Development
The exact development of craniopagus parasiticus isn't well known. However, it is known that the underdeveloped twin is a parasitic twin. Parasitic twins are known to occur in utero when monozygotic twins start to develop as an embryo, but the embryo fails to completely split. When this happens one embryo will dominate development while the other's development is severely altered. The key difference between a parasitic twin and conjoined twins is that in parasitic twins, one twin stops development during gestation, the parasite, while the other twin develops completely, the autosite.
In normal monozygotic twin development one embryo is fertilized by a single sperm. The egg will then completely split into two, normally at the two cell stage. If the egg splits in the early blastocyst stage two inner cell masses well be present. Eventually leading to the twins sharing the same chorion and placenta, but they will have a separate amnion. However, the egg can split into two, but still have one blastocyst. This will lead to one inner cell mass, and one blastocyst. Then as the twins develop they will share the same placenta, chorion, and amnion. This is thought to be the most likely reason why conjoined twins occur, and could possibly play a role in the development of craniopagus parasiticus.
One hypothesis is that craniopagus parasiticus starts with the development of two fetuses from a single zygote that fail to separate at the head region around the second week of gestation. Some however believe it occurs later in development, around the fourth week of gestation. It is thought that the two embryos fuse together at the fourth week of gestation near the anterior open neuropore.
Another hypothesis is that there is joining of the somatic and placental vascular system of the twins and also a degeneration of the umbilical cord the parasitic twin. This suggests that caniopagus parasiticus develops due to the lack of blood supply to one of the twins.
The condition also seems to be more predominate in males than females, with seven of the nine reported cases being male. With the number of cases being so small it is hard to say whether this data has any significance. However, it does provide some insight to the possibility of a genetic predisposition to the condition.
Read more about this topic: Craniopagus Parasiticus
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