Human Reproduction
Twins
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Human Reproduction
Twins
LECTURE 11
Human Reproduction
Twins
➔ Twins are two offspring produced by the same pregnancy.
Human Reproduction
Twins
Monozygotic Twins Dizygotic Twins
Human Reproduction
Monozygotic/ Identical Twins
➔ They develop from one zygote, which splits and forms two embryos.
Human Reproduction
Dizygotic/Fraternal Twins
➔ Each twin develops from a separate egg and each egg is fertilized by its own
sperm cell.
Human Reproduction
Monozygotic Twins
➔ These develop from a single fertilized ovum.
➔ They contain exactly the same genetic material and are mostly of the same sex.
Human Reproduction
Monozygotic (MZ)/ Identical Twins
➔ Monozygotic (MZ) or identical twins occur when a single egg is fertilized to form
one zygote (hence, "monozygotic") which then divides into two separate
embryos.
Human Reproduction
Monozygotic Twins
➔ These arise from separation of developing cells into two embryos, which in 99%
of the cases occurs before 8 days have passed.
➔ Separations that occur later than 8 days are likely to produce conjoined twins.
Human Reproduction
Dizygotic/Fraternal Twins
➔ These are produced from independent release of two secondary oocytes and
the subsequent fertilization of each by different sperm.
➔ They are of the same age and in the uterus at the same time, but genetically they
are as dissimilar as any other siblings.
➔ Dizygotic twins may or may not be the same sex.
Human Reproduction
Dizygotic (DZ)/ Fraternal twins
➔ Also referred to as "non-identical twins", "dissimilar twins", "biovular twins", and,
informally in the case of females, "sororal twins" usually occur when two
fertilized eggs are implanted in the uterus wall at the same time.
➔ When two eggs are independently fertilized by two different sperm cells,
fraternal twins result.
Human Reproduction
Conjoined Twins
➔ Separations that occur later than 8 days are
likely to produce conjoined twins, a situation in
which the twins are joined together and share
some body structures.
Monozygotic twins
Conjoined Twins
➔ Non-conjoined monozygotic twins form up to day 14 of
embryonic development, but when twinning occurs after 14
days, the twins will likely be conjoined.
Human Reproduction
Twins
➔ Multiple pregnancies are much less likely to carry to full term than single births.
➔ Twin pregnancies last on average 37 weeks, three weeks less than full term.
Human Reproduction
Twins
➔ Women who have a family history of fraternal twins have a higher chance of
producing fraternal twins themselves, as there is a genetically linked tendency to
hyper-ovulate.
➔ There is no known genetic link for identical twinning.
Human Reproduction
Factors which Increase Chances of having Fraternal Twins
Maternal age
Fertility drugs and other fertility treatments
Nutrition
Prior births
Human Reproduction
Factors which Increase Chances of having Fraternal Twins
➔ Some women intentionally turn to fertility drugs in order to conceive twins.
Human Reproduction
Types of Fraternal Twins
Female–female twins(sororal twins-25%)
Male–male twins (fraternal twins-25%)
Male–female twins (50%)
Human Reproduction
Mechanism
➔ Regarding spontaneous or natural monozygotic twinning, a recent theory
proposes that monozygotic twins are probably formed when a blastocyst
contains two inner cell masses (ICM), each of which will lead to a
separate fetus, rather than by the embryo splitting while hatching from the
zona pellucida.
Monozygotic twins
Genetic Similarity
➔ Monozygotic twins are genetically nearly identical and they are always
the same sex unless there has been a mutation during development.
➔ The children of monozygotic twins test genetically as half-siblings.
Monozygotic twins
Genetic Similarity
➔ Identical twins do not have the same fingerprints.
➔ Within the confines of the womb, the fetuses touch different parts of
their environment, giving rise to small variations in their corresponding
prints and thus making them unique.
Monozygotic twins
Genetic Similarity
➔ Monozygotic twins, although genetically very similar, are not
genetically exactly the same.
➔ Another cause of difference between monozygotic twins is
epigenetic modification, caused by differing environmental
influences throughout their lives.
Monozygotic twins
Epigenetics
➔ It refers to the level of activity of any particular gene.
➔ A gene may become switched on, switched off, or could become
partially switched on or off in an individual.
Human Reproduction
Vanishing Twins
➔ A vanishing twin, also known as twin resorption, is a fetus in a multigestation
pregnancy that dies in utero and is then partially or completely reabsorbed.
➔ In some instances, the dead twin is compressed into a flattened, parchment-like
state known as fetus papyraceus.
Human Reproduction
Parasitic Twins
➔ A parasitic twin, also known as an asymmetrical or unequal conjoined
twin, is the result of the processes that also produce vanishing twins
and conjoined twins, and may represent a continuum between the two.
➔ Parasitic twins occur when a twin embryo begins developing in utero,
but the pair does not fully separate, and one embryo maintains
dominant development at the expense of its twin.
Human Reproduction
Conjoined Twins
➔ Conjoined twins (or the once-commonly used term "siamese") are monozygotic
twins whose bodies are joined together during pregnancy.
➔ This occurs when the zygote starts to split after day 12 following fertilization and
fails to separate completely.
Human Reproduction
Conjoined Twins
➔ Most conjoined twins are now evaluated for surgery to attempt to separate them
into separate functional bodies.
➔ The degree of difficulty rises if a vital organ or structure is shared between twins,
such as the brain, heart or liver.
Human Reproduction
Conjoined Twins
➔ Conjoined twins, also known as Siamese twins, are identical twins
joined in utero.
➔ An extremely rare phenomenon, the occurrence is estimated to
range from 1 in 49,000 births to 1 in 189,000 births, with a
somewhat higher incidence in Southwest Asia and Africa.
Human Reproduction
Conjoined Twins
➔ Approximately half are stillborn, and an additional one-third die
within 24 hours.
➔ Most live births are female, with a ratio of 3:1.
➔ Conjoined twins share a single common chorion, placenta, and
amniotic sac.
Human Reproduction
Siamese Twins
➔ Chang and Eng Bunker (1811-1874), Thai
brothers born in Siam – now Thailand, traveled
widely for many years and were labeled as The
Siamese Twins.
➔ Chang and Eng were joined at the torso by a
band of flesh, cartilage, and their fused livers.
Human Reproduction
Siamese Twins
➔ In modern times, they could have been easily
separated.
➔ Due to the brothers' fame and the rarity of the
condition, the term "Siamese twins" came to
be used as a synonym for conjoined twins.
Human Reproduction
Separation of Conjoined Twins
Conjoined Twins
Separation
➔ Surgery to separate conjoined twins may range from very easy to very
difficult depending on the point of attachment and the internal parts
that are shared.
➔ Most cases of separation are extremely risky and life-threatening.
➔ In many cases, the surgery results in the death of one or both of the
twins, particularly if they are joined at the head or share a vital organ.
Conjoined Twins
Separation
➔ Recent successful separations of conjoined twins include that of the separation
of Ganga and Jamuna Shreshta in 2001, who were born in Kathmandu, Nepal,
in 2000.
➔ The 97-hour surgery on the pair of craniopagus twins was a landmark one
which took place in Singapore.
Conjoined Twins
Separation
➔ The surgery left Ganga with brain damage and Jamuna unable to walk.
➔ Seven years later, Ganga Shrestha died at the Model Hospital in Kathmandu in
July 2009, at the age of eight, three days after being admitted for treatment of a
severe chest infection.
Conjoined Twins
Separation
➔ In 2003, two 29-year-old women from Iran,
Ladan and Laleh Bijani, who were joined at the
head but had separate brains (craniopagus)
were surgically separated in Singapore, despite
surgeons' warnings that the operation could be
fatal to one or both.
Conjoined Twins
Separation
➔ Their complex case was accepted only because technologically advanced
graphical imagery and modelling would allow the medical team to plan the
risky surgery.
➔ However, an undetected major vein hidden from the scans was discovered
during the operation.
➔ The separation was completed but both women died while still in surgery.
Conjoined Twins
Separation
➔ Lakshmi Tatma is an Indian girl born in 2005 in a village in Araria district,
Bihar, with "4 arms and 4 legs."
Conjoined Twins
Separation
➔ She was actually one of a pair of ischiopagus conjoined twins one of
which was headless because its head had atrophied and chest had not
fully developed in the womb, causing the appearance of one child with
four arms and four legs.
➔ She underwent surgery to remove these extra limbs.
Twins
Development of Language
➔ There have been many studies highlighting the development of language in
twins compared to single-born children.
➔ These studies have converged on the notion that there is a greater rate of delay
in language development in twins compared to their single-born counterparts.
➔ The reasons for this phenomenon are still in question; however, cryptophasia
was thought to be the major cause.
Human Reproduction
Idioglossia
➔ It is defined as a private language that is usually invented by young children,
specifically twins.
➔ Another term to describe what some people call "twin talk" is cryptophasia
where a language is developed by twins that only they can understand.
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