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Embryology > Twinning & Epigenetics > Flashcards

Twinning & Epigenetics Flashcards

1
Q

What is meant by “zygosity”?

A
  • the zygote is the first structure that is formed post-fertilisation
  • zygosity describes the number of zygotes that will be formed
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2
Q

Why has there been an upward trend in the prevalence of multiple births recently?

A
  • increasing age of mothers at the time of birth
  • increasing use of fertility treatments, including ARTs
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3
Q

What is meant by monozygotic, dizygotic and polyzygotic?

A

Monozygotic:

  • this describes multiple pregnancies from a single zygote
  • this produces identical twins (or more) as they originate from the same zygote

Dizygotic:

  • this describes multiple pregnancies from 2 different zygotes
  • this produces non-identical twins as 2 separate sperm have fertilised 2 separate oocytes to produce different zygotes

Polyzygotic:

  • this describes multiple pregnancies from 2 or more zygotes
  • this can produce a combination of both identical and non-identical twins
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4
Q

What % of twin pregnancies are dizygotic?

How does this occur?

A
  • 90% of twins are dizygotic or “fraternal”
  • there is simultaneous shedding of 2 oocytes and fertilisation by different spermatozoa
  • the twins have a completely different genetic make-up so have no more resemblance than other brothers and sisters
  • they can be the same or different sex
  • the 2 ova do not have to be fertilised during the same act of sexual reproduction - it can occur on 2 different occasions
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5
Q

What factors can make twinning more likely to occur?

A
  • increased levels of FSH can cause more than one dominant follicle to mature and ovulate

multiple oocytes are ovulated each menstrual cycle, making superfecundation more likely

superfecundation is the fertilisation of 2 or more ova from the same cycle by sperm from separate acts of sexual intercourse

  • the likelihood of twins increases with age - incidence doubles in those > 35
  • there is a correlation with increased parity (number of previous births)
  • fertility procedures, such as ARTs, make twinning more likely as more than one fertilised oocyte is introduced into the reproductive tract
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6
Q

What is heteropaternal superfecundation?

How does it occur?

A
  • this describes dizygotic twins that have different fathers
  • it occurs when the mother has had sex with more than one man during ovulation where 2 ova were released
  • more than one oocyte from the same cycle is fertilised (superfecundation) by 2 different sperm (dizygotic) to produce 2 zygotes
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7
Q

What is meant by superfoetation?

Why can it occur?

A
  • this occurs when a second zygote implants into the uterus after another pregnancy has already begun
  • these 2 different zygotes are technically not twins
  • fertilisation occurring at a slightly different time point can lead to implantation occurring at a slightly different time point
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8
Q

What are the potential problems associated with superfoetation?

In what situation are these problems more likely?

A
  • there is a marked delay in growth and maturation of the second foetus
  • uterine contractions for one birth will influence the other birth during gestation, which can cause preterm birth of the second foetus
  • if superfoetation occurs during the same menstrual cycle, both foetuses will be at a similar stage in development
  • it causes more problems when fertilisation of the second oocyte occurs during a separate menstrual cycle
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9
Q

What is meant by monozygotic twinning?

A
  • monozygotic twins occur when a single ovum is fertilised by a single sperm to produce one zygote
  • there is then splitting of the zygote, which can occur at various stages in development
    • It usually occurs at the early blastocyst stage
  • this produces 2 individuals with “identical DNA”
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10
Q

What causes monozygotic twinning?

What influences the stage at which the zygote splits?

A
  • the actual cause of monozygotic twinning is unknown
  • the stage at which the zygote splits determines the extent to which the foetuses share membranes
    • the later in development the splitting occurs, the more sharing of support structures there is
  • it is thought that damage to the ICM could possibly lead to splitting of the early zygote
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11
Q

What has findings from IVF research found about possible causes of monozygotic twinning?

A
  • damage to the zona pellucida prior to implantation may induce twinning
  • blastomeres may become discordant (e.g. some may contain a mutation) and migrate away from “foreign” material
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12
Q

What are the 3 possible outcomes of triplets?

A
  • fraternal / non-identical triplets occur when 3 separate ova are fertilised by 3 different sperm
  • identical triplets can occur if a single zygote splits into 3 (rare)
  • there can be one set of identical twins plus one that is non-identical

this occurs when there is fertilisation of 2 ova by 2 different sperm and then splitting of one of the zygotes

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13
Q

What are the possible combinations resulting in quads?

A
  • 4 separate ova being fertilised by 4 different sperm
  • 2 sets of identical twins - fertilisation of 2 ova which then both split
  • 1 set of identical triplets + 1 non-identical - fertilisation of 2 ova with 1 splitting into 3
  • 1 set of identical quads if a single zygote splits into 4
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14
Q

How is the extent of sharing of foetal membranes in monozygotic twins determined?

What is the risk of having only one amniotic cavity?

A
  • the extent of sharing of foetal membranes depends on when in development the zygote splits
  • if there is sharing of membranes, there needs to be adequate sharing of the blood supply to allow for perfusion of both twins
  • if there is only one amniotic cavity, this reduces the space available and can lead to compression of the umbilical cord of one of the twins
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15
Q

How do the twins implant if there is splitting of the zygote at the 2-cell stage (day 0-3)?

A
  • splitting early in development leads to the formation of 2 blastocysts
  • the blastocysts implant separately into the uterine lining
  • each blastocyst develops its own amniotic and chorionic cavity
  • there is development of 2 placentas
  • this is known as diamniotic and dichorionic twinning
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16
Q

What is the extent of membrane sharing if the zygote splits at the early blastocyst stage (day 4-7)?

A
  • the ICM splits within the same blastocyst cavity
  • this means that each embryo has its own amniotic cavity, but they share a chorionic cavity
  • there is a single shared placenta
  • this is a monochorionic and diamniotic pregnancy (most common)
17
Q

What is the outcome of membrane sharing if the zygote splits at the late blastocyst stage (day 7-14)?

A
  • this is rare and involves splitting at the bilaminar disc stage
  • the embryos share the same amniotic cavity and choronic cavity
  • there is a single shared placenta
  • this is a monochorionic and monoamniotic pregnancy
18
Q

What are the 2 possible outcomes of sharing of foetal membranes in dizygotic twins?

A
  • if the 2 separate blastocysts are far enough apart, they will implant separately into the uterine lining and remain separate

this produces a diamniotic and dichorionic pregnancy as each foetus develops its own amniotic and chorionic cavity

  • if the blastocysts implant close to each other then the chorion can fuse

this produces a diamniotic and monochorionic pregnancy

each foetus has its own amniotic cavity but there is a single shared placenta

19
Q

Why is inspection of foetal membranes an unreliable way to determine zygosity?

A
  • in dizygotic twinning, sharing of the chorion can occur if the blastocysts implant too close to each other

this produces a monochorionic pregnancy in which there is sharing of foetal membranes

  • the same outcome could have developed from a monozygotic pregnancy in which there is splitting of the zygote at the blastocyst stage
20
Q

What is the underlying cause of the risks associated with monochorionic twins?

A
  • many of the risks associated with monochorionic twinning comes from the fact that the twins share a placenta
21
Q

Why is the incidence of twinning thought to be much higher than the number observed at birth?

A
  • many twins die before birth
  • the term “vanishing twin” refers to the death of one foetus, which occurs in the first trimester or early in the second trimester
  • the disappearance results from resorption or formation of a foetus papyraceous
22
Q

How common is twin-twin transfusion syndrome (TTTS) and why does it occur?

A
  • occurs in 15% of monochorionic pregnancies
  • placental vascular anastomoses, which occur in a balanced arrangement in most monochorionic pregnancies, are formed
  • this leads to one twin receiving most of the blood flow and the other being underperfused
  • death of both twins occurs in 50-70% of cases
23
Q

Why is the mortality rate for both twins so high in TTTS?

A

Underperfused twin:

  • this twin becomes smaller, hypovolaemic and hypoxic

Overperfused twin:

  • this twin becomes larger and polycythaemic
  • they are also polyuric with polyhydraminos
    • ​they have more RBCs, are producing more urine and amniotic fluid and there is more rapid development of internal organs
  • as a result of increased blood flow, there is an increased risk of heart failure and this condition is often associated with congenital heart defects
24
Q

What happens in twin reverse arterial perfusion (TRAP)?

What is the outcome of this usually like?

A
  • it occurs when one of the twins fails to develop a heart - this is the acardiac monster
  • the acardiac monster acts as a parasite and causes the normal twin to pump blood into it - this is the “pump twin”
  • the normal “pump” twin has a high risk of mortality due to heart failure
    • there is increased pressure on the heart as it is essentially supplying blood to 2 individuals
  • the acardiac monster often has other abnormalities, such as absence of a head and/or limbs
25
Q

What are most of the risks associated with monoamniotic twinning due to?

A
  • the risks associated with monoamniotic twinning comes from the fact that they share an amniotic cavity
  • this is associated with increased mortality
  • there is limited space, which leads to entanglement or compression of the umbilical cord
26
Q

When does spontaneous foetal reduction (“vanishing twin”) occur usually?

What is a foetus papyraceus?

A
  • there is spontaneous abortion of one of the twins, leading to a single birth
  • the twin usually aborts early on in development and it is resorbed by the body
  • if death occurs during the first trimester, it can produce a “paper-doll” foetus found compressed onto the placenta

a mummified foetus is found flattened between the membranes of the other foetus and uterine wall

27
Q

Why does conjoined twinning occur?

A
  • conjoined twinning occurs as a result of partial splitting of the primitive node and streak at later stages of development
  • the nature and degree of their union depends on exactly when and to what extent the primitive streak split
28
Q

How does the extent of primitive streak splitting influence the extent of organ sharing?

A
  • the primitive streak forms during gastrulation, which is important in the development of internal organs
  • is the streak almost completely splits, there will be less sharing of internal organs
  • if the streak only slightly splits, there will be more sharing of structures
29
Q

How are conjoined twins classified?

What are the most common variations?

A
  • they are classified based on the nature and degree of their union
  • the 3 most common arrangements are:
  1. thoracopagus (20-40%)
  2. pyopagus (18-28%)
  3. omphalopagus (13-33%)
30
Q

Is it possible to split conjoined twins?

When is it absolutely necessary?

A
  • the extent of sharing of internal organs determines whether the twins can be split after birth or not
  • this is determined by the degree of fusion of vital organs and vasculature
  • emergency separation may be required in situations where one of the twins is dying / dead
    • otherwise some shared structures will begin to necrose and this will affect the surviving twin
31
Q

What is meant by parasitic twinning?

What does the parasitic twin usually consist of?

A
  • it is essentially a case of asymmetric conjoined twins in which the parasitic twin is entirely dependent on the normal twin
  • the parasitic twin stops development during gestation, but is physically attached to the normal twin
  • the parasitic twin never completes development
  • it usually consists of externally attached supernumerary limbs and they lack a functional heart or brain
32
Q

What is craniopagus parasiticus?

A
  • a rare form of parasitic twinning that occurs when a rudimentary head is attached to the cranium
  • the cranium is shared between the twins, but the parasitic twin is entirely dependent on the normal twin
33
Q

Is it always possible to operate on a parasitic twin?

A
  • the decision to operate depends on the extent of sharing as it can present a risk to the normal twin
  • if there is no nervous / neurological function in the parasitic twin then an operation is considered
34
Q

What is meant by fetus in fetu?

A
  • this occurs when a monozygotic, monochorionic, diamniotic twin becomes incorporated into the body of a host twin
  • a mass of tissue resembling a foetus forms inside the body of its twin
35
Q

What are the features of a fetus in fetu that allow it to be distinguished from a teratoma?

A
  1. enclosed within a distinct amniotic sac
  2. partially or completely covered by skin
  3. grossly recognisable anatomic parts are visible
  4. attached to the dominant twin by a pedicle that contains a few relatively large blood vessels (these allow the tissue to survive)
36
Q

Why may some twins develop the same medical conditions?

A
  • genes and environmental influences both have relative importance in the development of different traits / diseases
  • height has a large genetic influence
  • going down the list, genetic influence decreases but it still plays a role
37
Q

What is meant by epigenetics?

A
  • “the inheritance patterns of DNA and RNA that do not depend on the naked nucleoide sequence.*
  • By “inheritance” we mean a memory of such activity transmitted from one cell generation to the next through mitosis”*
  • an individual’s DNA will remain the same throughout life
  • but factors in both the internal and external environment can act on genes and alter their expression
  • this can lead to susceptibility to or protection from disease
38
Q

What is meant by a “modulator” in epigenetics?

A
  • a modulator is an epigenetic factor that influences a change on gene expression
  • it is thought that changes to DNA caused by epigenetic factors can be transmitted when a cell replicates by mitosis
39
Q

What are the 2 different modifications involved in epigenetics?

A

Methylation of DNA:

  • a methyl group can tag DNA and work to activate or inactivate genes
  • individuals have different baseline levels of methylation and different cell types show different levels of methylation

Histone acetylation:

  • this works to expand DNA, allowing for transcription
  • histones are proteins around which DNA can wind for compaction and gene regulation
  • the binding of epigenetic factors to histone “tails” alters the extent to which DNA is wrapped around histones and the availability of genes in the DNA to be activated

Embryology (20 decks)

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