12 Fertilisation and contraception Flashcards
Define fertilisation
A sequence of coordinated events that begin with a sperm and an oocyte (haploid, 23 chromosomes) and ends with intermingling of maternal and paternal chromosomes (diploid 46 chromosomes)
Where is the site of fertilisation?
Ampulla of fallopian tube
Describe the features of an ovulated oocyte
-
Granulosa cells= corona radiata suspended in hyaluron rich matrix
- releases progesterone and chemo attractants (aromatic aldehydes)
- Secondary oocyte obtains nutrients from cytoplasm
- first polar body degenerates
- zona pellucida remains as protective shell
What are the features of spermatozoa?
Acrosome: contains enzyme acrosin
plasma membrane
minimal cytoplasm
Mid piece has mitochondria
What are the features of the plasma membrane in spermatozoa
- Odorant receptors (similar to olfactory) react to chemo-attractants of oocyte
- 3 surface binding molecules- ADAM family
- Fertilin alpha
- fertilin beta
- Cyritestin
- can secrete hyaluronidase (degrades hyaluronic acid in corona radiata)
What is an ADAM
A disintegrin and metalloproteinase domain containing protein
Why is capacitation needed and when is full capacity reached?
To attain full fertilisation capacity
Full capacity reached by the term spermatozoa travels from isthmus to ampulla
Discuss movement of spermatzoa from vagina to oviduct
- Starts in vaginal environment when it is oestrogen primed: pH <5.7
Just prior to ovulation and 2-3 days after
- 99% spermatozoa do not enter cervix
3.
When can sperm penetrate into cervix/ oviduct
When there is thin mucus
absence of progesterone dominated mucus and oestrogen is dominant
What are the two changes when capacitation of spermatozoa occurs?
- Changes in movement characteristics
- Changes in spermatozoal surface by stripping of glycoproteins
What are the changes in movement characteristics during capatication
Hyperactivated motlility pattern
Regular wave like (vagina to cervix) changes to whiplash beats to swim from isthmus to ampulla
Sperm swims against tide of ciliated oviduct- selection pressure
What is a substance formed during ejaculation that is a potent capatication agent
Hydrogen peroxide-> exposure of atmospheric oxygen during ejaculation
What is the role of hydrogen peroxide with low pH during capacitation?
Changes in surface charge, macromolecular organisation, loss of cholesterol
- Reduced stability of plasma membrane and enhanced fusibility
- Increased permeability to calcium:
- Increased internal Ca2+ levels; loss calmodulin binding proteins
Increased cAMP
Phosphorylation of proteins- PKA
Increased phosphorylation of tyrosine kinase needed for acrosome reaction
Actin polymerisation (F-actin) between (acrosome) surface membrane and plasma membrane to prevent premature acrosome reaction
What needs to happen once capacitation is complete?
Must find oocyte rapidly or will die
metastable
What are the 3 steps of fertilisation?
- Penetration of corona radiata
- Penetration of zona pellucida
- Acrosome reaction
Fertilisation: penetration of corona radiatia
Secretion of hyaluronidase
digestion EC matrix
Active movement to reach ZP
Structure of ZP
- Sulphated glycoproteins: ZP1,2,3 4 (humans)
- ZP3 dominant binding role but only in conjunction ZP2
- ZP 2/3 human specific to prevent cross species fertilisation
Fertilisiation: penetration of ZP/ attachment
ZP3 R surface sperm head
ZP2R inner acrosomal membrane
Fertilisation: acrosome reaction
what does the binding of ZP3 and ZP3R lead to?
- Calcium influx-> depolarisation of F actin between acrosome and sperm head plasma membrane-> acrosome can expand
- Increase in calcium-> increased cAMP-> increase pH 7.1 to 7.5
fertilisation: acrosome reaction
what is the consequence of binding?
- Acrosome swelling
- Acrosome fusion with overlying plasma membrane -> vesiculated appearance
- Exteriorisation of contents and inner acrosomal membrane (IAM) by exocytosis-> Exposure of inner ZP2R
- ZP2 binds ZP2R
- Mechanical propulsion along ZP2 filaments until oocyte plasma membrane surface is reached
- Acrosin release aids digestion of a pathway through ZP
(mechanism shown in mice similar to human)
what happens after ZP penetration when gamete fuses
spermatazoon is tangential to oocyte
- oocyte microvilli envelope sperm head
2. sperm-oocyte binding
3. specific areas on egg are rich in intergrins to allow binding in correct areas to avoid triploidy
4. spermatazoon sinks into oocyte and zygote is formed
which adhesions molecules are involved in gamete fusion binding in sperm and egg
sperm:
- disintergrins from ADAM (fertilin alpha, beta)
- izumo 1
egg:
- Integrins (a6b1)
- Cd9 (large membrane spanning molecule)
post fusion events
- At fertilisation, oocyte arrested in Metaphase II
- Increase in Ca2+ after fusion -> exit from M phase
- One set chromosomes dispatched as 2nd polar body
- Other half set 23 unpaired chromosomes remain behind female pronucleus -> can unite 23 paternal chromosomes
how is gynogenetic triploidy avoided by the egg?
dispatching second polar body
how is triploigy avoided by spermatazoon binding to egg?
- spermatazoon does not bind to egg immediately overlying second metaphase spindle (devoid of binding proteins)
- avoid encounter of second polar body
what does triploidy result in?
fetal cell death
what reaction is needed to prevent polyspermy?
corticol reaction = release of corticol granules into perivitelline space
- enzymes cleave ZP2 and hydrolyses binding region of ZP3
- ZP2,3 and 4 no longer available for for further sperm binding
- Tyrosine residues on adjacent ZPs cross link
- zona becomes indissoluble and impenetrable to spermatozoa
what leads to the corticol reaction?
change in electrical activity or membrane potential of zygote -> Ca2+ released from internal stores
-> calcium spikes leading to corticol reaction
define aneuploidy
=abnormal number of chromosomes by loss or duplication
-loss of chromosomes is lethal
common trisomies
- Trisomy 21 (Downs Syndrome) – most common ;
- 18 (Edwards syndrome also common)
- 13 and X less common. XXX XXY XYY
can be identified using FISH
what does fertilising spermatazoon and oocyte contribute?
sperm: centriole, essential for kayro and cytokinesis
oocyte: cell membrane, cytoplasm, mitochondria
(maternal cytoplasmic inheritance)
when does activation of embyronic genome occur and what dominates prior to this stage?
4-8 cell stage
maternal RNA dominates prior, so any protein synth in zygote is maternally regulated in early stages
consequences of maternally derived mitochondria?
genetically based defects of mitochondrial function would be transmitted to offspring
define imprinted genes
= in a percentage of genes, only inherit one working copy, other is silent (copy from mother or father epigenetically silenced)
- Silencing: addition of methyl groups. epigenetic tags on imprinted genes remain for life of organism.
- direct phenotype expression of the allele that is not being silenced
parental imprinting: insuling life growth factor
IGF2: paternally expressed.
- It binds to its receptor- R1 to promote growth. present in the placenta and offspring. (Mums IGF2 copy is silent).
Dad is ensuring growth and good nutrient transfer to baby.
(theory of Intra-genomic conflict : “Dad wants you big whilst mum wants you small!”)
second receptor for IGF2 can bind to it (weaker affinity). This R2 is maternally expressed and can mop up IGF2 (competitive inhibition). Mum can influence growth
parental imprinting: suckling behaviour
smiling in babies- paternally expressed gene
post fertilisation transport of conceptus
Endocrine: rising ratio of progesterone to oestrogen.
Mechanical: Cilia of tubal epithelium pushes zygote towards uterus
Implantation occurs in uterine endometrium.
Conceptus continues cell divisions as it travels to uterus
hormonal contraception
Combined oral contraceptives
- Oestrogen: inhibits FSH release – inhibits development of follicle
- Progestogen: inhibits LH release, prevents ovulation, alters cervical mucus, motility in fallopian tube
- Endometrial changes do not occur reducing chance of implantation
Progestogen only oral contraceptive
alternative contraceptions
- Transdermal hormonal contraception
- Intramuscular injections
- Subdermal implants
- Morning after pill
- Intra-uterine implanted devices
- Barrier methods (male condoms, female condom)
- Diaphragm with spermicide; cervical cap
- Sterilization, Vasectomy (men)
- Natural Methods: The rhythm method, coitus interruptus, celibacy
- Hormonal contraceptives for men (still not available)
