2023-05-15 15:07:56
When twins share the same amniotic sac, they sometimes “merge” giving rise to surprising malformations.
When twins share the same amniotic sac in their mother’s womb, it happens in extremely rare cases that one of the two embryos “ car involuntarily the second. This developmental anomaly can only concern twins bathing in the same amniotic cavity, that is to say monozygotic twins or “identical twins”. This phenomenon drastically limits the development of the so-called twin. parasite without it disappearing, which gives rise to surprising malformations. What remains of the parasitic twin and can we live with it? Answers with Philippe Charlier, forensic pathologist, anatomo-pathologist, archaeo-anthropologist and French paleopathologist, author of Autopsy of historical hearts (Tallandier, 2023).
How common is this pathology?
Called “ischiopagus”, and more commonly “ fetus in fetus This anomaly is extremely rare and affects only one in 500,000 births. According to a study by the Hong Kong Medical published in 2015, only 200 cases have been described in the scientific literature worldwide. Historically, it is to the “father of modern surgery”, Ambroise Paré, that we owe one of the first descriptions, represented on a 16th century engraving.
How does the parasitic twin develop?
The fusion of these two entities takes place after fertilization from the moment the embryos undergo cellular differentiation. ” It is a sort of phenomenon of competition where the marginal individual is “swallowed up” by the one who becomes, so to speak, the main “, explains Philippe Charlier. The parasitic individual does not disappear completely but develops in vestigial form in the body of its twin. “ As long as this portion of the organ is nourished and vascularized, it will grow, but it will be very special and amazing. “, emphasizes Professor Chalier.
Generally the remaining parts of the twin appear as growths which correspond to abnormally developed limbs. In some cases, however, these buds will develop inside the host individual, manifesting as swellings. They can form at the back of the mouth (the floor of the soft palate), at the level of the vertex of the head, of the neck (at the location of the thyroid), in the central part of the thorax (the mediastinum), at the level of the testicles in men or the ovaries in women.
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What are the causes ?
The causes of this phenomenon remain misunderstood, especially since all cases of “ fetus in fetus» do not seem to be explained by a twin pregnancy. “ Other hypotheses suggest that the vestigial forms, which take on the appearance of limbs with sometimes even hair, teeth or organs, may come from a tumor that grows under the influence of very complex cellular and tissue processors. explains Philippe Charlier. “ We then speak of teratoma“. Unlike the parasitic twin, the teratoma is a rare tumor that forms from particular embryonic stem cells, the germ cells that are capable of forming gametes. Some believe that the pathological processes of “ fetus in fetus and teratoma overlap in several respects, which sometimes makes it difficult to distinguish between the two phenomena.
Does the parasitic twin have a conscience?
Removing the members that belong to the parasitic twin tends to shock the imagination because the question arises as to whether it has a conscience and therefore, if its integrity is affected. “ This is a question that has been much debated. At the time, in the absence of knowledge on the subject, the priests wondered if one should consider that it was about one or two births, if one should celebrate one or two baptisms and, at the time of the death of the main individual, organize a funeral for one or two people “says Professor Charlier. Thanks to important scientific advances, we now have that when only members of the parasitic twin develop, it is not considered a proper individual. “ Barring exceptions, he doesn’t have a conscience for the simple and good reason… that he doesn’t have a brain. “, emphasizes Professor Charlier. Its development indeed stopped well before the differentiation of certain stem cells into neurons.
Note that from the moment there is a part of a sensitive man, that is to say if we observe two distinct cerebral developments, we speak of Siamese twins and not of parasitic twins.
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Can you live with a parasitic twin?
Thanks to the examinations carried out during pregnancy (3D ultrasound or even fetal MRI), the parasitic twin can be detected very early. However, most of the time, the fetus in fetus » is diagnosed at birth or during childhood before the age of 18, without our realizing it. This is all the more true if the fetus ends up inside its twin. It is therefore possible to live relatively long with a parasitic twin. However, this type of malformation can also be very disabling. This is why, as soon as the diagnosis is made, doctors consider surgery to remove the remaining parts of the parasitic twin.
A little story of mythology
Before the revolution in modern medical techniques, the question of whether or not to remove the remains of limbs did not necessarily arise because it was a question of the desire of these atypical individuals to keep their “parasite”. “ Some did not have the means to do so, which is still the case today, others were exhibited in circuses, bought on “monster markets”, even prostituted for rather special customers. “says Philippe Charlier. Moreover, we know that already in antiquity, the observation of these unique cases inspired the monsters of Greek mythology such as the centaurs or Cerberus, a polycephalic dog guarding the entrance to the Underworld.
Is it dangerous to remove it?
It depends on how it developed. In all cases, these are heavy surgical operations which can prove to be very complicated anatomically. “ For questions of anastomoses, that is to say the communication established between the two fetuses, it is sometimes very difficult or even impossible to remove the parasitic twin without endangering its twin. Imagine for example that two cores are partially merged: in this case few options are possible “recalls Philippe Charlier.
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