Chimerism in Humans Chimerism Comes

Chimerism in Humans Chimerism comes from the term 'Chimera' which has been widely referenced in botanical regards for plants which have a couple or more tissues with dissimilar genetic arrangements. It results in an offspring which have two or more sets of cells which distinctly come from multiple zygotes. In mythological tales, the word 'Chimera' has also been to denote a fire breathing monster with the head of a lion, the body of a goat and the tail of a snake. Even though this is fictional, it explains the idea.

Chimerism in cattle is not an exceptional occurrence. When they give birth to twins, there are more chances than not, for them to develop anastomoses between their individual circulatory systems in early phases. The two fetuses growing side by side have the tendency to transfer blood amongst each other. Each fetus is associated with their own hematopoietic stem cells, separated from the other twin. This causes each of the offspring to be hematopoietically linked Chimeras.

Hematopoietic stem cells may consist of the Kupffer cells in the liver, peripheral cells in the blood, macrophages and lymphocytes in the lymph nodes and spleen. A fair portion of their body cells which come from these cells can be considered to be associated with the other twin. An interesting aspect of this form of exchange comes forth when the fetuses differ in sex. This causes them to be exposed to genes of the opposite sex. The female fetus on coming in touch with the male tends to get masculinized.

These kinds of female cattle are termed as freemartins. Being originally female, they do have genital parts resembling other females, except for the fact that their vaginas grow smaller with irregular and immature Uterian parts resulting in sterility. Due to lack of capacity of cattle to develop vascular anastomoses, cattle show more chances of growth of freemartins than others. Chimerism occurs naturally in other species as well, mostly in cases where the embryos unite. Several experiments on Chimerism have been conducted over the years.

They serve as important additions to the expanding arena of biomedical studies. Offsprings are developed with embryos in their early stages brought together. They need to contain the presence of cells from each of them for this to succeed. Experiments such as this have been conducted on various species of mice in laboratories. One instance of Chimerism occurred when embryos of a goat and a sheep were fused.

This animal termed as 'geep' has significant proofs of the occurrence of Chimerism with parts of the body covered with wool ( sheep charecteristics) and most of the rest of the body covered in hair (goat charecteristics). This technique has a definite probability of saving endangered species where different species can have their fetuses fused together. Endangered species can have their genes transferred via a Chimeric offsprings where it is fused with another species. Chimerism is associated with another occurrence which is termed as Mosaicism.

While Chimerism results when multiple zygotes unite resulting in different generic characteristics, Mosaicism occurs when we come across variations in genes from a single zygote. Fusions resulting in development of Chimera usually occur in early stages of the embryo. This is when the stem cells are in the process of being assigned what they are supposed to do. Regardless of the fact that individuals from such an embryo share DNA characteristics, they are not exactly the same.

Differences between such Chimeric twins and those who do not share any characteristics, can be pinpointed using genetic testing techniques. The presence of Hermaphrodism can be a result of such fusions in the zygote. One instance of human Chimerism, which gained a fair amount of popularity was the case of Karen Keegan. Genetic test results revealed her to be a Chimera. This was in 1998 when she required a kidney transplant. Testing results denied the fact that two of her three children were biologically her own.

This, when confirmed later was found to be due to the presence of Chimerism. The initial response to this was complete amazement. Each of her children had their tissue types analyzed. These kinds of genetic tests use a collection of genes termed as 'HLA complex'. These include distinct varieties of immune proteins such as those on the cellular surface. These are among some of the factors which differentiate the body's tissues from those which are from other individuals. HLA genes can occur in more than just one form.

In fact adaptations of such genes have several forms known as alleles. Variations of how these alleles combine and appear together are mostly unique from person to person. However groups of these genes unite on the chromosome 6, which causes their inheritance to proceed as chunks termed as halotypes. Every individual gets an HLA halotype from both parents. When people need transplants of a certain body part, the closeness of these HLA halotypes is what decides its success or failure.

As they get closer to each other, the success rate gets better with chances of that part getting rejected by the host body getting much less. Thus most people who need transplants should turn to those who are related to them with a matching HLA halotype. Studies have shown a 25% chance of halotypes matching between siblings and a 50% chance of such a match with the immediate offspring. Thus the first step on dealing with Keegan's situation was testing her HLA halotypes along with those of her family members.

Keegan's halotypes were marked as 1 and 3 while her husband's were numbered as 5 and 6. On analyzing their sons, it revealed that each of them had a contribution from the father's genes. However, only one son had a contribution of Keegan's genes. The remaining two sons had presence of another halotype which was unidentified and marked as 2. Several conclusions had already been drawn before the tests were conducted. One of them was how Keegan could have got someone else's baby.

This was discarded due to the fact that each son had the father's genes. This unusual situation spurred further tests on other members of Keegan's family. It was found that her brother carried the type 2 halotype which was found in one of the sons. This strengthened the belief that the two sons who did not have Keegan's genes were related to her family after all. Irregularity of these halotypes caused them to conduct even more studies. Keegan was tested using tissues in her hair, thyroid gland and mouth.

This brought forth some interesting outcomes. Some parts had the presence of a combination of types 1 and 3. Others had a combination of types 2 and 4. This led the researchers to the possibility that Keegan was an individual who was composed of two different individuals. The most likely explanation regarding this occurrence was that her mother was probably going to have twin girls, who had their embryos fused together at an early phase of development. This caused the formation of one embryo which had characteristics of both.

The medical term associated to this is called 'tetragametic chimera'. It occurs when two separate branches of cells are obtained from four gametes. Tissue types from only one twin in the fused embryo showed up in Keegan's blood as the dominant type. Cell types from both twins however exist in other parts such as the ovaries and do not reject each other. This explains the generic features of her sons where one has cells which dictate most of Keegan's blood and the others from the cells resembling the twins.

Chimerism can cause offsprings to have irregular features. For example -- the eye colors of a child being different from one another and the presence of both male and female reproductive features due to fusion of the sexes at the embryonic level. Such cases are pretty rare and most irregularities which occur are normally too weak to be noticed. Instances of Chimerism have shown a rise in recent years. This is due to developments in the medical field which enhance twinning during the process of fertilization.

A woman can be made to discharge multiple eggs at one time, which can unite in the process of different stages of transfer. Such a case did occur once in 1998 where embryos of different sexes accidentally fused with each other. It resulted in an infant who had the external features of a male except one side of his reproductive system showed the presence of an ovary along with a fallopian tube.

The current status of research predicts every child who is born, to be a Chimera to some extent with the existence of cells which could be from past generations. Regardless of how protected the fetus is inside the womb, some cells from the mothers blood tend to creep in and some from the fetus escape outside. Studies have shown over 80% of women (while pregnant) to be carrying some cells or DNA resembling those of her child. This continues even after childbirth for around 50% women.

The medical term assigned to this phenomenon is called 'microchimerism'. In the case of more children being born, this can lead to some of the older child's DNA to be transferred from the mother to the younger child's fetus. In an attempt to study microchimerism, J. Lee Nelson, an immunologist from the Fred Hutchinson Cancer Research Center in Seattle conducted an experiment with assistance from his colleague Natalie. Blood samples from 32 healthy women revealed over 20% of them to bear white blood cells which belonged to their mother.

It is hard to understand how these cells reside there instead of being rejected immediately by the host body. One assumption stated by Nelson indicates this to be a way to enhance the mother's immunity to prepare for the development of the fetus which can be deemed as a foreign organ on its own. They might even actively participate in the reproductive process by repairing some tissues helping the embryo in the womb to survive.

The study also revealed how cells stored from previous generations might enter the placenta as well, resulting in changes in.