Conjoined twins have recently made the headlines as surgeons attempt their intricate separation. This phenomenon--the merging of tissues from two separate individuals--happens because R factors are identical for identical twins and if tissues from one individual come in intimate contact with the other, the tissues will not know the difference between self and non-self and will continue their development as if they were one individual. Conjoining cannot happen between fraternal twins because R factors are different and one fraternal twin is able to recognize the other fraternal twin's cells as foreign. Of course, fraternal twins also have separate placentas keeping them apart but I do not believe that their embryonic tissues would merge even if placed in close proximity to each other.
They do not merge because of an ancient ploy used by multicellular organisms in their attempt to thwart parasites. If everyone had the same R factors, parasites would eventually find out what those R factors were and successfully invade by disguising themselves as host tissue.
Somewhere in the genome there must be a repertoire, if you will, of R factors (I strongly suspect the so-called selfish DNA said to have no known purpose). There is, I believe, a developmental mechanism that draws on this collection of R factors as needed. It's as if you had Lego blocks from different manufacturers and you could build roughly identical objects but you couldn't share the pieces once the manufacturer (the developmental mechanism) decided on a certain means of attachment.
This uniqueness of the R factors is the reason why transplants--in other than identical twins--is nearly impossible without immune suppression. Even if you look just like your father and mother, your R factors will be different and will not allow for transplantation to occur readily.
It is often said that there are two types of twins: identical and fraternal. I submit that there exists a much greater variety. There are the fraternal twins but with identical twins you will have virtually indistinquishable twins and you will have twins that are sufficiently different to enable one to "tell them apart." If I am correct, this occurs because the separation that results in two embryos may occur at different times in development--the earlier it occurs, the more identical the twins are.
[a note about the experiment I mentioned at the beginning of the blog telling how adult cancer cells were put into an embryo and the cells became normal. This was possible because biologists use highly inbred animals that have nearly identical R factors. If you tried this with humans, the experiment would probably fail in all but identical twins.]
R factors play a role in many biological processes, and I hope to elucidate others as we move further along this bioblog.
They do not merge because of an ancient ploy used by multicellular organisms in their attempt to thwart parasites. If everyone had the same R factors, parasites would eventually find out what those R factors were and successfully invade by disguising themselves as host tissue.
Somewhere in the genome there must be a repertoire, if you will, of R factors (I strongly suspect the so-called selfish DNA said to have no known purpose). There is, I believe, a developmental mechanism that draws on this collection of R factors as needed. It's as if you had Lego blocks from different manufacturers and you could build roughly identical objects but you couldn't share the pieces once the manufacturer (the developmental mechanism) decided on a certain means of attachment.
This uniqueness of the R factors is the reason why transplants--in other than identical twins--is nearly impossible without immune suppression. Even if you look just like your father and mother, your R factors will be different and will not allow for transplantation to occur readily.
It is often said that there are two types of twins: identical and fraternal. I submit that there exists a much greater variety. There are the fraternal twins but with identical twins you will have virtually indistinquishable twins and you will have twins that are sufficiently different to enable one to "tell them apart." If I am correct, this occurs because the separation that results in two embryos may occur at different times in development--the earlier it occurs, the more identical the twins are.
[a note about the experiment I mentioned at the beginning of the blog telling how adult cancer cells were put into an embryo and the cells became normal. This was possible because biologists use highly inbred animals that have nearly identical R factors. If you tried this with humans, the experiment would probably fail in all but identical twins.]
R factors play a role in many biological processes, and I hope to elucidate others as we move further along this bioblog.
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