo-attractants, cilia beats towards uterus, sperm swims against tide = selective pressure) & changes in membrane properties of spermatozoon
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What changes happen in the spermatozoal surface?
Glycoproteins stripped, reduced membrane stability, enhanced fusibility. Increase internal Ca levels, loss of calmodulin-binding proteins (more responsive to Ca), increase cAMP (phosphorylation of proteins for acrosome reaction) & protein kinase A
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What happens during fertilisation?
Penetration of corona radiata - secretion of hyaluronidase, digestion of extracellular matrix & active movement to reach zona pellucida
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What is the structure of the zona pellucida?
3 sulphated glycoroteins (ZP 1/2/3). ZP1 is minor non-essential component for binding spermatozoa. ZP3 has dominant binding role but only if it's in conjunction with ZP2. The ZP2/3 3D framework is species specific & blocks cross species fertilisation
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What happens during penetration of the zona pellucida?
Ligand-receptor interactions: ZP2 & ZP3 binding sites/receptors are present of different membrane components of spermatozoa. Receptor for ZP3, ZP3R is on the surface of sperm head. Receptor for ZP2, ZP2R is on the inner acrosomal membrane
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What happens in the acrosome reaction?
Binding of ZP3 to its receptor on sperm head, leads to: Ca2+ influx, increased Ca -> depolymerization of F-actin between acrosome & spermhead plasma membrane allowing expansion of acrosome, increased cAMP, pH raised 7.1 -> 7.5
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What is the consequence of binding during the acrosome reaction?
Acrosome swelling, fusion with overlying surface plasma membrane, exteriorisation of inner acrosomal membrane+contents to outer surface, release of acrosin to digest zona pellucida, ZP2 receptor on inner membrane reveal, spermatozoon can now bind
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What happens during gamete fusion?
Oocyte microvilli envelop sperm head, bind using adhesion molecules disintegrin (sperm) & integrin (egg). Ca needed for fusion of both membranes (fusion proteins in egg). Spermatozoon sinks into oocye -> zygote formed
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What post-fusion events happen?
Establishment of diploid genetic constitution, prevention of polyspermy & avoidance of gynogenetic triploidy
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How is polyspermy prevented?
Changes in electrical activity in egg: Ca2+ waves & spikes. Cortical reaction by egg: release of cortical granules, cleavage of ZP2, hydrolysis of ZP3 (no longer available for binding), cross linking of tyrosine residues-> impenetrable zona pellucida
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How is gynogenetic triploidy avoided?
Rise in Ca in eggs allows completion of 2nd meiotic division: nucleus exits M phase, dispatches one set of chromosomes as 2nd polar body, other half remains behind in pronucleus to unite with paternal chromosomes, thus avoiding gynogenetic triploidy
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What do the spermatozoon & oocyte contribute to the zygote?
Sperm: centriole (for karyokinesis & cytokinesis, without it cell division in early development is compromised). Oocyte: cell membrane, cytoplasm, cell organelles & mitochondria (maternal cytoplasmic inheritance)
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What is parental imprinting?
Packaging of chromosomes for transmission to zygote, influences gene organisation & ability to become transcriptionally active. The wrapping (chromatin) not actual genetic code is affected. Both parental imprints needed to be fully functional
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What is gene silencing?
A subset of mammalian genes where one parent actively silences their own gene, so there is a parent-of-origin specific monoallelic expression of the gene in the offspring
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How do combined oral contraceptives work?
Oestrogen inhibits FSH release & thus follicle development. Progesterone inhibits LH release, prevents ovulation, alters cervical mucus & motility in fallopian tube. Both alter endometrium, reducing chance of implantation
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Other cards in this set

Card 2

Front

What pre-ovulatory changes happen in the oocyte?

Back

LH surge, first meiotic division completed, unequal cell division: secondary oocyte (half the chromosomes, almost all the cytoplasm, 2nd meiotic division) & first polar body (remaining chromosomes, small bag of cytoplasm, degenerates subsequently)

Card 3

Front

What is the structure of the oocyte?

Back

Preview of the front of card 3

Card 4

Front

What is the structure of the spermatozoa?

Back

Preview of the front of card 4

Card 5

Front

How do spermatozoa move from the ****** to oviduct?

Back

Preview of the front of card 5
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