[boatbums]

"But in Mary's womb, the placenta effectively isolates each from the other."

So it was believed, but erroneously. We now know about microchimerism (link) and other processes by which stem cells and other factors cross the placenta. Some of the mother's cells find they way into the gestating child, and some of the child's cells into the mother. Therefore, some of Mary's cells would be found in Jesus, and --- even more intriguing ---- some of Jesus' cells found in Mary! (Recent research shows that fetal stem cells help the mother's body repair damage. Something we moms can thank our kids for!)

I went to the link you provided and read the Wikipedia article concerning the microchimerism link and, though I am no medical expert, I don't think it means exactly what you imply it does. According to the article at http://en.wikipedia.org/wiki/Microchimerism:

Microchimerism (abbreviated Mc) is the presence of a small number of cells that originate from another individual and are therefore genetically distinct from the cells of the host individual. This phenomenon may be related to certain types of autoimmune diseases; however, the mechanisms responsible for this relationship are unclear.

In humans (and perhaps in all Placentals) the most common form is fetomaternal microchimerism (also known as fetal cell microchimerism or fetal chimerism) whereby cells from a fetus pass through the placenta and establish cell lineages within the mother. Fetal cells have been documented to persist and multiply in the mother for several decades.[1][2] The exact phenotype of these cells is unknown, although several different cell types have been identified, such as various immune lineages, mesenchymal stem cells, and placental-derived cells.[3] The potential health consequences of these cells are currently unknown. One hypothesis is that these fetal cells might trigger a graft-versus-host reaction leading to autoimmune disease. This offers a potential explanation for why many autoimmune diseases are more prevalent in middle-aged women.[4] The other main theory is that fetal cells home to injured or diseased maternal tissue where they act as stem cells and participate in repair.[5][6] It is also possible that the fetal cells are merely innocent bystanders and have no effect on maternal health.[7]

After giving birth, about 50-75% of women carry fetal immune cell lines. Maternal immune cells are also found in the offspring yielding in maternal→fetal microchimerism, though this phenomenon is about half as frequent as the former .[8]

Microchimerism had also been shown to exist after blood transfusions to a severely immunocompromised population of patients who suffered trauma.[9]

Animal[edit source | edit]Microchimerism occurs in most pairs of twins in cattle. In cattle (and other bovines), the placentae of fraternal twins usually fuse and the twins share blood circulation, resulting in exchange of cell lines. If the twins are a male-female pair, the male hormones from the bull calf have the effect of partially masculinising the heifer (female), creating a martin heifer or freemartin. Freemartins appear female, but are infertile and so cannot be used for breeding or dairy production. Microchimerism provides a method of diagnosing the condition, because male genetic material can be detected in a blood sample.[10]

Relationship with autoimmune diseases and breast cancer[edit source | edit]Microchimerism has been implicated in autoimmune diseases. Independent studies repeatedly suggested that microchimeric cells of fetal origin may be involved in the pathogenesis of systemic sclerosis.[2][11] Moreover, microchimeric cells of maternal origin may be involved in the pathogenesis of a group of autoimmune diseases found in children, i.e. juvenile idiopathic inflammatory myopathies (one example would be juvenile dermatomyositis).[12] Microchimerism has now been further implicated in other autoimmune diseases, including systemic lupus erythematosus.[13] Contrarily, an alternative hypothesis on the role of microchimeric cells in lesions is that they may be facilitating tissue repair of the damaged organ.[14]

Moreover, fetal immune cells have also been frequently found in breast cancer stroma as compared to samples taken from healthy women. It is not clear, however, whether fetal cell lines promote the development of tumors or, contrarily, protect women from developing breast carcinoma.[15][16]

I do not get the impression that this exchange of certain cells between mothers and their offspring is either as common or widespread nor as beneficial as stated. Something you say mothers should thank their kids for can turn out to be not such a good thing when autoimmune diseases are caused more often than facilitating tissue repair or when tumors are promoted instead of prevented. It sounds as if the jury is still out on this relatively new subject of biology.

[Mrs. Don-o]

It's pretty new research, and you're right that "the jury is still out" --- in fact, we're still just in the initial evidence-gathering stage.

Here's one study "Male Microchimerism in the Female Human Brain" (Link) that states:

"During pregnancy, genetic material and cells are bi-directionally exchanged between the fetus and mother... We report that 63% of the females (37 of 59) tested harbored male microchimerism in the brain....In conclusion, male microchimerism is frequent and widely distributed in the human female brain."

This male microchimerism in the female brains resulted from the women's pregnancies with male fetuses.

This particular study also suggests a connection to both favorable and unfavorable consequences for the female: on the one hand, a possible elevated risk for autoimmune complications; on the other, a possible enhanced protective effect for tissue repair and immune surveillance (improved immune function.)

I just love this kind of stuff!! The Creator of the human race is so clever!