Question: Where do extraembryonic fetal cells originate?
About the article "Stem cell", section "Fetal".

Candidate passages:
The primitive stem cells located in the organs of fetuses are referred to as fetal stem cells.
There are two types of fetal stem cells:
 Fetal proper stem cells come from the tissue of the fetus proper, and are generally obtained after an abortion.
These stem cells are not immortal but have a high level of division and are multipotent.
 Extraembryonic fetal stem cells come from extraembryonic membranes, and are generally not distinguished from adult stem cells.
These stem cells are acquired after birth, they are not immortal but have a high level of cell division, and are pluripotent.
The formation of the chorionic cavity (Extra-embryonic coelom) and the yolk sac (umbilical vesicle) is still up for debate.
The thought of how the yolk sac membranes are formed begins with an increase in production of hypoblast cells, succeeded by different patterns of migration.
On day 8, the first portion of hypoblast cells begin their migration and make what is known as the primary yolk sac, or Heuser's membrane (exocoelomic membrane).
By day 12, the primary yolk sac has been disestablished by a new batch of migrating hypoblast cells that now contribute to the definitive yolk sac.
While the primary yolk sac is forming, extraembryonic mesoderm makes its way into the blastocyst cavity to fill it with loosely packed cells.
When the extraembryonic mesoderm is separated into two portions, a new gap arises called the chorionic cavity, or the extra-embryonic coelom.
This new cavity is responsible for detaching the embryo and its amnion and yolk sac from the far wall of the blastocyst, which is now named the chorion.
When the extraembryonic mesoderm splits into two layers, the amnion, yolk sac, and chorion follow its lead and also become double layered.
The chorion and amnion are composed of extraembryonic ectoderm and mesoderm, where as the yolk sac is made of extraembryonic endoderm and mesoderm.
When day 13 rolls around, the connecting stalk, a dense portion of extraembryonic mesoderm, restrains the embryonic disc in the chorionic cavity. 
The extraembryonic coelom (or Chorionic cavity) is a portion of the conceptus consisting of a cavity between Heuser's membrane and the Trophoblast.
During formation of the primitive yolk sac, some of the migrating Hypoblast cells transdifferentiate into mesenchymal cells that fill the space between Heuser's membrane and the Trophoblast, forming the extraembryonic mesoderm.
As development progresses, small lacunae begin to form within the extraembryonic mesoderm which become larger and form the extraembryonic coelom.
It divides the extraembryonic mesoderm into two layers: extraembryonic splanchnopleuric mesoderm, which lies adjacent to Heuser's membrane around the outside of the primitive yolk sac, and extraembryonic somatopleuric mesoderm, which lies adjacent to the cytotrophoblast layer of the embryo.
The extraembryonic coelemic cavity is also called the chorionic cavity, and is enclosed by the chorionic plate, which is composed of an inner layer of somatopleuric mesoderm and an outer layer of trophoblast cells. 
Placental mechanisms.
The placenta functions as an immunological barrier between the mother and the fetus, creating an immunologically privileged site.
For this purpose, it uses several mechanisms: It secretes Neurokinin B containing phosphocholine molecules.
This is the same mechanism used by parasitic nematodes to avoid detection by the immune system of their host.
Also, there is presence of small lymphocytic suppressor cells in the fetus that inhibit maternal cytotoxic T cells by inhibiting the response to interleukin 2.
The placental trophoblast cells do not express the classical MHC class I isotypes HLA-A and HLA-B, unlike most other cells in the body, and this absence is assumed to prevent destruction by maternal cytotoxic T cells, which otherwise would recognize the fetal HLA-A and HLA-B molecules as foreign.
On the other hand, they do express the atypical MHC class I isotypes HLA-E and HLA-G, which is assumed to prevent destruction by maternal NK cells, which otherwise destroy cells that do not express any MHC class I.
However, trophoblast cells do express the rather typical HLA-C.
It forms a syncytium without any extracellular spaces between cells in order to limit the exchange of migratory immune cells between the developing embryo and the body of the mother (something an epithelium will not do sufficiently, as certain blood cells are specialized to be able to insert themselves between adjacent epithelial cells).
The fusion of the cells is apparently caused by viral fusion proteins from endosymbiotic endogenous retrovirus (ERV).
An immunoevasive action was the initial normal behavior of the viral protein, in order to avail for the virus to spread to other cells by simply merging them with the infected one.
It is believed that the ancestors of modern viviparous mammals evolved after an infection by this virus, enabling the fetus to better resist the immune system of the mother.
Still, the placenta does allow maternal IgG antibodies to pass to the fetus to protect it against infections.
However, these antibodies do not target fetal cells, unless any fetal material has escaped across the placenta where it can come in contact with maternal B cells and make those B cells start to produce antibodies against fetal targets.
The mother does produce antibodies against foreign ABO blood types, where the fetal blood cells are possible targets, but these preformed antibodies are usually of the IgM type, and therefore usually do not cross the placenta.
Still, rarely, ABO incompatibility can give rise to IgG antibodies that cross the placenta, and are caused by sentitization of mothers (usually of blood type 0) to antigens in foods or bacteria. 
The link between a pregnant mother and the fetus is such that “at all sites of direct cell-to-cell contact, maternal tissues (decidua and blood) are juxtaposed to extraembryonic cells (trophoblast).” According to Cunningham, "after conception, a biomolecular communication system is established between the fetus and mother that is operative from before the time of nidation and continues through and beyond the time of parturition."
This communication system is essential to all facets of the pregnancy.
"Physiological processes such as fetal nutrition and fetal development progress directly from embryonic/fetal tissue-directed modifications of maternal responses."
Notably, the placenta is the principal site of transfer between mother and fetus.
A fetus is exposed via the placenta to all substances which are present in the peripheral circulation of the mother.
Overall, the abundance of toxins contained within cigarette smoke that is inhaled by the mother exerts a direct impact by altering the placental and fetal cell proliferation and differentiation.
The vital balance of cellular activity is disrupted.
"The association of in utero exposure to such carcinogens and the subsequent development of cancer has been reported for all childhood cancers combined and particularly for childhood acute lymphoblastic leukemia, lymphoma, and brain tumors."