ps418
January 12, 2003, 10:49 AM
Most humans of course have a body consisting of cells descended from a single zygote. A small number of humans (and other mammals) are genetic chimeras, possessing cells of two (or more, theoretically) distinct genotypes. According to one source, as many as 8% of fraternal twin pairs (n=415), and 21% of fraternal triplets (n=57) may be chimeric with regards to blood type (van Dijk et al., 1996). This can happen when fraternal twins transfer blood prenatally. (Identical twins may share blood too, but they would not be chimeras, since they already descend from a single cell line.) In some cases, an individual with blood type chimerism will have both XY and XX cells. The proportional of "other cells' is usually very small.
There are also singletons who are chimeric. How could that happen if they were born as singletons? It can happen because not all twin pregnancies end in twin births. In fact, many co-twins "vanish" before birth, and before the first ultrasound. Many people who began life as a twin were not born as a twin. According to one source, twins vanish in 21-30% of multifetal gestations (Vanishing Twin Syndrome (http://www.emedicine.com/med/topic3411.htm)). Some individuals have learned to their surprise that they were originally part of an opposite-sex twin pair, and even though their twin was never born, their twin's blood is literally pumping through their veins (e.g. Bird et al., 1982).
There are rare cases where individuals are not only chimeric with respect to blood cells, but with respect to tissues as well. This happens when seperate embryos actually fuse very early in development, resulting in the development of an individual with 2 distinct cell lines, descended from two embryos and 4 gametes. This sort of chimera can be produced in mice (e.g. Method used to generate embryo-aggregated chimeric mice (http://www-ermm.cbcu.cam.ac.uk/99001465h.htm)), such as the one pictured here. The light and dark patches on the mouse derive from different embryos.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/W/Wegmann100.jpg
One example of chimerism in humans was described in the New England Journal of Medicine (Strain et al., 1998), in a "phenotypically male child who was found to have ovarian tissue when his left inguinal region was explored because of an undescended testis." The child's cells descend from two distinct embryonic cell lines -one male XY and one female XX. Both skin fibroblast cells and blood cells were found to be chimeric. Its not clear what other cell types are chimeric in this individual.
Another case of true human chimerism involves a 52 year old woman who was undergoing blood tests in anticipation of a kidney transplant (Yu et al., 2002). Skin cells, hair follicles, thyroid cells, bladder cells were tested and found to be chimeric. Both genotypes were XX. Her chimeric status was only discovered because blood type compatability tests initially suggested that she could not be the mother or one of her 4 sons.
References
Bird GW, Wingham J, Nicholson GS, Battey DA, Koster HG, Webb T., 1982. Another example of haemopoietic (twin) chimaerism in a subject unaware of being a twin. Journal of Immunogenetics 5, pp. 317-22.
Strain, L., Dean, J. C. S., Hamilton, M. P. R. & Bonthron, D. T., 1998. A true hermaphrodite chimera resulting from embryo amalgamation after in vitro fertilization. New England Journal of Medicine 338, pp. 166 - 169.
van Dijk, B. A., Boomsma, D. I., and de Man, A. J. M., 1996. Blood group chimerism in human multiple births is not rare. American Journal of Medical Genetics 61, pp. 264 - 268.
Yu, et al., 2002. Disputed Maternity Leading to Identification of Tetragametic Chimerism. New England Journal of Medicine 346: 20, pp. 1545-52.
Patrick
There are also singletons who are chimeric. How could that happen if they were born as singletons? It can happen because not all twin pregnancies end in twin births. In fact, many co-twins "vanish" before birth, and before the first ultrasound. Many people who began life as a twin were not born as a twin. According to one source, twins vanish in 21-30% of multifetal gestations (Vanishing Twin Syndrome (http://www.emedicine.com/med/topic3411.htm)). Some individuals have learned to their surprise that they were originally part of an opposite-sex twin pair, and even though their twin was never born, their twin's blood is literally pumping through their veins (e.g. Bird et al., 1982).
There are rare cases where individuals are not only chimeric with respect to blood cells, but with respect to tissues as well. This happens when seperate embryos actually fuse very early in development, resulting in the development of an individual with 2 distinct cell lines, descended from two embryos and 4 gametes. This sort of chimera can be produced in mice (e.g. Method used to generate embryo-aggregated chimeric mice (http://www-ermm.cbcu.cam.ac.uk/99001465h.htm)), such as the one pictured here. The light and dark patches on the mouse derive from different embryos.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/W/Wegmann100.jpg
One example of chimerism in humans was described in the New England Journal of Medicine (Strain et al., 1998), in a "phenotypically male child who was found to have ovarian tissue when his left inguinal region was explored because of an undescended testis." The child's cells descend from two distinct embryonic cell lines -one male XY and one female XX. Both skin fibroblast cells and blood cells were found to be chimeric. Its not clear what other cell types are chimeric in this individual.
Another case of true human chimerism involves a 52 year old woman who was undergoing blood tests in anticipation of a kidney transplant (Yu et al., 2002). Skin cells, hair follicles, thyroid cells, bladder cells were tested and found to be chimeric. Both genotypes were XX. Her chimeric status was only discovered because blood type compatability tests initially suggested that she could not be the mother or one of her 4 sons.
References
Bird GW, Wingham J, Nicholson GS, Battey DA, Koster HG, Webb T., 1982. Another example of haemopoietic (twin) chimaerism in a subject unaware of being a twin. Journal of Immunogenetics 5, pp. 317-22.
Strain, L., Dean, J. C. S., Hamilton, M. P. R. & Bonthron, D. T., 1998. A true hermaphrodite chimera resulting from embryo amalgamation after in vitro fertilization. New England Journal of Medicine 338, pp. 166 - 169.
van Dijk, B. A., Boomsma, D. I., and de Man, A. J. M., 1996. Blood group chimerism in human multiple births is not rare. American Journal of Medical Genetics 61, pp. 264 - 268.
Yu, et al., 2002. Disputed Maternity Leading to Identification of Tetragametic Chimerism. New England Journal of Medicine 346: 20, pp. 1545-52.
Patrick