Xenopregnancy
Encyclopedia
Interspecific pregnancy is the pregnancy
involving an embryo
or fetus
belonging to another species
than the carrier. Strictly, it excludes the situation where the fetus is a hybrid of the carrier and another species, thereby excluding the possibility that the carrier is the biological mother of the offspring. Strictly, interspecific pregnancy is also distinguished from endoparasitism, where parasite offspring grow inside the organism of another species, not necessarily in the womb.
It has no known natural occurrence, but can be achieved artificially by transfer of embryos
of one species into the womb of the female of another.
s carrying human fetuses to term as a potential yet ethically controversial alternative to surrogate mothers or artificial uteri
for gay male couples
or mothers with damaged uteri. It would also provide a sober, drug-free and nonsmoking carrier. For animals, it could be a valuable tool in preservation programs of endangered species
, providing a method of ex-situ conservation
. It could also avail for recreation of extinct species.
pathways.
Interspecies compatibility is related to the type of placentation
, as mothers of species having the more invasive hemochorial placentation (such as humans) must create a stronger downregulation of maternal immune responses, and are thereby more receptive to fetuses of other species, compared to those with endotheliochorial (e.g. cats and dogs) or epitheliochorial placentation (e.g. pigs, ruminant
s, horse
s, whale
s) where there is no contact between the maternal blood and the fetal chorion.
Other potential hazards include incompatibility of nutrition or other support system. Notably, there is a risk of inappropriate interactions between the trophoblast
of the fetus and the endometrium
of the mother. For example, the placental glycosylation pattern at the fetomaternal interface should optimally be similar to that of the host species.
Yet, for some species, such as a Bactrian camel
embryo inside a Dromedary
, pregnancy can be carried to term with no other intervention than the embryo transfer. This is possible for gaur
embryos inside cattle
as well, but with severe intrauterine growth restriction
, with uncertainty of how much is caused by the IVF procedure itself, and how much is caused by interspecies incompatibility.
The ability of one species to survive inside the uterus of another species is in many cases unidirectional, that is, pregnancy would not necessarily be successful in the inverse situation where a fetus of the other species would be transferred into the uterus of the first one. For example, horse embryos survive in the donkey uterus, but donkey embryos perish in the uterus of an untreated mare. Deer mouse
embryos survive in the uterus of the white-footed Mouse
, but the reciprocal transfer fails.
Methods to artificially stimulate gestational immune tolerance towards a xeno-fetus include concomitantly introducing a component of a normal allogeneic pregnancy. For example, embryos of the species Spanish Ibex
are aborted when inserted alone into the womb of a goat
, but when introduced together with a goat embryo, they may develop to term. This technique has also been used to grow panda fetuses in a cat, but the cat mother died of pneumonia before completed term. Also, murine embryos of Ryukyu Mouse
(Mus caroli) will survive to term inside the uterus of a house mouse
(Mus musculus) only if enveloped in Mus musculus trophoblast cells. Such enveloping can be created by first isolating the inner cell mass
of blastocysts of the species to be reproduced by immunosurgery
, wherein the blastocyst is exposed to antibodies toward that species. Because only the outer layer, that is, the trophoblastic cells, are exposed to the antibodies, only these cells will be destroyed by subsequent exposure to complement
. The remaining inner cell mass can be injected into a blastocele of the recipient species to acquire its trophoblastic cells. It has been theorized that the allogeneic component prevents the production of maternal lymphocytes and cytotoxic anti-fetal antibodies, but the mechanism remains uncertain.
On the other hand, immune suppression with ciclosporin
has shown no effect for this purpose. Pre-transfer immunization
with antigens from the species providing the embryo has promoted more rapid and uniform failure of the interspecies pregnancy in mice, but increased survival in horse-donkey experiments.
(SCNT) into an egg cell of another species, creating a cloned embryo that transferred into the uterus of yet another species. This technique was used for the experiment of panda fetuses in a cat mentioned in techniques for evercoming rejection. In this experiment, nuclei from cells taken from abdominal muscles of giant panda
s were transferred to egg cells of rabbit
s and, in turn, transferred into the uterus of cat
together with cat embryos. Concomitant use of SCNT and interspecific pregnancy has also been speculated to potentially recreate the mammoth
species, for example by taking genetic material from mammoth specimens preserved in permafrost
and transferring it into egg cells and subsequently the uterus of an elephant
.
Pregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
involving an embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
or fetus
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
belonging to another species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
than the carrier. Strictly, it excludes the situation where the fetus is a hybrid of the carrier and another species, thereby excluding the possibility that the carrier is the biological mother of the offspring. Strictly, interspecific pregnancy is also distinguished from endoparasitism, where parasite offspring grow inside the organism of another species, not necessarily in the womb.
It has no known natural occurrence, but can be achieved artificially by transfer of embryos
Embryo transfer
Embryo transfer refers to a step in the process of assisted reproduction in which embryos are placed into the uterus of a female with the intent to establish a pregnancy...
of one species into the womb of the female of another.
Potential applications
Potential applications include pigPig
A pig is any of the animals in the genus Sus, within the Suidae family of even-toed ungulates. Pigs include the domestic pig, its ancestor the wild boar, and several other wild relatives...
s carrying human fetuses to term as a potential yet ethically controversial alternative to surrogate mothers or artificial uteri
Artificial uterus
An artificial uterus is a theoretical device that would allow for extracorporeal pregnancy or extrauterine fetal incubation by growing an embryo or fetus outside of the body of a female organism that would normally internally carry the embryo or fetus to term.An artificial uterus, as a...
for gay male couples
LGBT parenting
LGBT parenting refers to lesbian, gay, bisexual, and transgender people parenting one or more children. This includes children raised by same-sex couples , children raised by single LGBT parents, and children raised by an opposite-sex couple where at least one partner is LGBT.LGBT people can...
or mothers with damaged uteri. It would also provide a sober, drug-free and nonsmoking carrier. For animals, it could be a valuable tool in preservation programs of endangered species
Endangered species
An endangered species is a population of organisms which is at risk of becoming extinct because it is either few in numbers, or threatened by changing environmental or predation parameters...
, providing a method of ex-situ conservation
Ex-situ conservation
Ex-situ conservation means literally, "off-site conservation". It is the process of protecting an endangered species of plant or animal outside of its natural habitat; for example, by removing part of the population from a threatened habitat and placing it in a new location, which may be a wild...
. It could also avail for recreation of extinct species.
Causes of failure
Immunologically, an embryo or fetus of an interspecific pregnancy would be equivalent to xenografts rather than allografts, putting a higher demand on gestational immune tolerance in order to avoid an immune reaction toward the fetus. Some mice experiments indicate an imbalance between Th1 and Th2 helper cells with a predominance of Th1 cytokines. However, other mice experiments indicate that a immune response towards xeno-fetuses does not belong to classical cytotoxic T lymphocyte or natural killer cellNatural killer cell
Natural killer cells are a type of cytotoxic lymphocyte that constitute a major component of the innate immune system. NK cells play a major role in the rejection of tumors and cells infected by viruses...
pathways.
Interspecies compatibility is related to the type of placentation
Placentation
In biology, placentation refers to the formation, type and structure, or arrangement of placentas. The function of placentation is to transfer nutrients from maternal tissue to a growing embryo...
, as mothers of species having the more invasive hemochorial placentation (such as humans) must create a stronger downregulation of maternal immune responses, and are thereby more receptive to fetuses of other species, compared to those with endotheliochorial (e.g. cats and dogs) or epitheliochorial placentation (e.g. pigs, ruminant
Ruminant
A ruminant is a mammal of the order Artiodactyla that digests plant-based food by initially softening it within the animal's first compartment of the stomach, principally through bacterial actions, then regurgitating the semi-digested mass, now known as cud, and chewing it again...
s, horse
Horse
The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...
s, whale
Whale
Whale is the common name for various marine mammals of the order Cetacea. The term whale sometimes refers to all cetaceans, but more often it excludes dolphins and porpoises, which belong to suborder Odontoceti . This suborder also includes the sperm whale, killer whale, pilot whale, and beluga...
s) where there is no contact between the maternal blood and the fetal chorion.
Other potential hazards include incompatibility of nutrition or other support system. Notably, there is a risk of inappropriate interactions between the trophoblast
Trophoblast
Trophoblasts are cells forming the outer layer of a blastocyst, which provide nutrients to the embryo and develop into a large part of the placenta. They are formed during the first stage of pregnancy and are the first cells to differentiate from the fertilized egg...
of the fetus and the endometrium
Endometrium
-Function:The endometrium is the innermost glandular layer and functions as a lining for the uterus, preventing adhesions between the opposed walls of the myometrium, thereby maintaining the patency of the uterine cavity. During the menstrual cycle or estrous cycle, the endometrium grows to a...
of the mother. For example, the placental glycosylation pattern at the fetomaternal interface should optimally be similar to that of the host species.
Yet, for some species, such as a Bactrian camel
Bactrian camel
The Bactrian camel is a large, even-toed ungulate native to the steppes of central Asia. It is presently restricted in the wild to remote regions of the Gobi and Taklamakan Deserts of Mongolia and Xinjiang. A small number of wild Bactrian camels still roam the Mangystau Province of southwest...
embryo inside a Dromedary
Dromedary
The dromedary or Arabian camel is a large, even-toed ungulate with one hump on its back. Its native range is unclear, but it was probably the Arabian Peninsula. The domesticated form occurs widely in North Africa and the Middle East...
, pregnancy can be carried to term with no other intervention than the embryo transfer. This is possible for gaur
Gaur
The gaur , also called Indian bison, is a large bovine native to South Asia and Southeast Asia. The species is listed as vulnerable on the IUCN Red List since 1986 as the population decline in parts of the species' range is likely to be well over 70% over the last three generations...
embryos inside cattle
Cattle
Cattle are the most common type of large domesticated ungulates. They are a prominent modern member of the subfamily Bovinae, are the most widespread species of the genus Bos, and are most commonly classified collectively as Bos primigenius...
as well, but with severe intrauterine growth restriction
Intrauterine growth restriction
Intrauterine growth restriction refers to poor growth of a baby while in the mother's womb during pregnancy. The causes can be many, but most often involve poor maternal nutrition or lack of adequate oxygen supply to the fetus....
, with uncertainty of how much is caused by the IVF procedure itself, and how much is caused by interspecies incompatibility.
The ability of one species to survive inside the uterus of another species is in many cases unidirectional, that is, pregnancy would not necessarily be successful in the inverse situation where a fetus of the other species would be transferred into the uterus of the first one. For example, horse embryos survive in the donkey uterus, but donkey embryos perish in the uterus of an untreated mare. Deer mouse
Peromyscus maniculatus
Peromyscus maniculatus is a rodent native to North America. It is most commonly called the Deer Mouse, although that name is common to most species of Peromyscus and is fairly widespread across the continent, with the major exception being the southeast United States and the far north.Like other...
embryos survive in the uterus of the white-footed Mouse
White-footed mouse
White-footed Mouse is a rodent native to North America. It ranges from Ontario, Quebec, Labrador and the Maritime Provinces to the southwest USA and Mexico. It is also known as the Woodmouse, particularly in Texas.Adults are in length, not counting the tail, which can add another . A young adult...
, but the reciprocal transfer fails.
Overcoming rejection
Spanish ibex
The Iberian ibex, Spanish ibex, Spanish wild goat, or Iberian wild goat is a species of ibex with four subspecies. Of these, two can still be found on the Iberian Peninsula, but the remaining two are now extinct. The Portuguese subspecies became extinct in 1892 and the Pyrenean subspecies became...
are aborted when inserted alone into the womb of a goat
Goat
The domestic goat is a subspecies of goat domesticated from the wild goat of southwest Asia and Eastern Europe. The goat is a member of the Bovidae family and is closely related to the sheep as both are in the goat-antelope subfamily Caprinae. There are over three hundred distinct breeds of...
, but when introduced together with a goat embryo, they may develop to term. This technique has also been used to grow panda fetuses in a cat, but the cat mother died of pneumonia before completed term. Also, murine embryos of Ryukyu Mouse
Ryukyu Mouse
The Ryukyu Mouse is a species of rodent in the family Muridae.It is found in Cambodia, China, Indonesia, Japan, Laos, Malaysia, Taiwan, Thailand, and Vietnam.-References:* Baillie, J. 1996. . Downloaded on 19 July 2007....
(Mus caroli) will survive to term inside the uterus of a house mouse
House mouse
The house mouse is a small rodent, a mouse, one of the most numerous species of the genus Mus.As a wild animal the house mouse mainly lives associated with humans, causing damage to crops and stored food....
(Mus musculus) only if enveloped in Mus musculus trophoblast cells. Such enveloping can be created by first isolating the inner cell mass
Inner cell mass
In early embryogenesis of most eutherian mammals, the inner cell mass is the mass of cells inside the primordial embryo that will eventually give rise to the definitive structures of the fetus...
of blastocysts of the species to be reproduced by immunosurgery
Immunosurgery
Immunosurgery is a method of removing an external cell layer of an object. Immunosurgery is to expose the specimen to a solution containing antibodies against the object, remove that solution and subsequently expose the object to complement...
, wherein the blastocyst is exposed to antibodies toward that species. Because only the outer layer, that is, the trophoblastic cells, are exposed to the antibodies, only these cells will be destroyed by subsequent exposure to complement
Complement system
The complement system helps or “complements” the ability of antibodies and phagocytic cells to clear pathogens from an organism. It is part of the immune system called the innate immune system that is not adaptable and does not change over the course of an individual's lifetime...
. The remaining inner cell mass can be injected into a blastocele of the recipient species to acquire its trophoblastic cells. It has been theorized that the allogeneic component prevents the production of maternal lymphocytes and cytotoxic anti-fetal antibodies, but the mechanism remains uncertain.
On the other hand, immune suppression with ciclosporin
Ciclosporin
Ciclosporin , cyclosporine , cyclosporin , or cyclosporin A is an immunosuppressant drug widely used in post-allogeneic organ transplant to reduce the activity of the immune system, and therefore the risk of organ rejection...
has shown no effect for this purpose. Pre-transfer immunization
Immunization
Immunization, or immunisation, is the process by which an individual's immune system becomes fortified against an agent ....
with antigens from the species providing the embryo has promoted more rapid and uniform failure of the interspecies pregnancy in mice, but increased survival in horse-donkey experiments.
Embryo creation
Embryos may be created by in vitro fertilization (IVF) with gametes from a male and female of the species to be reproduced. They may also be created by somatic cell nuclear transferSomatic cell nuclear transfer
In genetics and developmental biology, somatic-cell nuclear transfer is a laboratory technique for creating a clonal embryo, using an ovum with a donor nucleus . It can be used in embryonic stem cell research, or, potentially, in regenerative medicine where it is sometimes referred to as...
(SCNT) into an egg cell of another species, creating a cloned embryo that transferred into the uterus of yet another species. This technique was used for the experiment of panda fetuses in a cat mentioned in techniques for evercoming rejection. In this experiment, nuclei from cells taken from abdominal muscles of giant panda
Giant Panda
The giant panda, or panda is a bear native to central-western and south western China. It is easily recognized by its large, distinctive black patches around the eyes, over the ears, and across its round body. Though it belongs to the order Carnivora, the panda's diet is 99% bamboo...
s were transferred to egg cells of rabbit
Rabbit
Rabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world...
s and, in turn, transferred into the uterus of cat
Cat
The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...
together with cat embryos. Concomitant use of SCNT and interspecific pregnancy has also been speculated to potentially recreate the mammoth
Mammoth
A mammoth is any species of the extinct genus Mammuthus. These proboscideans are members of Elephantidae, the family of elephants and mammoths, and close relatives of modern elephants. They were often equipped with long curved tusks and, in northern species, a covering of long hair...
species, for example by taking genetic material from mammoth specimens preserved in permafrost
Permafrost
In geology, permafrost, cryotic soil or permafrost soil is soil at or below the freezing point of water for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of...
and transferring it into egg cells and subsequently the uterus of an elephant
Elephant
Elephants are large land mammals in two extant genera of the family Elephantidae: Elephas and Loxodonta, with the third genus Mammuthus extinct...
.