Feto–Maternal Microchimerism and the Brain: Mechanisms, Neurological Implications, and Translational Perspectives

ABSTRACT

Problem

Microchimerism in the brain is a common phenomenon, where in cells cross between mother and fetus during pregnancy, and persist for decades. It has been studied primarily within reproductive immunology and transplantation medicine. The relevance of microchimerism to central nervous system biology, neurological disease, and experimental chimeric modelling has received comparatively little systematic attention. This review sought evidence across the biology of feto–maternal microchimerism, its association with neurological disease, and the emerging field of experimental chimeric brain modelling, to extrapolate a cohesive mechanistic framework.

Method

Research articles in reproductive immunology, neurodevelopment, and neurodegeneration were gathered to assess the potential roles of fetal microchimeric cells (FMc) in brain health and disease. By combining natural microchimerism with experimental chimeric models, a framework for understanding how nonself cells influence the maternal brain was extrapolated and critical mechanisms identified. Searches were conducted across PubMed/MEDLINE, Scopus, and Google Scholar using a dual‐concept Boolean.

Results

The literature reviews show that microchimeric cells cross the blood‐brain barrier (BBB), adopt neural and glial phenotypes in the maternal brain parenchyma, and exhibit injury‐responsive recruitment in preclinical models. Reciprocally, maternal microchimeric cells (MMc) are present in the offspring brain, where they have been found adopting neural and immune‐lineage phenotypes in experimental models. These findings raise the possibility that bidirectional microchimerism influences susceptibility to neurological disease, modulates neuroimmune signaling, and contributes to endogenous repair, although causal mechanisms remain unresolved. Experimental chimeric brain models have extended these principles into therapeutic contexts. Establishing the functional mechanism and directionality requires more prospective longitudinal cohort studies and transcriptional profiling at single‐cell resolution in microchimeric brain‐resident populations.

1. Introduction

The long‐term persistence of genetically distinct cells acquired during pregnancy is increasingly recognized as a biologically active component of maternal physiology. The discovery of feto–maternal microchimerism dates back to Georg Schmorl in 1893, although historically studied in reproductive immunology, recent evidence demonstrates that fetal microchimeric cells (FMc) cross the blood brain barrier, adopt neural phenotypes, and persist for decades in the maternal brain [1]. These findings challenge classical concepts of CNS immune privilege and raise the possibility that FMc contributes to neuroimmune signaling, neuroplasticity, and disease susceptibility. The reciprocal arm of this exchange has been directly demonstrated by experimental and clinical findings, most recently by Kanaan et al. [2], who identified microchimeric cells in the CNS of children (individuals who have never been pregnant), establishing that these cells are of maternal origin. This finding reinforces the view that microchimerism in the CNS represents a bidirectional, life‐long cellular legacy rather than a strictly maternal phenomenon.

Chimerism is different from mosaicism. Mosaicism results from genetic variability arising within a single organism due to post‐zygotic mutations in a clonal cell lineage, whereas chimerism involves the presence of two or more genetically distinct cell populations that originate from separate zygotes [3]. Human chimerism includes three main categories. Natural chimerism occurs spontaneously and is often found by chance during genetic testing. It consists of feto–maternal microchimerism, which is the bidirectional transfer of cells between a pregnant woman and her fetus [4, 5]. Another aspect is twin chimerism, where dizygotic co‐twins exchange blood cells through connected placental blood vessels [5, 6, 7]. Lastly, there is fusion chimerism (tetragametism), which results from the early fusion of two separately fertilized embryos [7, 8]. In contrast, iatrogenic chimerism occurs due to medical procedures, mainly organ and bone marrow transplants [9, 10]. Here, the recipient permanently takes in genetically different donor cells [5].

The frequency of natural chimerism in the general population depends on its form. For example, tetragametic chimerism is uncommon and often clinically silent, frequently going undiagnosed [5]. Feto–maternal Microchimerism, by contrast, is likely universal, as probably everyone contains microchimeric cells from their mothers: maternal microchimeric cells (MMc) have been detected in fetal tissues as early as the second trimester (22 weeks) and persist into adult (93 years) life across multiple organs and tissues [2, 11] Also Stevens et al. [12] found both FMc and MMc to be common in the blood of healthy controls and suggests that Mc within tissues may be a normal phenomenon. FMc include stem and progenitor cells that can move, settle, develop into various cell types and traffic bidirectionally during pregnancy within maternal tissues [13]. These cells have been found in maternal blood, bone marrow, liver, skin, heart, and lung. Importantly, they can remain for decades after delivery, sometimes over 30 years [7, 13]. Nelson [14] was the first to connect fetal microchimerism to systemic maternal disease. She suggested that these long‐lasting foreign cells could contribute to the higher rates of autoimmune disease in women and the changes in autoimmune conditions during pregnancy.

The relevance of chimerism in neuroscience extends beyond naturally acquired microchimerism. A growing body of experimental research has intentionally used chimerism, implanting human neural and glial progenitor cells into rodent brains to create human‐animal chimeric models. These models were used to study neurological diseases like Alzheimer's, Parkinson's, and Huntington's disease, as well as conditions that standard animal models do not adequately reflect [15]. This review aims to create a unified framework that highlights the important connection between feto–maternal microchimerism and neurological diseases.

2. Methods

Research on feto–maternal chimerism and its link to the central nervous system, neurological disease, and experimental chimeric brain modeling was combined. Three main areas were searched: reproductive immunology, clinical neuroscience, and experimental stem cell biology. Searches were conducted using PubMed/MEDLINE, Scopus, and Google Scholar from their inception until January 2026. A two‐concept Boolean structure was used, with concept 1 focusing on microchimerism and concept 2 focusing on the brain, and neurological disease, linked by “AND,” and “OR.” Studies were included if they were peer‐reviewed primary research, systematic reviews, or case reports published in English; with direct relevance to neurological disease, or neuroimmunology; or established foundational studies on the mechanisms of feto–maternal microchimerism. Studies were excluded if they focused solely on chimerism in nonneural peripheral tissues without relevance to neurology (Green and Glasgow 2006; Ferrari, 2015) [16, 17].

3. Chimerism and Types

3.1. Feto–Maternal Microchimerism

The perinatal period is crucial for offspring development, where environmental factors can have lasting effects on health. Feto–maternal microchimerism is the most common form of natural chimerism in humans. During pregnancy, there is a two‐way exchange of cells and genetic material across the placenta, allowing fetal cells to enter the mother's bloodstream and maternal cells to move into the fetus. This transfer is natural and asymmetric, with more fetal‐to‐maternal movement than the opposite, and it is not limited to cell‐free DNA. It also includes intact, nucleated cells such as hematopoietic progenitors, mesenchymal stem cells, trophoblasts, and lymphocyte subsets, each with their own migratory and differentiation abilities. The key difference between cell‐free fetal DNA (cffDNA) and cellular fetal microchimerism (cFMc) is that cffDNA mostly comes from dying placental cells and is quickly cleared after birth, while intact cFMc cells have properties similar to stem cells, allowing them to integrate long‐term and interact continuously with the maternal immune system. The bidirectionality of feto–maternal cell exchange carries distinct implications for CNS biology in each generation. In the maternal brain, fetal progenitor cells of hematopoietic and mesenchymal origin engraft and persist as demonstrated by Y‐chromosome‐based detection in female autopsy specimens [1]. In the offspring brain, MMc are prevalent, as demonstrated by Kanaan et al. [2] who identified maternal Mc in 70% of pediatric epilepsy brain specimens using polymorphism‐specific qPCR with quantities ranging as high as 458.89 genome equivalents per 100 000 cell equivalents (gEq/10⁵) and a distribution spanning the temporal, frontal, parietal lobes, and hippocampus. Critically, maternal Mc prevalence in healthy neurotypical brains (78%) was comparable to that in epilepsy specimens, indicating that CNS microchimerism is a normative biological feature rather than a disease artefact, and this evidence positions the brain as an organ in which genetically nonself‐cells are routinely accommodated across the life course.

3.2. Other Forms of Chimerism: Twin, Tetragametic, and Iatrogenic

Beyond feto–maternal microchimerism, three other forms of chimerism are relevant to this review. Twin chimerism arises through vascular anastomoses in dizygotic twin gestations, enabling bidirectional blood cell exchange first described by Owen [18]. Tetragametic chimerism results from the early fusion of separately fertilized embryos, distributing two diploid lineages across organ systems including, potentially, the nervous system [7, 8]. Iatrogenic chimerism, the most clinically established form, occurs through hematopoietic stem cell transplantation (HSCT) or solid organ transplant, and provides direct evidence that genetically foreign cells can cross the blood‐brain barrier (BBB), adopt microglial identities, and influence neurological disease trajectories [19, 20].

Human studies show that blood chimerism occurs in ∼8% of dizygotic twins, 21% of triplets, and over 90% of monochorionic dizygotic twin, often linked to assisted reproductive therapy (ART), with chimeric lymphocyte levels ranging from 1–99% and persisting into early childhood [21]. Iatrogenic chimerism affects the central nervous system, with 8%–42% of allogeneic HSCT recipients showing CNS‐specific donor–recipient chimerism, including cases where cerebrospinal fluid (CSF) reveals loss of donor alleles despite full donor chimerism in blood. This may point to the fact that the CNS can function as a semi‐independent chimeric compartment [19, 22, 23, 24]. In parallel, HSCT‐derived donor microglia can cross the blood–brain barrier and replace dysfunctional host microglia in metabolic leukodystrophies, demonstrating that genetically foreign cells can integrate into the human brain and directly modify neurodegeneration and neuroimmune function as reported by Zimmerman and Shenoy [20].

3.3. Mechanisms of Trafficking, CNS Entry, and Neural Differentiation

Fetal microchimerism was first confirmed when Lo et al. (1989) [25] detected Y‐chromosome sequences in the blood of women carrying male fetuses, and later work showed that fetal progenitor cells can persist in maternal blood for decades [4]. The report establishes fetal microchimerism as a stable biological phenomenon rather than transient contamination. Subsequent studies using PCR and Y‐chromosome fluorescence in Situ hybridization (FISH) identified microchimeric cells across multiple maternal organs, including liver, thyroid, skin, heart, kidney, and the brain, where Chan et al. [1] found male DNA in 63% of female autopsy brains and demonstrated that these cells can adopt neuronal, astrocytic, oligodendroglial, and microglial phenotypes. feto–maternal microchimerism arises through bidirectional cellular trafficking, allowing transfer of hematopoietic stem cells, mesenchymal progenitors, lymphocytes, and trophoblast‐derived cells as early as 5weeks of gestation [26, 27]. Transfer increases during placental disruption, and maternal cells likewise enter fetal tissues, where they can persist into adulthood [11, 28]. Of mechanistic significance is the nonrandom distribution of microchimeric cell fates identified by Kanaan et al. [2], who studied surgically resected brain specimens from 52 pediatric epilepsy patients using polymorphism‐specific quantitative PCR targeting HLA‐region loci. Maternal Mc was detected in 70% of patients, distributed across temporal, frontal, parietal, and hippocampal regions, with quantities ranging up to 458.89 genome equivalents per 100 000 cells. Rather than mirroring the composition of the host brain's indigenous cell population, maternal Mc was enriched in two specific subpopulations: layer 2/3 intratelencephalic (L2/3 IT) excitatory cortical projection neurons, predominantly in younger brains, and microglial/perivascular macrophage‐like cells, more prevalent in older specimens. L2/3 IT neurons are among the latest‐born cortical cell types, generated during the final waves of corticogenesis, suggesting that rare maternal progenitor cells may become incorporated during developmental windows in which the local microenvironment favors upper‐layer neuronal identity. The microglial enrichment is consistent with experimental models in which maternal Mc preferentially acquires myeloid fates in murine brain [1] and with observations of donor‐derived microglial replacement following allogeneic HSCT in humans. The analysis revealed Mc across all major neural and glial populations, including neurons, oligodendrocytes, astrocytes, microglia, and endothelial cells, demonstrating a degree of multipotency encompassing both neuroectodermal and myeloid lineage fates [2].

The BBB, made up of tightly linked endothelial cells, astrocyte end‐feet, and pericytes, restricts the movement of cells and large molecules. Still, microchimeric cells are found in the brain tissue, and several potential mechanisms exist. First, microchimeric cells that are hematopoietic or mesenchymal stem cells may use the same passage methods as leukocytes [27]. They can cross the BBB during times when the barrier becomes more permeable, such as during fever, inflammation, eclampsia, and neuroinflammation around birth when tight junctions loosen. The choroid plexus epithelium offers a less restrictive barrier between blood and CSF (Figure 1). Mouse studies support these pathways, showing that fetal cells preferentially accumulate in brain regions with high vascular permeability and increase in number when the BBB is experimentally disrupted [29]. Although not yet directly tested in fetal microchimerism, the choroid plexus is recognized as an immunological surveillance hub [30], making it a biologically plausible conduit for microchimeric cell entry and residence within the maternal CNS. CD34⁺ functions as the primary hematopoietic stem/progenitor marker that demonstrates FMc include genuine, lineage‐capable stem cells rather than terminally differentiated cells. These findings support the idea that fetal progenitor cells can take on neural roles once they move into the maternal brain tissue [31].

FIGURE 1.

Fetal microchimeric cell dynamics in the maternal brain. (A) entry routes from maternal circulation to brain parenchyma via diapedesis, choroid plexus, and peripartum BBB remodeling. (B) Mechanisms of long‐term immune evasion and persistence. Teal: source/input cells; purple: supported mechanisms; coral: disputed mechanisms; blue: observed outcomes; grey: structural barriers. BBB, blood‐brain barrier; NeuN → Neuronal Nuclei (also known as RBFOX3); GFAP → Glial Fibrillary Acidic Protein (Astrocytic differentiation markers) [2, 32, 33].; Chan et al. [1]; Khosrotehrani et al. [34], and Bianchi et al. [4]). Illustrations were created using BioRender.com.

Khosrotehrani et al. [34] proposed true transdifferentiation driven by local neural signals, a view supported by Zeng et al. [35] and Nguyen et al. [36], who found microchimeric cells expressing neuronal, astrocytic, and oligodendroglial markers in the maternal brain. Other studies suggest partial phenotype acquisition without full lineage conversion, where environmental cues induce neural features [37, 38]. Alternative explanations attribute neural marker expression to immunostaining artifacts or intercellular material transfer [39, 40]. The persistence of these cells is supported by CNS immune‐privileged features such as reduced MHC expression, local immunosuppressive factors, and limited dendritic cell presence [41], in combination with long‐lasting pregnancy‐induced tolerance mechanisms [42], that enables microchimeric cells to survive and remain integrated within maternal brain tissue [43].

3.4. Functional Roles of FMc in the Brain

3.4.1. Neurogenic Contribution and Structural Plasticity

Zeng et al. [35] showed in a mouse model that pregnancy‐associated progenitor cells (PAPCs) demonstrated progressive growth and development over time, exhibiting markers including those for neural stem cells, immature neurons, and eventually, mature neurons, following a path similar to that of natural adult brain cell development. Therefore, it can be suggested that fetal‐derived cells can integrate into the maternal brain and exhibit neuron‐like development. Their actual functional impact in humans remains incompletely understood; however, the identification of MMc adopting L2/3 intratelencephalic neuronal fates in the human brain by single‐nucleus transcriptomics [2] provides the first direct evidence of neuroectodermal lineage acquisition by naturally acquired Mc in living human tissue, lending mechanistic credence to earlier experimental transdifferentiation models. This preferential adoption of L2/3 IT neuronal fate by microchimeric cells in younger brains, as characterized by Kanaan et al. [2], is consistent with the hypothesis that maternal progenitor cells incorporate during late corticogenesis when local microenvironmental competence favors upper‐layer neuronal identity.

3.4.2. Cerebral Immunosurveillance

Fetal‐derived immune‐lineage cells seem able to function as a surveillance system in the maternal brain, positioning themselves to monitor and respond to injury, these cells are selectively recruited to damaged regions, where their presence is linked to lower cell death and better repair [44].

3.4.3. Matrescence‐Related Neural Remodeling

The timing of fetal microchimeric cell engraftment with the peak of neuroplasticity associated with becoming a mother has led to the deductions that fetal cells play a role in this remodeling [45, 46]. The FMc may serve as an immune reserve that responds to injury in the maternal brain. Recruitment mediated by CCL2 is linked to decreased cell death and improved repair in postpartum models [29, 47].

3.5. Feto–Maternal Chimerism and Neurological Disease

Over the past two decades, research has begun to map associations between microchimerism and several neurological conditions. These links suggest roles from immune support to tissue repair, and in some cases, possible contributions to pathology. The growing body of evidence encourages further investigation into how microchimeric cells interact with neuroinflammation, neurodegeneration, and the central nervous system's immune response. Major neurological diseases associated with microchimerism include stroke, autism, brain tumors, Parkinson's disease, Alzheimer's disease, and multiple sclerosis (MS).

3.5.1. Alzheimer Disease

The most compelling evidence connecting FMc to neurological disease comes from studies on Alzheimer's disease (AD). Chan et al. [1] conducted a postmortem analysis of brain tissue from 59 women, including 26 without neurological disease as controls and 33 with Alzheimer's disease. Using real‐time quantitative PCR targeting the Y‐chromosome‐specific DYS14 gene, they demonstrated that the frequency of chimeric cells in AD brains is significantly lower compared to healthy controls (p = 0.03 for prevalence, p = 0.06 for concentration). Two main hypotheses explain this connection. The neuroprotective hypothesis proposes that microchimeric cells, potentially differentiating into microglia‐like phagocytes as observed by O'Donoghue et al. [31], support amyloid‐β clearance and neuroimmune surveillance (Figure 2); their relative absence in AD may reflect a failure of this protective mechanism. The research used XY fluorescence in‐situ hybridization to identify male cells in bone marrow‐derived mesenchymal stem cells and rib sections from women with male pregnancies. In their controlled study of 16 marrow samples and 6 rib sections, they found male cells in all 9 women who had male pregnancies but in none of the 5 controls who had only daughters or had never been pregnant (9/9 vs 0/5, p = 0.0005), demonstrating that fetal stem cells can engraft and persist in maternal tissues throughout life. The depletion hypothesis offers a different view, proposing that microchimeric cells are not protective but rather vulnerable: the neuroinflammatory AD microenvironment actively eliminates these foreign cells via activated microglia or astrocytes [33], making reduced FMc an outcome of disease rather than a cause. Distinguishing between these models requires longitudinal data, which postmortem studies cannot provide [1]. This interpretive challenge is further informed by Kanaan et al. [2], who observed reduced Mc in epileptic brain tissue relative to neurotypical controls, suggesting that pathological CNS microenvironments may broadly impair microchimeric cell maintenance, consistent with the depletion model.

FIGURE 2.

Mechanistic framework linking Alzheimer's disease and Microchimerism. Top panel: proposed protective functions of fetal microglia‐like cells in the maternal brain, including immune surveillance, amyloid‐β clearance, and neurotrophic support. Middle panel: Hypothesis 1 (protective/causal): reduced FMc impairs neuroprotection, promoting Alzheimer pathology; Hypothesis 2 (depletion/consequence): Alzheimer neuroinflammation eliminates FMc cells as a secondary effect. Lower panel: convergent Alzheimer disease pathology including amyloid plaques, neuroinflammation, neurodegeneration, and cognitive decline. Illustrations were created using BioRender.com.

3.5.2. MS

MS affects women about three times more often than men, with the highest onset during reproductive years. The PRIMS (Pregnancy in Multiple Sclerosis) prospective cohort: 254 women with relapsing MS across 12 centers, quantified the pregnancy effect precisely. Annualized relapse rate declined from approximately 0.7 per year at baseline to 0.2 per trimester in the third trimester, then rebounded sharply to 1.2 per trimester in the first three months postpartum [48]. While this pattern has conventionally been attributed to systemic immunosuppression during pregnancy (Figure 3), emerging data implicate microchimeric mechanisms as an additional contributor. The primary histopathological evidence comes from Snethen et al. [49], who examined postmortem brain tissue from 6 male MS patients and 6 male controls using X/Y chromosome FISH and found chimeric cells in active MS lesions. These cells expressed both neuronal and immune markers. Although, no significant difference in prevalence between MS and controls was demonstrated.

FIGURE 3.

Proposed immunological pathway linking FMc to demyelination in MS. (A) Fetal cell entry and CNS persistence. (B) HY antigen presentation pathway. (C) Effector mechanisms: molecular mimicry and bystander activation. (D) Convergent outcome: demyelination. Evidence for Y‐chromosome‐positive microchimeric cells in proximity to MS lesions [27]. FMc, fetal microchimeric cells; FY, fetal Y‐encoded antigens; HLA, human leukocyte antigen; HY, male‐specific Y‐encoded antigen; Treg, regulatory T cell; CNS, central nervous system; MS, multiple sclerosis. Illustrations were created using BioRender.com.

The immunological basis for FMc pathogenicity in MS has been hypothesized through the HY antigen mechanism: women who have borne male children carry persistent Y‐chromosome‐bearing microchimeric cells, and chronic low‐level exposure to these HY‐expressing cells may prime or sustain an HY‐directed immune response capable of cross‐reacting with CNS autoantigens through molecular mimicry or bystander activation [50]. These epidemiological and immunological findings could mean that FMc carrying Y chromosome‐based HY antigens may trigger reactions when CNS autoantigens are presented, impacting immune responses relevant to MS [50].

3.5.3. Parkinson Disease

Direct evidence of FMc's role in Parkinson's disease (PD) is limited. Unlike studies on AD and MS, PD research has not systematically examined FMc levels in affected brain areas. The substantia nigra, the main area for dopamine neuron loss in PD, is not typically included in autopsy panels for microchimerism, which target more accessible regions for standard neuropathological sampling. However, several indirect factors suggest a possible connection between fetal microchimerism and Parkinson's disease (PD). First, PD shows notable differences between sexes, with men experiencing a higher incidence but women facing a faster decline in function. These patterns might relate to hormonal and immune factors that connect to FMc biology [51]. Second, historical studies of fetal mesencephalic cell transplantation show that foreign neural cells can survive, integrate, and provide benefits in the parkinsonian striatum, suggesting that naturally occurring FMc cells might also integrate in vulnerable areas [52, 53]. Direct experimental support comes from a mice model 6‐OHDA lesion model in which FMc, tracked by eGFP expression and confirmed by Y‐chromosome FISH, showed a 2.3‐fold increase in the lesioned hippocampus at postpartum day 7 compared with non‐lesioned controls. However, this recruitment was transient: by postpartum day 60, PAPCs (pregnancy‐associated progenitor cells), a fetal microchimeric progenitor population frequency had declined significantly in both hemispheres, indicating that the PD‐like microenvironment promotes early attraction but does not support long‐term engraftment or survival of microchimeric cells [35]. Third, neuroinflammation caused by activated microglia is now recognized as a significant factor in dopamine neuron loss in PD [54]. This phagocytic and immune‐modulating behavior linked to FMc in Alzheimer's disease implies potential similar roles in PD [1]. Higher parity has been associated with modest reductions in PD risk in some cohorts, though the evidence is insufficient to implicate FMc specifically as opposed to the hormonal effects of multiple pregnancies [54], therefore, more studies need to be done to connect the underlying mechanism.

3.5.4. Brain Tumors

FMc carrying a male (XY) karyotype have been identified within glioblastoma multiforme (GBM) tissue of female patients, demonstrating that these cells can traffic to and persist within the CNS tumor microenvironment [32]. This research reviewed medical records of 205 female patients with glioblastoma (GBM) or meningioma, identifying 57 tumor specimens from women with documented histories of male pregnancy. Using quantitative PCR for the Y‐chromosome marker DYS14, with female genomic DNA as a negative control and a sensitivity threshold requiring ≥50% of replicates to amplify, male DNA was detected in 25 of 32 GBM cases (78.1%) and 12 of 25 meningioma cases (48.0%). This difference was significant (p < 0.05, Fisher's exact test). Fluorescence in situ hybridisation (FISH) confirmed intact XY cells within tumor tissue in a subset of cases. Male cell frequency ranged from <1 to 46.1 per 100 000 female cells in GBM, and <1 to 7.3 per 100 000 in meningioma. Importantly, Y‐positive cells were also identified in histologically normal brain parenchyma adjacent to meningiomas, indicating that microchimeric cells are not exclusively recruited by tumor signals but may represent constitutive brain residents enriched within the tumor microenvironment as it expands. These cells did not contribute to the neoplastic clone, their frequency and karyotype identify them as stromal constituents and their phenotypic characterization within CNS tumors, whether immune, endothelial, or mesenchymal, remains to be established [32]. It is unclear whether these cells entered due to tumor‐associated signals or if they are normal brain residents that became more abundant as the tumor disrupted the Blood Brain Barrier. Two competing interpretations have been proposed for the role of Mc in brain tumor. The first holds that microchimeric cells augment antitumor immune surveillance, consistent with epidemiological evidence linking FMc to altered cancer risk [55, 56] and a trend toward improved survival in FMc‐positive GBM patients [32]. The second proposes that shared HLA antigens between fetal‐derived cells and maternal tumor tissue exploit immune tolerance, potentially facilitating immune evasion [57, 58]. More recently, Kanaan et al. [2] observed a trend toward reduced total microchimerism in epileptic versus neurotypical brains, raising the broader question of whether disease‐associated neuroinflammation disrupts the microchimeric niche. Resolving these roles is a critical open question, particularly given the expanding use of immunotherapy in malignant brain tumors.

3.5.5. Stroke, Traumatic Brain Injury (TBI), and Neurological Repair

The acute neurological injury context gives a distinct rationale for FMc involvement in the brain. Ischemic stroke and TBI rapidly disrupt the BBB, creating an open window for cell movement that does not usually happen. In animal models using bone marrow chimeras, where donor‐derived cells are distinct from host cells by genetic or fluorescent markers, BBB disruption consistently leads to increased infiltration of donor‐derived cells into the injured area [59]. Mezey et al. (2003) [60] examined postmortem brain tissue from female patients who had received male bone marrow transplants), identifying Y‐chromosome‐positive cells co‐labeled with neuronal markers by FISH and immunohistochemistry; male‐transplant recipients without neurological injury served as the key comparator, establishing migration rather than artefact as the operative mechanism. Clinical observations that some women exhibit spontaneous neural recovery after stroke that exceeds statistical expectation have prompted speculation that FMc might contribute to repair; although direct evidence is lacking, a tractable strategy would be systematic examination of FMc prevalence and phenotype in peri‐infarct postmortem tissue from women with documented stroke histories, building on studies that have demonstrated fetal microchimeric engraftment and lineage potential in maternal organs [31, 61].

3.5.6. Autism Spectrum Disorder (ASD) and Other Neurodevelopmental Conditions

The relationship between FMc and ASD is speculative but has gained attention in recent years, particularly due to the shared immunological aspects. Epidemiological evidence connects prenatal infections to an increased ASD risk [62]. FMc connects with this hypothesis in two ways: FMc in the maternal brain might influence maternal immune responses in future pregnancies, and materno‐fetal microchimerism (maternal cells entering the fetal brain) might directly affect fetal brain development. Evidence for this direct effect includes findings that MMc can engraft and differentiate within infant tissues from Stevens et al. [12], who performed simultaneous FISH and immunohistochemistry on archived tissues from seven autopsied male infants. Maternal (XX) cells were identified across all subjects and all organs examined: liver, kidney, pancreas, lung, bladder, skin, and spleen, constituting 0.017% to 1.9% of parenchymal cells per tissue. Critically, these cells expressed organ‐appropriate markers: cytokeratin in liver, CAM5.2/AE3 in renal tubules, and insulin in pancreatic islets, with anti‐CD45 co‐staining confirming the non‐hematopoietic identity of differentiated cells. Their presence in non‐inflamed tissues indicates that maternofetal microchimerism is a constitutive developmental phenomenon. While the brain was not examined in this study, the demonstration that maternal cells adopt tissue‐specific functional identities in multiple organ systems provides a plausible cellular basis for investigating whether similar engraftment occurs in the fetal brain. No study has directly enumerated microchimeric cells in postmortem or living ASD brain tissue; the mechanistic hypothesis therefore rests on indirect epidemiological evidence, representing a critical gap for future investigation.

3.5.7. Postpartum Neuropsychiatric Conditions

Evidence from literature proposes that fetal microchimerism may contribute to postpartum depression and psychosis, suggesting that the postpartum period involves swift changes in the immune system and a shift away from the pregnancy‐related tolerance state, disrupting fetal chimerism and causing abrupt changes in circulating cytokine profiles. Corwin and colleagues in 2008, documented elevated IL‐6, TNF‐α, and other pro‐inflammatory markers in postpartum women who develop depressive episodes [63].

3.6. Bidirectional Microchimerism and Neurodevelopment

Maloney and colleagues established that maternal cells persist in adult offspring tissues [11]. In rat models, Kaplan and Land, [64] and Dawe and colleagues [65] demonstrated that MMc can be identified in the brains of offspring following normal pregnancy, and that these cells express both glial and neuronal markers in the offspring brain tissue [65]. Mold et al. [66] showed that maternal alloantigen in utero promotes the development of fetal regulatory T cells, establishing a state of peripheral tolerance to nonself‐cells that shapes the immune phenotype in the offspring's early life. This finding was further corroborated in a murine model experiment by Kinder et al. [67], demonstrating that MMc established in female offspring during development drive systemic accumulation of NIMA‐specific FOXP3+ regulatory T cells, conferring cross‐generational reproductive advantage. This confers a functional significance of microchimeric Treg interactions beyond the index pregnancy [67]. Importantly, the stability of these tolerance phenotypes is not permanent: Shao et al. [68] showed in mice models that preexisting microchimeric cells are displaced by new fetal cells during subsequent pregnancy, with FMc tonic stimulation essential for maintaining protective Treg expansion; NIMA‐specific tolerance was functionally erased by new pregnancy despite persistence of partner‐specific immune resilience, demonstrating context‐dependent plasticity of microchimeric Treg phenotypes [68]. Schepanski et al. [46] also demonstrated that MMc in the developing fetal brain express a unique set of sensome markers, regulate microglial homeostasis, prevent excessive presynaptic elimination, and support the development of prefrontal‐hippocampal circuits and behavioral abilities. Another recent comprehensive human evidence for maternal‐to‐offspring microchimerism in the brain is provided by Kanaan et al. [2], using multimodal genetic analysis of surgically resected pediatric epilepsy specimens and publicly available neurotypical brain datasets to establish that Mc is not randomly distributed across neural cell types; hierarchical clustering of Mc quantities by cell‐type composition identified two dominant fate categories: L2/3 intratelencephalic (IT) excitatory neurons, more prevalent in young brains, and microglial/perivascular macrophage‐like cells, more prevalent in older brains, suggesting that the developmental microenvironment shapes the lineage commitment of integrating maternal cells. Also, a birth order effect was observed: first‐born offspring were significantly more likely to harbor detectable maternal Mc than second‐ or later‐born children, consistent with murine sibling competition models in which later‐born offspring encounter a brain microenvironment already partially occupied by prior microchimeric grafts [1]. Therefore, maternal microchimerism has potential implications for neurodevelopment, immune maturation, and neurological disease susceptibility across the life course. The links between maternal immune activation, exposure to inflammatory cytokines during pregnancy, and increased risks of neurodevelopmental disorders in offspring are well documented [69, 70, 71]. Some Mechanistic work has shown that specific cytokine pathways, including IL‐6 and IL‐17A, can alter fetal brain development and produce long‐lasting behavioral phenotypes [72, 73].

3.7. Experimental Chimeric Brain Model as Research Tool

As earlier stated in the previous sections that naturally acquired FMc can persist and adopt neural phenotypes in the maternal brain. A parallel body of experimental research has used the same principles; neural plasticity, BBB permeability, and immune tolerance of foreign cells, to build chimeric brain models as research tools and therapeutic testbeds. These experiments have been conducted and has established a link between natural chimerism and the experimental model.

3.7.1. Stem Cell‐Derived Neural Grafts

Human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) can now be guided into specific neural subtypes, enabling experimental neural chimerism without using fetal tissue. Protocols that create ventral midbrain dopaminergic progenitors achieve over 80% purity, and transplantation studies in rodent and nonhuman primate Parkinson's models show survival, integration, and behavioral recovery [74]. Beyond dopaminergic neurons, stem‐cell–derived grafts have been developed for spinal cord injury, retinal degeneration, Huntington disease, and stroke, creating chimeric CNS environments where human donor cells must integrate into rodent or primate circuits. These grafts extend axons, mature according to human developmental timelines, and form synapses with host neurons, as shown in xenotransplantation studies [75, 76]. This reflects the broader principle established by Le Douarin's foundational chimera work.

3.7.2. Cerebral Organoid Transplantation

Cerebral organoids, which are self‐organizing neural tissues derived from human pluripotent stem cells, replicate early cortical development, including ventricular zone structure, cortical layering, and various neuronal lineages. However, in vitro, they lack blood vessels and stall at early fetal stages. Transplantation into rodent cortex overcomes these limitations: human organoids become vascularized by host vessels, experience less tissue death, continue maturation, generate new neurons, and form functional synaptic connections with host circuits [77].

Further refinements using neonatal hosts, immunodeficient animals, or injury‐primed cortical environments have also improved graft survival and integration [78]. These advancements show that human cortical tissue can be maintained in a living, vascularized, and functionally integrated state within an animal brain, allowing the study of human‐specific neurodevelopment and disease mechanisms.

3.7.3. Fetal Cell Transplantation in Parkinson Disease

Fetal ventral mesencephalic tissue was historically transplanted into the striatum of patients with Parkinson disease based on the premise that developing dopaminergic neurons could replace lost nigrostriatal input. Open‐label trials showed that grafted fetal dopaminergic neurons survived for more than a decade, integrated into host striatal circuits, and released dopamine as confirmed by PET imaging [79]. Later discoveries of Lewy body pathology within graft‐derived neurons provided evidence for prion‐like transmission of misfolded α‐synuclein from host to graft, highlighting a form of neural chimerism within the human brain [80, 81]. These findings established proof of concept for long‐term human neural integration while encouraging the shift toward standardized stem cell‐derived dopaminergic progenitors.

4. Discussion

Evidence reviewed across natural and experimental systems indicates that microchimerism is a biologically active phenomenon with potential relevance to brain cytoarchitecture and function. FMc cells can enter the maternal brain, persist for decades, and adopt neural or immune phenotypes, suggesting possible roles in neuroimmune modulation, injury response, and disease susceptibility. Across neurological diseases, the strength of association varies. Alzheimer's disease currently has the most robust human data: male microchimerism is detectable in a majority of female brains, with lower prevalence in AD cases, though findings remain cross‐sectional. MS provides the clearest histopathological evidence, with fetal‐derived cells identified within active demyelinating lesions, yet their functional protective, pathogenic, or neutral role remain unresolved. Associations with Parkinson's disease, stroke, and brain tumors are supported primarily by small studies or indirect mechanistic reasoning. These differences highlight the need for systematic, disease‐specific investigation. Methodological limitations also constrain interpretation.

Experimental chimeric brain models, provide biological proof‐of‐concept for what FMc studies observe associatively: that nonself‐cells can enter the brain, survive long‐term, and participate in neural function. The principle of xenogeneic neural integration, demonstrated across dopaminergic grafts, organoid transplants, and glial progenitor replacement models lends mechanistic credibility to the natural microchimerism observations and suggests that the brain is more tolerant of genetically foreign cellular populations than classical immunological models assumed [82]. Advancements have also demonstrated that the cell‐type composition of microchimeric populations shifts with age: neuronal fates dominating in young brains and microglial fates predominating in older individuals, suggesting that distinct developmental windows and tissue niches may differentially shape microchimeric cell fate across the lifespan [2].

Cell densities used in the murine experimental chimerism studies typically exceed those observed in human postmortem tissue [1]. HLA‐specific real‐time qPCR offers superior sensitivity over FISH for low‐frequency Mc quantification, as demonstrated in Scleroderma‐specific (SSc) tissue studies where blood‐based sampling returned false negatives in the same individuals [83]. Also, human FMc brain studies identified in this review that rely on Y‐chromosome detection as a proxy for fetal origin, which had restricted inference to women who have carried male fetuses and precluding detection of XX FMc in women who carried only female offspring. Beyond this, findings of Kanaan et al. [2] expand the conceptual scope of CNS microchimerism, integrating maternal genotyping with quantitative HLA‐polymorphism‐specific qPCR and single‐nucleus transcriptomics. The study provides the first quantitative, region‐resolved assessment of microchimerism in human brain tissue and demonstrates that microchimeric signals can be detected alongside cell‐type‐resolved nuclear transcriptomes. There is therefore the need for research that is methodical, long‐term, and ethically focused to understand how microchimeric cells influence neurological health, disease vulnerability, and the potential for regeneration. The absence of data from low‐ and middle‐income countries which may limit generalizability.

Future progress in this field requires coordinated methodological and conceptual advances. First, prospective longitudinal cohort studies incorporating sensitive FMc quantification, including sex‐independent markers and single‐cell genomic approaches are essential for establishing temporal relationships between FMc dynamics and neurological outcomes. Second, single‐cell transcriptomic and epigenomic profiling of microchimeric cells in postmortem human brain tissue will be critical for defining their lineage identity, functional states, and interactions with local neural and immune networks. Third, mechanistic animal models enabling selective depletion, augmentation, or lineage tracing of FMc populations will allow causal inference regarding their roles in neuroprotection, neuroinflammation, and disease progression. Together, these approaches can transform FMc from an observational curiosity into a mechanistically grounded component of neurobiology.

5. Conclusion

Fetal‐MMc are bidirectional, capable of crossing the BBB, adopting neural and glial phenotypes, responding to injury signals, and potentially influencing neuroinflammation, repair, or vulnerability in disease contexts. Although, this biology has been mechanistically established, its functional significance in humans remains unresolved. At the associative level, significant inverse relationships between microchimeric cell prevalence and Alzheimer disease, with histopathological detection of chimeric cells in MS lesions and brain tumor microenvironments, suggesting that these cells are not neurologically inert. Conceptually, they establish a unifying biological principle that CNS can sustain, integrating and potentially benefiting from genetically foreign cellular populations beyond the explanations that current models of CNS immunology provide. This insight provides implication for future research and the question of whether naturally acquired fetal cells in the human brain contribute to neurological resilience, vulnerability, or repair is one of the more biologically tractable and clinically consequential inquiries in modern neuroimmunology.

Funding

This research received no grant from any funding agency or not‐for‐profit sectors.

Conflicts of Interest

The author declares no competing interests.

Declaration of AI‐Assisted Technology

During the preparation of this manuscript, author used AI assistance for grammar correction of some sentences, and author take full responsibility for the content of the published article.

Data Availability Statement

The authors have nothing to report.