12 Fertilisation and contraception

Question 1

Define fertilisation

Answer 1

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)

Question 2

Answer 2

Question 3

Where is the site of fertilisation?

Answer 3

Ampulla of fallopian tube

Question 4

Describe the features of an ovulated oocyte

Answer 4

• 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

Question 5

What are the features of spermatozoa?

Answer 5

Acrosome: contains enzyme acrosin

plasma membrane

minimal cytoplasm

Mid piece has mitochondria

Question 6

What are the features of the plasma membrane in spermatozoa

Answer 6

• 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)

Question 7

What is an ADAM

Answer 7

A disintegrin and metalloproteinase domain containing protein

Question 8

Why is capacitation needed and when is full capacity reached?

Answer 8

To attain full fertilisation capacity

Full capacity reached by the term spermatozoa travels from isthmus to ampulla

Question 9

Discuss movement of spermatzoa from vagina to oviduct

Answer 9

1. Starts in vaginal environment when it is oestrogen primed: pH <5.7

Just prior to ovulation and 2-3 days after

2. 99% spermatozoa do not enter cervix

3.

Question 10

When can sperm penetrate into cervix/ oviduct

Answer 10

When there is thin mucus

absence of progesterone dominated mucus and oestrogen is dominant

Question 11

What are the two changes when capacitation of spermatozoa occurs?

Answer 11

1. Changes in movement characteristics
2. Changes in spermatozoal surface by stripping of glycoproteins

Question 12

What are the changes in movement characteristics during capatication

Answer 12

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

Question 13

What is a substance formed during ejaculation that is a potent capatication agent

Answer 13

Hydrogen peroxide-> exposure of atmospheric oxygen during ejaculation

Question 14

What is the role of hydrogen peroxide with low pH during capacitation?

Answer 14

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

Question 15

What needs to happen once capacitation is complete?

Answer 15

Must find oocyte rapidly or will die

metastable

Question 16

What are the 3 steps of fertilisation?

Answer 16

1. Penetration of corona radiata
2. Penetration of zona pellucida
3. Acrosome reaction

Question 17

Fertilisation: penetration of corona radiatia

Answer 17

Secretion of hyaluronidase

digestion EC matrix

Active movement to reach ZP

Question 18

Structure of ZP

Answer 18

• 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

Question 19

Fertilisiation: penetration of ZP/ attachment

Answer 19

ZP3 R surface sperm head

ZP2R inner acrosomal membrane

Question 20

Fertilisation: acrosome reaction

what does the binding of ZP3 and ZP3R lead to?

Answer 20

• 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

Question 21

fertilisation: acrosome reaction

what is the consequence of binding?

Answer 21

1. Acrosome swelling
2. Acrosome fusion with overlying plasma membrane -> vesiculated appearance
3. Exteriorisation of contents and inner acrosomal membrane (IAM) by exocytosis-> Exposure of inner ZP2R
4. ZP2 binds ZP2R
5. Mechanical propulsion along ZP2 filaments until oocyte plasma membrane surface is reached
6. Acrosin release aids digestion of a pathway through ZP

(mechanism shown in mice similar to human)

Question 22

what happens after ZP penetration when gamete fuses

Answer 22

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

Question 23

which adhesions molecules are involved in gamete fusion binding in sperm and egg

Answer 23

sperm:

• disintergrins from ADAM (fertilin alpha, beta)
• izumo 1

egg:

• Integrins (a6b1)
• Cd9 (large membrane spanning molecule)

Question 24

post fusion events

Answer 24

1. At fertilisation, oocyte arrested in Metaphase II
2. Increase in Ca2+ after fusion -> exit from M phase
3. One set chromosomes dispatched as 2^nd polar body
4. Other half set 23 unpaired chromosomes remain behind female pronucleus -> can unite 23 paternal chromosomes

Question 25

how is gynogenetic triploidy avoided by the egg?

Answer 25

dispatching second polar body

Question 26

how is triploigy avoided by spermatazoon binding to egg?

Answer 26

* spermatazoon does not bind to egg immediately overlying second metaphase spindle (devoid of binding proteins) * avoid encounter of second polar body

Question 27

what does triploidy result in?

Answer 27

fetal cell death

Question 28

what reaction is needed to prevent polyspermy?

Answer 28

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

Question 29

what leads to the corticol reaction?

Answer 29

change in electrical activity or membrane potential of zygote -\> Ca2+ released from internal stores -\> calcium spikes leading to corticol reaction

Question 30

define aneuploidy

Answer 30

=abnormal number of chromosomes by loss or duplication -loss of chromosomes is lethal

Question 31

common trisomies

Answer 31

* Trisomy 21 (Downs Syndrome) – most common ; * 18 (Edwards syndrome also common) * 13 and X less common. XXX XXY XYY can be identified using FISH

Question 32

what does fertilising spermatazoon and oocyte contribute?

Answer 32

sperm: centriole, essential for kayro and cytokinesis oocyte: cell membrane, cytoplasm, mitochondria (maternal cytoplasmic inheritance)

Question 33

when does activation of embyronic genome occur and what dominates prior to this stage?

Answer 33

4-8 cell stage maternal RNA dominates prior, so any protein synth in zygote is maternally regulated in early stages

Question 34

consequences of maternally derived mitochondria?

Answer 34

genetically based defects of mitochondrial function would be transmitted to offspring

Question 35

define imprinted genes

Answer 35

= 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_

Question 36

parental imprinting: insuling life growth factor

Answer 36

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 I*GF2 *(competitive inhibition). Mum can influence growth*

Question 37

parental imprinting: suckling behaviour

Answer 37

smiling in babies- paternally expressed gene

Question 38

post fertilisation transport of conceptus

Answer 38

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

Question 39

hormonal contraception

Answer 39

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

Question 40

alternative contraceptions

Answer 40

* 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)