Do the Baby and Mother's Blood Mix in the Placenta
Jail cell Adh Migr. 2007 Jan-Mar; 1(1): 19–27.
Cell Migration from Baby to Mother
Gavin Due south Dawe
oneDepartment of Pharmacology; Yong Loo Lin Schoolhouse of Medicine; National University of Singapore; Singapore
Xiao Wei Tan
2Establish of Molecular and Prison cell Biology; Singapore
Zhi-Cheng Xiao
2Institute of Molecular and Cell Biology; Singapore
threeDepartment of Clinical Research; Singapore General Infirmary; Singapore
ivDepartment of Anatomy; Yong Loo Lin School of Medicine; National Academy of Singapore; Singapore
Received 2007 February 27; Accepted 2007 Feb 28.
Abstruse
Fetal cells migrate into the mother during pregnancy. Fetomaternal transfer probably occurs in all pregnancies and in humans the fetal cells can persist for decades. Microchimeric fetal cells are found in diverse maternal tissues and organs including claret, os marrow, skin and liver. In mice, fetal cells accept besides been found in the brain. The fetal cells likewise announced to target sites of injury. Fetomaternal microchimerism may have important implications for the immune status of women, influencing autoimmunity and tolerance to transplants. Further agreement of the power of fetal cells to cross both the placental and claret-brain barriers, to migrate into diverse tissues, and to differentiate into multiple cell types may also advance strategies for intravenous transplantation of stem cells for cytotherapeutic repair. Hither we hash out hypotheses for how fetal cells cross the placental and blood-brain barriers and the persistence and distribution of fetal cells in the mother.
Cardinal Words: fetomaternal microchimerism, stem cells, progenitor cells, placental barrier, blood-brain barrier, adhesion, migration
Microchimerism is the presence of a modest population of genetically distinct and separately derived cells inside an private. This commonly occurs following transfusion or transplantation.1 – iii Microchimerism can besides occur between mother and fetus. Pocket-sized numbers of cells traffic across the placenta during pregnancy. This exchange occurs both from the fetus to the female parent (fetomaternal)4 – 7 and from the mother to the fetus.8 – ten Similar substitution may also occur between monochorionic twins in utero.11 – 13 In that location is increasing bear witness that fetomaternal microchimerism persists lifelong in many changeable women.7 , fourteen The significance of fetomaternal microchimerism remains unclear. Information technology could exist that fetomaternal microchimerism is an epiphenomenon of pregnancy. Alternatively, it could exist a mechanism by which the fetus ensures maternal fitness in order to raise its own chances of survival. In either case, the occurrence of pregnancy-acquired microchimerism in women may have implications for graft survival and autoimmunity. More detailed agreement of the biology of microchimeric fetal cells may as well accelerate progress towards cytotherapeutic repair via intravenous transplantation of stem or progenitor cells.
Trophoblasts were the first zygote-derived prison cell blazon constitute to cantankerous into the mother. In 1893, Schmorl reported the advent of trophoblasts in the maternal pulmonary vasculature.15 Later, trophoblasts were also observed in the maternal circulation.16 – 20 Subsequently various other fetal cell types derived from fetal blood were too plant in the maternal circulation.21 , 22 These fetal jail cell types included lymphocytes,23 erythroblasts or nucleated red blood cells,24 , 25 haematopoietic progenitors7 , 26 , 27 and putative mesenchymal progenitors.14 , 28 While it has been suggested that small numbers of fetal cells traffic across the placenta in every human pregnancy,29 – 31 trophoblast release does not appear to occur in all pregnancies.32 Likewise, in mice, fetal cells have likewise been reported in maternal blood.33 , 34 In the mouse, fetomaternal transfer also appears to occur during all pregnancies.35
Anatomy of the Placenta
Human and rodent placentation is hemochorial, the fetomaternal interaction betwixt the two blood circulations involving straight physical interaction between maternal claret and the chorionic trophoblasts.36 The fetal and maternal blood circulates in channels lined by these zygote-derived cells within the placental region known as the labyrinth in mice or the fetal placenta in humans (Fig. 1). In the human, the channels through which the fetal blood flows, the chorionic villi, form copse with numerous branches and sub-branches terminating in villous blunt-endings. The maternal claret flows in the relatively open intervillous infinite. In contrast in the mouse, the maternal blood flows through a labyrinthine network of interconnected cavities or lacunae.36 A layer of trophoblast cells forms the interface between the maternal blood and the fetal tissues. It is these trophoblast cells that form the placental bulwark between maternal and fetal circulation. In the human, this interface consists of a syncytium of syncytiotrophoblasts directly contacting the maternal claret (Fig. 2B). In the starting time trimester, there is also a layer of replicating mononuclear cytotrophoblasts beneath the syncytiotrophoblasts. In dissimilarity, in mice there are iii layers of trophoblasts. The outer layer consists of mononuclear cytotrophoblasts while the eye and inner layers are syncytiotrophoblastic.36 Between the trophoblasts and the fetal blood at that place are a trophoblastic basement membrane, in some but not all interfaces a core of extracellular matrix and/or pericytes, an endothelial basement membrane, and fetal capillary endothelial cells36 (Fig. 2B). Fetal blood enters and leaves the fetal placenta/labyrinth via the umbilical cord, whereas maternal blood enters and leaves the fetal placenta/labyrinth via the utero-placental apportionment.
A simplified diagrammatic representation of the structure of the human placenta (adapted from Georgiades et al.36) and hypothesized mechanisms of fetomaternal cell traffic. From the end of the first trimester, maternal blood flows into the fetal placenta via the maternal spiral arteries, through the intervillous infinite bathing the branches of the villous trees and out through the maternal veins (red arrows on left-paw side). The fetal blood enters via the umbilical cord and circulates to the fetal capillaries in the villous trees. A layer of zygote-derived trophoblasts, in humans a syncytium of syncytiotrophoblasts, on the surface of the villous copse (dark dark-green) forms the barrier between the fetal tissues and the maternal claret. Zygote-derived trophoblasts also progressively invade the placental bed and line the maternal vasculature. By the third trimester the maternal screw arteries are lined through to the (im), while the maternal veins are lined to the border between the decidua basalis (db) and basal plate (bp). In the mouse, the counterpart of the fetal placenta is labyrinthine and the trophoblastic invasion of the maternal blood vessels does non extend beyond the junctional zone analogous to the basal plate. Hypothesized mechanisms of fetomaternal jail cell traffic include (i) deportation of trophoblasts lining the maternal vessels and intervillous space; (ii) microtraumatic hemorrhage; and (iii) cell adhesion and transmigration across the placental barrier.
Simplified diagrammatic representations of blood-brain and placental barriers and hypothesized molecular mechanisms of cell adhesion and transmigration. (A) A simplified diagrammatic representation of multistep lymphocyte recognition and capture from claret at the blood brain bulwark (adapted from Engelhardt48). Cells expressing α4β1 are captured by VCAM-i expressed past endothelial cells. In that location is a rapid activation stage (seconds) that may involve lymphoid chemokines CCL19/ELC and CCL21/SLC. At that place is a prolonged adhesion stage (hours) followed past dull transmigration (hours) dependent upon binding of LFA-ane to ICAM-1 and/or ICAM-2 on the endothelial cells. It is hypothesized that a like molecular mechanism may explain fetal jail cell migration across the claret-brain barrier and the placental bulwark. (B) A simplified diagrammatic representation of the human placental barrier showing a hypothetical machinery of fetal cell capture, adhesion and transmigration. The placental barrier comprises of fetal capillary endothelial cells (fcec), an endothelial basement membrane (ebm), the villous core (vc) which at some interfaces contains pericytes (p) and extracellular matrix, a trophoblastic basement membrane (tbm), in the kickoff trimester a layer of proliferative cytotrophoblasts (ct), and a multinucleated syncytium of syncytiotrophoblasts (ss). In the mouse, the trophoblastic layers differ in that in that location are 2 syncytiotrophoblastic layers and the cytotrophoblastic layer is outermost facing the intervillous interface. It is hypothesized that fetal cells may adhere and transmigrate across the placental bulwark in a like mode to that past which lymphocytes cross the blood-brain bulwark.
The zone bordering the maternal surface of the fetal placenta/labyrinth is known as the basal plate in humans and the junctional zone or spongiotrophoblast zone in mice. This region is not perfused by fetal blood but is crossed past maternal blood channels lined by zygote-derived trophoblast cells through which the maternal blood flows in and out of the fetal placenta/labyrinth.36 This zone in turn is bordered past the maternal uterine tissue on the maternal side. The maternal uterine tissue becomes progressively invaded by zygote-derived trophoblast cells. In particular, these cells line the maternal blood vessels in the maternal uterine tissue. The maternal uterine tissues of this region, known as the placental bed in humans, tin can exist divided into the decidua basalis next to the basal plate/junctional zone and the myometrium on the maternal side. In humans, trophoblast invasion extends to the inner tertiary of the myometrium but in mice, trophoblast invasion is shallow and is limited to the decidua basalis.36 , 37 Even within the decidua basalis, maternal arteries and veins remain lined by maternal endothelium rather than trophoblasts in the mouse.38 , 39 While in the human being the trophoblasts stimulate arterial remodeling in the mouse uterine natural killer cells are more important.39 – 41
The cells of the placenta itself contain both zygote-derived and maternal cells. In mice, the zygote-derived cells include trophoblasts derived from the polar trophectoderm of the outer cell mass; fetal blood vessels and mesenchyme derived from the allantoic mesenchyme, which in turn is derived from the primitive ectoderm of the inner cell mass; and fetal blood cells of mesodermal lineage. Meanwhile, the maternal cells of the mouse placenta include uterine cells and cells coming from the maternal blood.36 It is mostly assumed that the origin of human placental cells is similar to those in the mouse, although not lineage studies accept been performed on human placentae.36 However, at that place is debate over whether the human allantoic vasculature, through which the fetal blood passes, is of trophectodermal or epiblast/hypoblast origin.36 , 42
The similarities in the anatomy of placentation and placental claret flow in mice and humans36 , 39 and the function of coordinating genes in mouse and human placentation43 make mouse placentation a good model for many aspects of human placentation. Nevertheless, there are important anatomical differences,36 , 39 in particular the difference between the villous nature of the human being fetal placenta and the labyrinthine nature of the coordinating mouse labyrinth and the greater role of invasion past zygote-derived trophoblasts in the maternal circulation in the human placenta.
Cell Traffic Beyond the Placenta
The mechanism by which cells are exchanged across the placental barrier is unclear. Possible explanations include deportation of trophoblasts, microtraumatic rupture of the placental blood channels or that specific cell types are capable of adhesion to the trophoblasts of the walls of the fetal blood channels and migration through the placental bulwark created past the trophoblasts (Fig. 1i–1iii). Intervillous thrombi containing mixed maternal and fetal cells occur in the fetal placenta/labyrinth.44 , 45 Histological defects in the continuity of the trophoblasts lining the vasculature of the placenta are also reported.46 , 47 Together these observations suggest the possibility that fetomaternal hemorrhage within the fetal placenta/labyrinth may allow exchange of cells betwixt the fetal and maternal circulation. Microtraumatic dislodgment of trophoblasts from the trophoblast-lined blood channels through which the maternal blood passes may too explain why trophoblasts announced in maternal circulation. The microtraumatic hypothesis of cell exchange does non appear consistent with the hypothesis that fetomaternal microchimerism may exist of adaptive value to the fetus but fits well with the hypothesis that fetomaternal microchimerism is an epiphenomenon of pregnancy with potential pathological consequences.
An alternative hypothesis is that cells cross the placental bulwark by mechanisms akin to the agile adhesion and transmigration that occurs across high endothelial venule (HEV) endothelium in peripheral lymph nodes and at the claret-brain barrier (BBB).48 Intriguingly, in the mouse at least some of the fetal cells that enter the mother are likewise capable of crossing the blood encephalon barrier into the brain.35 , 49
At the BBB and HEV, lymphocyte migration across the endothelial membrane involves a multistep process of recognition and recruitment from the claret involving tethering/rolling or capture, activation, adhesion and finally transmigration (Fig. 2A). In both HEV endothelium and BBB, the final stage of transmigration involves binding of LFA-1 expressed past the lymphocytes to ICAM-1 in HEV endothelium and to ICAM-one and/or ICAM-two at the BBB.48 , l , 51 In the HEV endothelium, the ICAM-1 also appears to be involved in the adhesion preceding transmigration, whereas at the blood brain barrier VCAM-1 is involved in lymphocyte capture and adhesion. Fetal cells crossing the placental barrier must transmigrate both the fetal capillary endothelial cell layer and the trophoblast cell layers (Fig. 2B).
The fetal capillary endothelial cell layer expresses a number of prison cell adhesion molecules including PECAM-one and ICAM-1.52 , 53 While, there is VCAM-1 expression in umbilical cord endothelium in that location appears to be no evidence for VCAM-one expression on fetal capillary endothelium in normal placenta at term.52 , 53 As PECAM-1 plays a function in integrin-mediated neutrophil adhesion and endothelial transmigration,54 – 56 including migration of CD34+ positive cells57 such equally the fetal cells in maternal claret,vii we hypothesize that it is also a candidate for contribution to fetal cell transmigration across the fetal capillary endothelium (Fig. 2B). The functional ligand for PECAM-1 in transmigration is unknown, just it is possible that it is an αvβ3 integrin.58 It is possible that multiple fetal prison cell types cantankerous the placental barrier by different mechanisms.
Once the fetal cells take crossed the fetal capillary endothelium, they must next cross the trophoblast layer. Trophoblasts express ICAM-1 in vitro and in vivo59 – 61 and monocytes bind to ICAM-1 expressed by trophoblasts in an LFA-1-dependent manner.lx Similarly, the migration of Toxoplasma gondii beyond epithelial barriers, including the placental bulwark comprised of trophoblast cells, involves interaction of the parasite adhesion molecule, MIC2, with the intercellular adhesion molecule ane (ICAM-1).62 Together these studies suggest that the molecular apparatus for maternofetal transmigration may be nowadays at the placental barrier. Although at that place is testify for greater in vivo expression of ICAM-one on the apical surface of the villous syncytiotrophoblasts exposed to the maternal blood,60 ICAM-1 is also present throughout the stroma of the chorionic villi,60 , 61 although it has not been conspicuously established that information technology is expressed on the basal surface of the trophoblasts facing the villous cadre. Trophoblasts as well express VCAM-1.63 – 65 Thus the molecular apparatus for fetomaternal transmigration of fetal cells expressing LFA-1 may also be present at the trophoblast cell layer. In one case the fetal cells have crossed the fetal capillary endothelial cell layer, we hypothesize that they cantankerous the trophoblast jail cell layer again in a manner similar to that in which lymphocytes cantankerous the BBB (Fig. 2B).
We hope that this speculative hypothesis regarding the mechanisms of fetomaternal cell traffic may stimulate further enquiry and that time to come studies volition make up one's mind whether agile fetomaternal adhesion and transmigration occurs and elucidate the molecular mechanisms involved.
Timing of Onset of Fetomaternal Traffic
In mice, fetal cells generally first appear in the mother in the second calendar week of pregnancy35 (see besides Fig. 3). Numbers of fetal cells are nowadays in maternal blood past GD10 to GD12 days (gestational days, the twenty-four hour period of vaginal plug detection existence designated GD0) in pregnancies from syngenic and allogenic crosses; even so the cells do not appear in blood in until GD13 to GD16 in pregnancies from outbred crosses.66 The advent of fetal cells in maternal blood at GD10 to GD12 in syngenic and allogenic crosses is consequent with the establishment of uteroplacental circulation. Maternal blood first appears in the labyrinth between GD9 and GD10 and extensive fetal capillary formation occurs by GD12.39 , 67 This coincides with the onset of fetal circulation on the completion of organogenesis at GD9 to GD10.36 In humans, fetal DNA has been detected in maternal blood as early as four weeks and five days after formulation and both fetal cells and Deoxyribonucleic acid are consistently detected from 7 weeks.68 , 69 Thus in humans, the first advent of fetal cells in maternal blood occurs slightly earlier the completion of fetal organogenesis, the onset of fetal circulation to the placenta, and the appearance of maternal blood within the fetal placenta. Plugs of invading trophoblast cells, which block the tips of the uteroplacental spiral arteries, are progressively dislocated afterward 10–12 weeksseventy and blood only becomes axiomatic in the intervillous space of the fetal placenta after x weeks gestation.71 Effective arterial apportionment of the placenta is not established until around the 12th week of gestation39 , 72 , 73 when the human embryo has largely completed the organogenesis stage.36 In the mouse, the timing of the appearance of fetal cells in maternal claret is consistent with the hypothesis that fetomaternal exchange occurs between fetal and maternal blood at the placental barrier in the fetal placenta/labyrinth. In the fetal placenta/labyrinth, the maternal claret comes into directly contact with the zygote-derived trophoblast and information technology has been proposed these may also be deported into the maternal circulation.66 The fetal placenta/labyrinth is also very rich in fetal hematopoietic stem cells74 – 76 and it has been suggested that these cells might able to migrate into the maternal claret.66 The earlier advent of fetal cells in maternal blood in humans may suggest more than active migration of certain fetal cells. Potentially there may exist multiple cell types and phases of migration involved. More detailed investigation of the fourth dimension course of the appearance of maternal blood in the placenta and the appearance of fetal cells in maternal claret in humans may be informative.
Time class of fetal prison cell engraftment and persistence in the mouse brain. Developed female person mice received intraventricular injection of the excitotoxic NMDA to produce a diffuse brain lesion or were untreated. The mice were crossed with adult male enhanced light-green fluorescent poly peptide (EGFP) transgenic Green Mice. Fetomaternal microchimerism in the encephalon was assayed at various time points: gestational days (GD) 7 and 14, the day of parturition (P0), and at seven days (P7), four weeks (P4W) and eight weeks (P8W) post partum (n = 3–8 per grouping at each time point). The number of fetal cells relative to total cells present in a encephalon block centered virtually the site of the injection was quantified by real-time PCR for the EGFP gene in genomic DNA. Procedures were every bit previously described.49 There are groovy individual differences, however, in those mothers in which fetal cells were detected in the brain, the number of fetal cells detected in the encephalon increases by 4 weeks mail partum and declines over again past eight weeks post partum. Overall, in those mothers in which fetal cells persist at four weeks and eight weeks post partum, at that place are greater numbers of fetal cells in the lesioned brains.
The reason for the filibuster in the appearance of fetal cells in maternal blood in outbred mouse crosses is at nowadays unknown. Outbred crosses were also observed to result in delayed and reduced trophoblast invasion of the decidua basalis.66 It may be that the appearance of fetal cells in maternal blood on outbred crosses is due to a more aggressive immune response; alternatively the delay may be due to a filibuster in the maturation of the placenta and maternal apportionment to the labyrinth. Information technology is hoped that further studies may elucidate the issue.
Intriguingly in syngenic pregnancies, fetal cells were detected in mouse lungs and to a lesser extent spleen and kidney in the first week of gestation before they robustly appear in detectable numbers in maternal circulation.35 , 66 Ane explanation might be that, consistent with the appearance of trophoblasts in maternal lungs in humans,15 these cells are trophoblasts. Thus one might hypothesis that the earliest phase of fetomaternal microchimerism involves deportation of zygote-derived trophoblasts as they invade the decidua basalis to line the maternal blood vasculature. In particular, the fate of the trophoblasts that plug the ends of the maternal arteries of the uteroplacental circulation may be to get dislodged into maternal apportionment as maternal blood period begins to intermission through into the fetal placenta/labyrinth. Trophoblasts existence large are rapidly cleared from maternal blood every bit they become lodged in the microvasculature of the lung and to a lesser extent other organs. While the studies discussed hither have made important contributions to establishing the time course of fetomaternal traffic, the question of whether different zygote-derived cell types show different time courses of traffic has not been investigated in depth. It is hoped that futurity studies will address this important issue.
Frequency and Persistence of Fetomaternal Microchimerism
Fetomaternal microchimerism appears to occur with groovy frequency post-obit human pregnancy. It has been suggested that fetomaternal traffic occurs in all pregancies.14 Moreover fetal cells are reported to persist in the mother for decades. Male cells have been found in maternal blood even decades after pregnancy,vii , 77 including in one case in which the women was final pregnant with a male child 27 years before.7 Fetal cells also may persist for fifty-fifty longer after engrafting maternal os marrowxiv and possibly other organs. By engrafting into niches such as the bone marrow, fetal cells may also exist able to proliferate and reinfiltrate blood or other tissues later. There is strong evidence that fetal cells with the characteristics of mesenchymal cells do engraft the os marrow. Male DNA was detected in 48% of CD34-enriched apheresis products from nonpregnant female marrow donors.1 Male person cells were besides detected in all bone marrow samples from women who had previously been significant with males, including one woman who was last meaning with a son 51 years earlier.14
The absenteeism of Y chromosome markers in samples from women who had never born sons in some studies14 strongly supports the argument that the male person cells observed originate from the fetus. Even so, it is important to note that there are crucial caveats in the apply of the Y chromosome alone as a marker for fetomaternal microchimerism that may have led to over estimation of the incidence and persistence of fetomaternal microchimerism in humans. Male cells take been found in the claret of women without sons.78 , 79 Male cells may occur in the blood of as many as 8–ten% of healthy women without sons and no known history of ballgame.79 It has been speculated that the male cells arise from unrecognized spontaneous abortions, vanished male person twins, an older brother transferred by the maternal circulation, or sexual intercourse. However, a history of unrecognized spontaneous abortions or sexual intercourse cannot explain all cases of the presence of male cells in females equally another study detected the presence of the Y chromosome in normal liver from seven of 11 female fetuses and v of six female person children.fourscore Such microchimerism may be best explained, by fetofetal transfer from an undetected vanishing male twin or maternofetal transfer of male cells harbored past the female parent. Estimates of the frequency of vanishing twins range from 3.7–100% of pregnancies81 however not all twins share continued placenta vasculature, particularly at the early stages of evolution at which many twins disappear. Maternofetal transfer to the mother may also have occurred if the mother's mother had a history of blood transfusion, transplantation or previous pregnancy with a male fetus. It is difficult to guess how frequently male cells in females could arise as a result of fetofetal or maternofetal transfer. Although 1 might look such events to be rare, the incidence may exist loftier enough to have biased estimates of the incidence of fetomaternal microchimerism in humans. While the possibility that the Y chromosome could besides enter the mother via microchimerism every bit a consequence of previous claret transfusion or transplantation has been considered in most studies, the possibility that male cells detected in the female parent may have arrived via fetofetal or maternofetal transfer to the mother in utero has not be systematically excluded. Conclusive proof of fetomaternal microchimerism in humans would require the use of other paternal markers that differentiate betwixt the male parent of the fetus and the male parent of the mother. One scenario might exist to investigate cases where the female parent and the mother'southward begetter share a genetic mutation or polymorphism non carried past the father of the fetus. In such cases, evidence of genetic markers derived from the father of the fetus in the mother could provide more conclusive testify of fetomaternal microchimerism in humans. If the genetic mutation or polymorphism acquired affliction the presence of fetal cells in the diseased tissue could as well offer show of the potential of fetomaternal tissue repair.
In contrast, to the suggestion that fetal cells are retained for decades subsequently nigh every human pregnancy,vii , 14 the retentiveness of fetal cells in mice appears more sporadic and rarely persists for more than than a few weeks mail partum. The use of mice bearing unique genetic markers such equally, the cytogenetic marker chromosome, T626 , 33 and more than recently transgenic mice bearing genetic markers such as enhanced green fluorescent protein (EGPF)35 , 49 , 66 has conclusively demonstrated fetomaternal microchimerism. The number of mice in which fetal cells can be detected in maternal claret and the number of fetal cells in maternal blood declines towards the end of gestation, at least in syngenic and allogenic crosses.66 Across the first week postpartum, fetal cells are rarely detected in maternal blood;35 , 66 although they have been found in some mice at 21 days mail partum following allogenic crosses and at 42 days post partum, but not 21 days postal service partum, following outbred crosses.66 Likewise, in maternal bone marrow, spleen, liver, eye, lung and kidney fetal cells practise not appear to be retained past maternal mice beyond the first week mail service partum.35 Fifty-fifty within the first week postal service partum, the retention of fetal cells is sporadic and highly variable between individuals.35 Our own observations suggest that there might exist greater retention of fetal cells within the encephalon equally although fetal cell numbers are depression, cells persist to 4 weeks post partum49 (see also Fig. three). However, by half-dozen–8 weeks post partum, the number of fetal cells has fallen beneath the limits of detection in blood and all organs studied, including uninjured encephalon66 (meet also Fig. iii). Although the numbers of fetal cells present were very low, fetal cells did persist at 8 weeks post partum in some of the lesioned maternal brains (Fig. 3). Together, these data suggest the possibility that, although fetal cells are cleared from the blood and some organs within a few weeks postpartum in mothers of syngenic and allogenic crosses, some fetal cells may remain harbored longer-term in certain niches. In dissimilarity, fetal cells have been detected in the blood of some mice at 42 days postal service partum post-obit outbred crosses.66 Additionally, there is express evidence that in some, but not all mice, repeated pregnancies may lead to greater memory of fetal cells,35 , 49 which may suggest that in some mothers there is longer-term retentiveness of fetal cells. Nonetheless, the elapsing of fetal cell retention in those few mice in which fetal cells do persist has not been systematically investigated. The reasons for the large individual differences in the numbers of fetal cells retained and the duration of retention are not known.
During pregnancy the mother develops immune tolerance to the fetus just later on pregnancy this suppression of the maternal immune response to the fetus is lifted.82 It is conceivable that, although fetomaternal cell traffic probably occurs in every pregnancy, persistence of microchimeric fetal cells after pregnancy depends upon the immunocompatibility betwixt the mother and fetus. This might explain why fetomaternal microchimerism does not persist in all mothers. The greater preservation of fetal cells in the brain than the blood would be consistent with an allowed rejection hypothesis, the brain being an immune privileged site.83 However, it is difficult to reconcile the hypothesis that immune rejection explains the great inter-private variability and low charge per unit of fetal prison cell persistence in syngenically crossed mice66 as there is less allowed rejection on transplantation betwixt syngenic mice. Although some differences between the mother and fetus may exist an advantage every bit it has been noted that, despite reducing placental expression of major histocompatibility complex (MHC) genes, major histocompatibility circuitous expression is ofttimes reestablished in the near invasive trophoblast cells and may contribute to an immunoprotective result on the fetus.84
In conclusion, although it has not been studied systematically and there are obvious methodological differences between the mouse and human studies, there appears to greater likelihood of long-term retention of microchimeric fetal cells in humans than in mice. This departure in the retention of fetal cells may be consistent with the hypothesis that fetomaternal microchimerism has developed as a mechanism by which the fetus ensures maternal fitness. As mice wean their offspring by iii–iv weeks postpartum, there would be no need for the fetal cells to continue to survive. In dissimilarity, human mothers nurse their offspring for many months and thereafter go along to nurture their offspring for many decades and so there may be an adaptive reward to fetal cell persistence. Alternatively, if fetal cells take adverse effects on the mother, information technology may be that rodents have developed greater maternal resistance to fetal jail cell infiltration equally they have far more offspring over a far shorter life span.
Intriguingly, in that location may in fact exist greater retention of fetal cells in outbred mice than in syngenic or allogenic crosses.66 That humans, who are generally outbred, retain fetal cells may be further prove against the immunocompatibility hypothesis for fetomaternal microchimeric persistence. It is hoped that futurity studies may investigate the determinants of fetal cell memory. The immunological hypothesis would predict that immunosuppression from tardily pregnancy and through the postal service-partum menstruum would increase fetomaternal microchimerism. Another hypothesis might exist that hormonal changes coinciding with the later stages of pregnancy and the post partum catamenia lead to rejection of fetal cells. This hypothesis would predict greater fetomaternal microchimerism in mother who did not consummate the normal hormonal sequela of commitment and peri- and post-partum hormonal changes. In humans, in that location is indeed evidence that spontaneous and induced abortions increment the frequency and level of male person microchimerism,79 , 85 but this may equally be explained by trauma associated with abortion leading to greater fetomaternal exchange.
Distribution of Microchimeric Fetal CellS
The microchimeric fetal cells in the female parent appear to be of multilineage potential. Y chromosome bearing cells have been identified in numerous tissues, including pare, liver, kidney and bone marrow, in healthy women and in women with autoimmune diseases86 – 92 and other none immune diseases such as hepatitis C93 and cervical cancer.94 In that location is at present a big literature on fetomaternal microchimerism, specially in autoimmune disease, and overall there appears to be testify of increased fetal cell presence in diseased tissues than healthy tissues.27 , 95 It is debatable whether microchimerism plays a office in triggering autoimmune illness,86 – 89 , 91 mayhap by stimulating graft-host disease or host-graft disease,96 or whether fetal cells home in on diseased tissue and contribute to tissue repair.27 , 96 In systematic lupus erythematosus, for case, it appears that microchimeric fetal cells are more likely to be institute in severe cases than in mild cases97 suggesting that the fetal cells are not causing the disease but rather are targeting the diseased maternal tissue once the damage reaches a threshold level.27 Similarly, in an animal model of excitotoxic brain injury nosotros institute greater numbers of fetal cells in the injured brain region.49 Fetal cells may also persist longer at sites of injury than in uninjured tissue (Fig. 3). This suggests the possibility that fetal cells may target to specific tissues and contribute to tissue repair or part.
There are various manners in which fetal cells might come to target damaged tissue. Sometimes the machinery by which the zygote-derived cells are sequestered in detail tissues may exist mechanical as has been hypothesized for the entrapment of large trophoblast cells in the capillaries of the microvasculature of the lung.15 Likewise, targeting of injured tissues may just be a mechanical procedure whereby tissue impairment is associated with micro-harm to the blood vessels and cells of all types are more likely to leak out into the damaged tissue. Some other hypothesis is that fetal cells invade all maternal tissues but but find a niche conducive to survival in damaged tissues. Alternatively, if this is a procedure that has evolved to allow the fetus to treat the female parent to enhance fetal survival, the fetal cells may actively invade the damaged tissue past a physiological machinery of adhesion and transmigration beyond the blood vessel walls followed by active migration through the tissue to sites of damage.
Recently, Khosrotehrani and colleagues98 have used in vivo bioluminescence imaging of fetal cells in which the paternal marker was VEGF receptor 2 promoter controlled luciferase factor expression to demonstrate that fetal cells contribute to neoangiogenesis. This in vivo bioimaging approach will exist extremely valuable in determining the extent to which fetal cells invade damaged tissues. Tracking genetically modified fetal cells or the behaviour of fetal cells in genetically modified mothers it may be possible to accost important questions about the mechanisms by which fetal cells engraft maternal tissues and home in on injured tissue.
Types of Fetal Cells Involved in Fetomaternal Microchimerism
The fetal cell type or types responsible for fetomaternal microchimerism are unknown. Candidates include all jail cell types in fetal blood and trophoblasts. However, considerable testify points towards the conclusion that fetal stem or progenitor cells may also exist involved. Subsequent pregnancies appear to trigger further proliferation and mobilization to maternal blood of fetal cells acquired during previous pregnancies.34 The very fact that fetal cells can exist detected decades afterward pregnancyseven , 14 , 99 is stiff evidence that these cells are replicating in the mother. Moreover, women with older sons take a greater number of male cells suggesting proliferation over time.93 Although fetal cells were non detected in all ex-breeder mice those mice that had fetal cells in the brain tended to accept higher numbers than in mice that had only delivered one litter suggesting accumulation or proliferation of fetal cells.49 The numbers of fetal cells detected in the maternal encephalon also showed marked postnatal increase between the last day of gestation and 4 weeks post partum (Fig. iii). This evidence that fetal cells can proliferate in the mother is adequately persuasive, but the alternative possibility that the fetal cells engraft in one niche and and so subsequently remobilize to some other niche without increasing in number has not been excluded.
Fetal cells announced indistinguishable from maternal tissues years subsequently pregnancy and can bear epithelial, leukocyte, hematopoietic, hepatocytic, renal or cardiomyocytic markers.27 , 95 , 100 That microchimeric fetal cells also appear to be able to differentiate to adopt cellular characteristics of various host organs suggests that they may be stalk or progenitor cells. In injured mouse encephalon, we have constitute fetal cells expressing various morphologies, localization and immunocytochemically stained protein markers characteristic of diverse brain cell types including perivascular macrophages, neurons, astrocytes and oligodendrocytes.49 While the evidence for differentiation may appear persuasive, important culling hypotheses accept yet to be excluded. Notably at that place have even so to be articulate-cutting examples of functional differentiation of microchimeric fetal cells. For case, it would exist important to show that apparent neuronal differentiation does non just involve location, morphology and expression of a few protein markers merely instead that this differentiation leads to functional neuronal characteristics such equally the chapters to fire activity potentials and synaptic connectivity to repair damaged circuitry. Likewise in the case of apparent oligodendrocytic differentiation, morphology and poly peptide expression should exist accompanied past functional wrapping of axons, and recovery of motor function in demyelination models.
At present there is little testify for or against fusion as a mechanism of the apparent differentiation in microchimeric fetal cells. While a binucleated fetal jail cell was observed juxtaposed to a blood vessel in the brain in a niche in which other fetal cells adopted a perivascular macrophage-similar graphic symbol,49 it is unclear whether this represents a fusion event, a prison cell division upshot, or a multinucleated cell type. Systematic and careful report of fusion events in fetomaternal microchimerism volition exist important in interpreting whether apparent differentiation of fetal cells is in fact the consequence of cell fusion. Typically prison cell fusion in iatrogenic microchimerism following transplantation has been studied past fluorescent in situ hybridization (FISH) for X and Y chromosome markers. The presence of multiple Ten chromosomes in the cells bearing Y chromosomes has been taken as evidence of fusion. However, the study of cell fusion by this method in fetomaternal microchimerism is complicated. Not but may the Y chromonsome non be a specific marker for fetal cells as discussed in a higher place, but the trophoblasts, one of the jail cell types which contribute to fetomaternal microchimerism, tin can be multinucleated and due to the mosaic nature of the placenta could naturally behave multiple Ten chromosomes together with the Y chromosome in cases of vanishing female twins or in the rodent model where most litters contain both male and female person offspring. Other strategies will be required to investigate fusion in fetomaternal microchimerism. For instance, combining labeling for paternal-specific and maternal specific markers (east.g., crossing male person EGFP transgenic mice with DsRed transgenic mice). Alternatively, Cre/lox recombination might be used to detect cell fusion events101 merely this approach would require in utero implantation of homozygous embryos, which may alter fetomaternal prison cell traffic.
If the multilineage differentiation chapters of microchimeric fetal cells does prove to exist genuine and functional this suggests that the fetal cells responsible are stalk cells. The type of stem prison cell or stalk cells involved is controversial. In that location is some show implicating haematopoietic stem cells. For case, male cells that persist in maternal claret after pregnancy are CD34+/CD38+,7 conduct like proliferative haematopoeitc progenitor cells in vitro civilisation,102 and in haematopoietic tissues, such as the lymph nodes and spleen, the bulk of microchimeric male cells express CD45.95 In contrast, in that location is besides evidence suggesting that fetal mesenchymal stem cells (fMSC) are involved. Fetal MSCs have been identified in maternal blood during pregnancy.28 , 103 Fisk and colleagues appear to favor the interpretation that these cells are fetal mesenchymal stalk cells considering, at least when found in the bone marrow, male cells in mothers were immunophenotypically mesenchymal.14 However, it has been pointed out that the extent of the multilineage differentiation of microchimeric male person cells argues confronting a strictly mesenchymal lineage.104 Indeed, unless one accepts the still controversial concept of stem cell plasticity and transdifferentiation, neither haematopoietic nor mesenchymal stem cells could explain the full range of differentiation, for example into neural cell types,49 that has been reported. The diversity of cell types into which microchimeric fetal cells can plainly differentiate suggests that, if a unmarried stalk or progenitor cell type is involved, information technology is a very early stem cell type.27 , 95 , 100 Bianchi and colleagues have referred to these cells as pregnancy-associated progenitor cells (PAPC) and announced to favor the estimation that they may be a relatively early stem prison cell blazon retaining multilineage potential.27 , 91 , 95 , 100 The alternative possibility that numerous cell types of unlike lineage enter the female parent has non been excluded. Perhaps the involvement of a number of jail cell types including diverse types of early stem cells could improve explain the diverseness of differentiation reported.
Conclusions and Future Prospects
Fetal cells showroom a remarkable power to migrate across the placenta into the female parent and to integrate with diverse maternal tissues and organs, plain homing in specially to sites of impairment and illness.49 , 97 Much remains to exist learned about the bones biology of fetomaternal microchimerism. The cell type or types involved have yet to be conclusively characterized. If various jail cell types are involved, it will be important to sympathize the fourth dimension course of the migration of the various cell types and their persistence in the mother. Studies of the process of cellular adhesion and migration that allow the cells to cross the placental barriers, infiltrate tissues and organs, cross the BBB and migrate to sites of impairment will exist peculiarly informative. Although long-term persistence of fetal cells may be less frequent in the mouse, the mouse appears to offer a useful model for investigating aspects of fetomaternal traffic during pregnancy.
In the longer-term, elucidation of the biology of fetomaternal microchimerism may take of import implications for understanding autoimmunity and graft-host interactions. Moreover, knowledge of the prison cell types and molecular mechanisms that let for the remarkable migratory and multilineage differentiation capacity of microchimeric fetal cells in the female parent may improve strategies for cytotherapeutic repair. Harnessing the capabilities of microchimeric fetal cells may enhance the prospects for minimally invasive intravenous delivery of stem cells.
Footnotes
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