Spatiotemporal development of the neuronal accumulation of amyloid precursor protein and the amyloid plaque formation in the brain of 3xTg-AD mice.

The amyloid plaque is a hallmark of Alzheimer's disease. The accumulation of the amyloid precursor protein (APP) in the neuronal structure is assumed to lead to amyloid plaque formation through the excessive production of ß-amyloid protein. To study the relationship between the neuronal accumulation of APP and amyloid plaque formation, we histologically analyzed their development in the different brain regions in 3xTg-AD mice, which express Swedish mutated APP (APPSWE) in the neurons. Observation throughout the brain revealed APPSWE-positive somata in the broad regions. Quantitative model analysis showed that the somatic accumulation of APPSWE developed firstly in the hippocampus from a very early age (<1 month) and proceeded slower in the isocortex. In line with this, the hippocampus was the first region to form amyloid plaques at the age of 9-12 months, while amyloid plaques were rarely observed in the isocortex. Females had more APPSWE-positive somata and plaques than males. Furthermore, amyloid plaques were observed in the lateral septum and pontine grey, which did not contain APPSWE-positive somata but only the APPSWE-positive fibers. These results suggested that neuronal accumulation of APPSWE, both in somatodendritic and axonal domains, is closely related to the formation of amyloid plaques.

Glycolytic activity is required for the onset of neural plate folding during neural tube closure in mouse embryos.

Physiological hypoxia is critical for placental mammalian development. However, the underlying mechanisms by which hypoxia regulates embryonic development remain unclear. We discovered that the expression of glycolytic genes partially depends on hypoxia in neuroepithelial cells of E8.25 mouse embryos. Consistent with this finding, inhibiting glycolysis during the early phase of neural tube closure (E8.0-8.5) resulted in a neural tube closure defect. In contrast, inhibiting the electron transport chain did not affect neural tube formation. Furthermore, inhibiting glycolysis affected cell proliferation, but not differentiation and survival. Inhibiting glycolysis repressed the phosphorylation of myosin light chain 2, and consequent neural plate folding. Our findings revealed that anaerobic glycolysis regulates neuroepithelial cell proliferation and apical constriction during the early phase of neural tube closure.

Influence of Immune System Abnormalities Caused by Maternal Immune Activation in the Postnatal Period.

The developmental origins of health and disease (DOHaD) indicate that fetal tissues and organs in critical and sensitive periods of development are susceptible to structural and functional changes due to the adverse environment in utero. Maternal immune activation (MIA) is one of the phenomena in DOHaD. Exposure to maternal immune activation is a risk factor for neurodevelopmental disorders, psychosis, cardiovascular diseases, metabolic diseases, and human immune disorders. It has been associated with increased levels of proinflammatory cytokines transferred from mother to fetus in the prenatal period. Abnormal immunity induced by MIA includes immune overreaction or immune response failure in offspring. Immune overreaction is a hypersensitivity response of the immune system to pathogens or allergic factor. Immune response failure could not properly fight off various pathogens. The clinical features in offspring depend on the gestation period, inflammatory magnitude, inflammatory type of MIA in the prenatal period, and exposure to prenatal inflammatory stimulation, which might induce epigenetic modifications in the immune system. An analysis of epigenetic modifications caused by adverse intrauterine environments might allow clinicians to predict the onset of diseases and disorders before or after birth.

LIF-IGF Axis Contributes to the Proliferation of Neural Progenitor Cells in Developing Rat Cerebrum.

In rodent models, leukemia inhibitory factor (LIF) is involved in cerebral development via the placenta, and maternal immune activation is linked to psychiatric disorders in the child. However, whether LIF acts directly on neural progenitor cells (NPCs) remains unclear. This study performed DNA microarray analysis and quantitative RT-PCR on the fetal cerebrum after maternal intraperitoneal or fetal intracerebral ventricular injection of LIF at day 14.5 (E14.5) and determined that the expression of insulin-like growth factors (IGF)-1 and -2 was induced by LIF. Physiological IGF-1 and IGF-2 levels in fetal cerebrospinal fluid (CSF) increased from E15.5 to E17.5, following the physiological surge of LIF levels in CSF at E15.5. Immunostaining showed that IGF-1 was expressed in the cerebrum at E15.5 to E19.5 and IGF-2 at E15.5 to E17.5 and that IGF-1 receptor and insulin receptor were co-expressed in NPCs. Further, LIF treatment enhanced cultured NPC proliferation, which was reduced by picropodophyllin, an IGF-1 receptor inhibitor, even under LIF supplementation. Our findings suggest that IGF expression and release from the NPCs of the fetal cerebrum in fetal CSF is induced by LIF, thus supporting the involvement of the LIF-IGF axis in cerebral cortical development in an autocrine/paracrine manner.

Hif1α-dependent hypoxia signaling contributes to the survival of deep-layer neurons and cortex formation in a mouse model.

Hypoxia-inducible factor 1 α (Hif1α) plays a crucial role in brain development. To study the function of Hif1α in early brain development, we generated neuroepithelial cell-specific Hif1α-knockout mice. Hif1α-knockout mice died soon after birth; these mice exhibited an abnormal head shape, indicating the presence of brain defects. Morphological analysis revealed that Hif1α ablation reduced the overall size of the brain, especially affecting the telencephalon. Neuronal apoptosis predominantly occurred in deep-layer neurons, consequently the alignment of cortical layers was severely disorganized in Hif1α knockout mice. Furthermore, we demonstrated that Vegf signaling contributes to the survival of deep-layer neurons as a downstream effector of Hif1α-dependent hypoxia signaling. Taken together, our findings demonstrate that Hif1α plays a critical role in the early stages of telencephalon development.

MC5R Contributes to Sensitivity to UVB Waves and Barrier Function in Mouse Epidermis.

MC5R is known for its role in the exocrine function of sebaceous glands, but other functions in the epidermis remain unclear. This study focused on the relationship between MC5R and homeostasis in the epidermis and examined the role of MC5R in mice whose skin was irradiated with UVB waves. UVB irradiation-induced skin ulcers and severe inflammation at lower doses in homozygotes of MC5R-deficient (i.e., MC5R -/- ) mice (150 mJ/cm2) than the doses in wild-type mice (500 mJ/cm2). Transepidermal water loss was increased (approximately 10-fold) in adult MC5R -/- mice compared with that in wild-type mice. In neonates, a dye exclusion assay showed no remarkable difference between MC5R -/- and wild-type mice. After UVB irradiation, compared with wild-type mice, MC5R -/- mice showed increased inflammatory cell infiltration in the dermis of the ulcerative region, significantly increased thickness of the epidermis in the nonulcerative region, significantly more prickle cells in the nonulcerative region, and increased serum IL-6 levels but decreased IL-10 levels. Transmission electron microscopy revealed fewer lamellar granules, less lipid secretion, and an expansion of the trans-Golgi network in the epidermis in MC5R -/- mice. This study elucidated the increased sensitivity to UVB irradiation and decreased barrier function in MC5R -/- mice.

Leukemia Inhibitory Factor Induces Proopiomelanocortin via CRH/CRHR Pathway in Mouse Trophoblast.

We previously showed that maternal leukemia inhibitory factor (LIF) induces placental production of adrenocorticotropic hormone (ACTH), which stimulates fetal nucleated red blood cells to further secrete LIF and promote neurogenesis in rodent brains. However, the underlying mechanism of LIF-dependent ACTH induction remains unclear. Recently, we found that LIF induces corticotropin-releasing hormone (CRH) in mouse trophoblast stem cells. This finding supports the results of a previous study that CRH, which is produced by the placenta, induces placental ACTH production. In this study, we examined whether the effects of LIF are mediated by the induction of Pomc via CRH upregulation in mouse trophoblast. In vivo, protein levels of LIF and CRH peak in mouse placenta at 13.5 days post coitum. In mouse placenta, Crh mRNA and protein levels significantly increased 3 h after intraperitoneal injection of LIF (5 µg/kg body weight) into dams at 13.5 days post coitum. We also examined the effect of LIF-induced CRH on the expression of Pomc induced by LIF in mouse trophoblast stem cells in vitro. After LIF supplementation for 3 days, we found that the increased expression of Crh-induced by new supplementation of LIF was earlier than that of Pomc. Furthermore, LIF-induced upregulation of Pomc in mouse trophoblast stem cells was attenuated by inhibition of the CRH/CRHR1 pathway, whereas LIF-induced secretion of ACTH was attenuated by inhibition of the JAK/STAT3 pathway. Therefore, LIF indirectly increases placental Pomc expression through the CRH/CRHR1 pathway, and placental ACTH secretion is induced directly by LIF via the JAK/STAT3 pathway.

Mid-pregnancy maternal immune activation increases Pax6-positive and Tbr2-positive neural progenitor cells and causes integrated stress response in the fetal brain in a mouse model of maternal viral infection.

Maternal immune activation (MIA) in midpregnancy is a risk factor for neurodevelopmental disorders. Improper brain development may cause malformations of the brain; maldevelopment induced by MIA may lead to a pathology-related phenotype. In this study, a single intraperitoneal injection of 20 mg/kg polyriboinosinic-polyribocytidylic acid [poly(I:C)] was administered to C57BL/6J mice on embryonic day (E) 12.5 to mimic maternal viral infection. Histopathological analysis of neurogenesis was performed using markers for Pax6, Tbr2, and Tbr1. In these fetuses, significant increases were observed in the proportion of Pax6-positive neural progenitor cells and Pax6/Tbr2 double-positive cells 24 h after poly(I:C) injection. There were no differences in the proportion of Tbr1-positive postmitotic neurons 48 h after poly(I:C) injection. At E18.5, there were more Pax6-positive and Tbr2-positive neural progenitor cells in the poly(I:C)-injected group than in the saline-injected group. Gene ontology enrichment analysis of poly(I:C)-induced differentially expressed genes in the fetal brain at E12.5 demonstrated that these genes were enriched in terms including response to cytokine, response to decreased oxygen levels in the category of biological process. At E13.5, activating transcription factor 4 (Atf4), which is an effector of integrated stress response, was significantly upregulated in the fetal brain. Our results show that poly(I:C)-induced MIA at E12.5 leads to dysregulated neurogenesis and upregulates Atf4 in the fetal brain. These findings provide a new insight in the mechanism of MIA causing improper brain development and subsequent neurodevelopmental disorders.

Cerebrospinal fluid may flow out from the brain through the frontal skull base and choroid plexus: a gold colloid and cadaverine injection study in mouse fetus.

PURPOSE: It has been commonly accepted for a long time that the cerebrospinal fluid (CSF) drains into arachnoid granulations from the subarachnoid space to the dural venous sinus unidirectionally. However, recently, periventricular capillaries and lymphatic concepts have been introduced. The CSF moves along the perivascular space and drains into the capillary vessels or meningeal lymphatic tissues. CSF is involved in removing brain waste out of the brain. In this study, we investigated the outflow mechanism of substances in the CSF from the brain. METHODS: We investigated the movement of CSF by injection of gold colloid conjugates (2, 40, and 200 nm) into the lateral ventricles of mouse fetuses and evaluated the deposition by silver stain with tissue transparency and electron microcopy. Cadaverine was also injected into the lateral ventricle to determine its movement tract. RESULTS: The gold particle deposition was mainly observed in the frontal skull base. Electron microscopic study showed that the gold particle deposition was observed on the choroid plexus and ependyma in the lateral ventricle and also red blood cells in the heart and liver. Two-nanometer particles were exclusively observed in the liver. Cadaverine injection study demonstrated that cadaverine was observed at the extracranial frontal skull base, choroid plexus, ependymal surface, and perivascular area in the brain white matter. CONCLUSION: The particles in the CSF were shown to move from the brain to the frontal skull base and also into the blood stream through the choroid plexus in the fetus. The outflow of particles in the CSF may be regulated by molecular size. This new information will contribute to the prevention of brain degeneration due to brain waste deposition.

Decidual cells are the initial target of polyriboinosinic-polyribocytidylic acid in a mouse model of maternal viral infection.

BACKGROUND: Maternal immune activation has been implicated in the pathophysiology of neurodevelopmental disorders such as autism spectrum disorders caused by maternal infection. It has been suggested that the placental origin of inflammatory cytokines leads to neurodevelopmental disorders. However, the identity of the initial immune-activated site in the placenta, in response to maternal viral infection, is not clear. METHODS: By cross-breeding male enhanced green fluorescent protein (EGFP) transgenic mice with wild-type females, the placental tissues of maternal origin can be distinguished from those of paternal origin by EGFP expression. Using this method, at embryonic day (E) 12.5, dams were administered an intraperitoneal polyriboinosinic-polyribocytidylic acid (poly [I:C]) injection. We quantitatively analyzed the levels of phosphorylated interferon (IFN) regulatory factor 3 (pIRF3) in the placenta, and investigated the distribution of pIRF3 positive cells. RESULTS: We show that maternally derived decidual cells are the initial target of maternal poly (I:C) through the toll-like receptor 3/TIR-domain-containing the adapter-inducing interferon-ß signaling pathway. We also show that the expression of interferon-ß was upregulated in the placenta after maternal injection with poly (I:C). CONCLUSION: These results suggest that maternally derived decidual cells are the initial target of maternal poly (I:C) and that this innate immune response is likely associated with a state of maternal immune activation.

Exposure to Maternal Immune Activation Causes Congenital Unfolded Protein Response Defects and Increases the Susceptibility to Postnatal Inflammatory Stimulation in Offspring.

BACKGROUND: A number of childhood diseases have been identified, such as severe infection or autoinflammatory disease, in which immune overreaction against inflammation is a possible underlying mechanism. Previous reports have demonstrated that fetal cells exposed to maternal immune activation (MIA) induced by polyriboinosinic-polyribocytidylic acid [poly(I:C)] exhibited hypersensitivity to inflammation in vitro. However, the details of this mechanism remain unclear. Therefore, this study aimed to reveal the reaction to inflammation in offspring exposed to MIA in the prenatal period, as well as its molecular mechanism, using a viral infection mouse model. MATERIALS AND METHODS: Pregnant mice at 12.5, 14.5, and 16.5 days post coitum were injected intraperitoneally with poly(I:C) 20 mg/kg body weight (BW) or saline. Offspring aged 3-4 weeks received the second injection of 20 mg/kg BW or 4 mg/kg BW poly(I:C) or saline. Serum and tissues were collected at 2, 24, 48, and 72 h after the postnatal injection. The cytokine profile, histopathology of organs, and unfolded protein response (UPR) in offspring were examined. RESULTS: The serum levels of interleukin (IL)-6, IL-17, and interferon-γ were significantly higher in the MIA group, and acute liver necrosis was detected. Moreover, failure in UPR was observed in the MIA group compared with that in the control group. CONCLUSION: Overall, MIA exposure in utero caused failure in UPR as well as immune overreaction to the second attack of inflammation in offspring. Our results suggested that prenatal exposure to MIA might contribute to the congenital inflammatory constitution after birth.

Rapid bone staining with hair removal (RAP-B/HR): a non-destructive and rapid whole-mount bone staining protocol optimized for adult hairy mice.

We developed a non-destructive and rapid whole-mount bone staining method for small fish, Xenopus laevis, and rodent fetuses (RAP-B). RAP-B does not require skin or soft tissue removal. However, RAP-B requires hair removal from hairy animals, such as adult mice and rats. In the present study, we investigated hair removal chemical treatments that did not result in soft tissue destruction. The hair removal effectiveness was investigated using a calcium mercaptoacetate or sodium mercaptoacetate solution on skin fragments obtained from the back of adult mice. A mixture of 2% sodium mercaptoacetate in 3% potassium hydroxide was found to be the most effective in complete hair removal from the skin. Using this hair removal treatment as a pretreatment for RAP-B, the preparation of fast-acting artifact-free whole-mount bone staining was possible without skin and soft tissue removal (RAP-B/HR). We performed a seamless observation from a low magnification wide-view to a high magnification without artifactacting artifacts using fluorescence zoom microscopy. Therefore, the combination of RAP-B/HR and fluorescent zoom microscopy is a novel platform for three-dimensional, wide-field, high-resolution pathological anatomical analysis.

The Impact of Ovariectomy on Olfactory Neuron Regeneration in Mice.

Estrogen has been shown to affect differentiation and proliferation as a mitogen in various neural systems. Olfactory receptor cells are unique within the nervous system, and have the ability to regenerate even after an individual has reached maturity. Olfactory receptor cells also regenerate after experimentally induced degeneration. The purpose of this study is to observe the influence of estrogen depletion induced by ovariectomy on olfactory nerve regeneration. Female mice underwent bilateral ovariectomy at 8 weeks of age and received intraperitoneal administration of methimazole 1 week later. At 2, 4, and 6 weeks after methimazole administration, the olfactory mucosa was analyzed histochemically to determine olfactory epithelium (OE) thickness, olfactory marker protein distribution, and Ki-67 immunoreactivity. Furthermore, 2 weeks after ovariectomy, trkA protein distribution in the OE and nerve growth factor (NGF) levels in the olfactory bulb were determined by immunohistochemistry and enzyme-linked immunosorbent assay, respectively. Our results showed that in ovariectomized mice OMP, Ki-67, and trkA-immunopositive cells expression decreased at 2 weeks after methimazole injection, a time point at which regeneration is underway. At this same time point, although NGF production in the olfactory bulb had increased before methimazole administration, no differences were observed between the ovx and control groups. These results suggest that estrogen depletion induces a suppressive effect on regeneration of olfactory neurons, and that estrogen may have a potential use in the treatment of sensorineural olfactory dysfunction.

Leukemia inhibitory factor induces corticotropin-releasing hormone in mouse trophoblast stem cells.

Previous studies have shown that some inflammatory cytokines promote the expression of corticotropin-releasing hormone (CRH) in trophoblasts during pregnancy and that placental CRH could induce the production of adrenocorticotropic hormone (ACTH) in humans. However, whether the same is true in rodent placenta remains unclear. In this study, we examined the effect of pro-inflammatory cytokine LIF on the induction of CRH in mouse trophoblast stem cells (mTSCs). During differentiation, the CRH levels in mTSCs gradually increased. On days 3 and 5 after LIF supplementation, Crh expression in the differentiated mTSCs was significantly increased with LIF treatment than those without LIF treatment. Moreover, the CRH concentration in the culture media increased. Thereafter, we examined the contribution of the downstream pathways of LIF to CRH induction in differentiated mTSCs. The LIF-induced upregulation of CRH was attenuated by inhibition of PI3K/AKT and MAPK phosphorylation but not by inhibition of JAK/STAT3. Therefore, in mTSCs, LIF increased Crh expression through activation of the PI3K/AKT and MAPK pathways but not by the JAK/STAT3 pathway. The present study suggests that mTSC is an ideal in vitro model for studying regulation and function of placental CRH.

Effects of Tokishakuyakusan on Regeneration of Murine Olfactory Neurons In Vivo and In Vitro.

Post-upper respiratory tract infection related olfactory dysfunction typically occurs due to neural damage after an upper respiratory tract infection associated with a common cold or influenza. At present, Tokishakuyakusan, a Japanese traditional Kampo medicine, has been found to be effective for post-viral olfactory dysfunction. However, the pharmacodynamics of Tokishakuyakusan in the treatment of post-viral olfactory dysfunction remains unresolved. We investigated the effects of Tokishakuyakusan on the regeneration of olfactory neurons and expression of nerve growth factor (NGF) in neural systems, using in vivo murine studies and in vitro cell culture studies. Eight-week-old BALB/C female mice were fed a pellet diet with or without Tokishakuyakusan. Degeneration of cells in olfactory epithelium was induced by intraperitoneal methimazole injection. Regeneration of olfactory neurons was observed by histological and immunohistochemical procedures. NGF expression in the olfactory bulb was measured by enzyme-linked immunosorbent assay. NGF gene and protein expression were measured using rat primary cultured astrocytes by real-time polymerase chain reaction and enzyme-linked immunosorbent assay. We found that olfactory marker protein, Ki-67, and NGF were more highly expressed in the olfactory epithelium during the regeneration period in mice receiving Tokishakuyakusan. In cultured astrocytes, Tokishakuyakusan as well as its individual components, Atractylodes lancea rhizome and Japanese angelica root, increased NGF expression. Screening assays revealed that NGF production was increased by atractylodin and levistolide A, which are ingredients in Atractylodes lancea rhizome and Japanese angelica root, respectively. These results suggest that Tokishakuyakusan promotes regeneration of olfactory neurons by increasing NGF expression in the olfactory bulb.

A new immunohistochemical method to evaluate the development of vestibular compensation after unilateral labyrinthectomy in rats.

BACKGROUND: Unilateral labyrinthectomy (UL) causes the disappearance of ipsilateral medial vestibular nuclear (ipsi-MVe) activity and induces spontaneous nystagmus (SN), which disappears during the initial process of vestibular compensation (VC). Ipsi-MVe-activity restores in the late process of VC. OBJECTIVE: We evaluated the late process of VC after UL in rats and examined the effects of thioperamide (H3 antagonist) on VC. MATERIALS AND METHODS: MK801 (NMDA antagonist)-induced Fos-like immunoreactive (-LIR) neurons in contra-MVe, which had been suppressed by NMDA-mediated cerebellar inhibition in UL rats was used as an index. RESULTS: The number of MK801-induced Fos-LIR neurons in contra-MVe gradually decreased to the same level as that of sham-operated rats 14 days after UL. Thioperamide moved the disappearance of the MK801-induced Fos-LIR neurons 2 days earlier. The number of MK801-induced Fos-LIR neurons in thioperamide-treated rats was significantly decreased, compared with that of vehicle rats on days 7 and 12 after UL. But, thioperamide did not influence the decline of SN frequency in UL rats. CONCLUSION: These findings suggested that the number of MK801-induced Fos-LIR neurons in contra-MVe was decreased in concordance with the restoration of ipsi-MVe-activity during the late process of VC after UL and that thioperamide accelerated the late, but not the initial process of VC.

Molecular mechanisms underlying the models of neurodevelopmental disorders in maternal immune activation relevant to the placenta.

The rapid rise in the prevalence of autism spectrum disorders (ASD) and other psychiatric disorders displaying similar traits has increased the need to elucidate their molecular mechanisms. Epidemiological studies have shown that maternal infection during mid-pregnancy is associated with increased risk of neurodevelopmental disorders such as ASD in offspring. Using maternal infection models, researchers have gathered evidence relevant to such disorders. A comprehensive summary of the changes in the brain structure, function, and behavior in offspring induced by maternal immune activation (MIA) has been reported. However, the molecular mechanisms underlying the association between MIA and improper brain development, which ultimately lead to neurodevelopmental disorders, have not been fully reviewed. This paper summarizes the currently known molecular mechanisms associated with the MIA model, with a special focus on the role of the placenta in fetal brain development.

Author Correction: A rapid and nondestructive protocol for whole-mount bone staining of small fish and Xenopus.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has not been fixed in the paper.

A rapid and nondestructive protocol for whole-mount bone staining of small fish and Xenopus.

Here we propose a new protocol for whole-mount bone staining, which allows the rapid preparation of highly cleared and nondestructive specimens. It only takes 3 days to complete whole procedure for small vertebrates, such as medaka, zebrafish, and Xenopus frogs. In this procedure, we used a newly developed fixative containing formalin, Triton X-100, and potassium hydroxide, which allows the fixation, decolorization, and transparentization of specimens at the same time. A bone staining solution containing alizarin red S with ethylene glycol and a clearing solution containing Tween 20 and potassium hydroxide also contributed the specificity and swiftness of this new system. As expected, although details of the skeletal system could be observed in specimens with high transparency, it was noteworthy that high-resolution fluorescence images acquired using zoom microscopes clearly delineated the shape of each bone. This new procedure would be expected to be widely used as a standard procedure for bone staining in the testing the developmental toxicity of chemicals and in the screening test of knockout or mutant animals.

Spinal nerve defects in mouse embryos prenatally exposed to valproic acid.

To examine in detail spinal nerve defects induced by prenatal exposure to valproic acid in mice, pregnant ICR mice were subcutaneously injected with a single dose of 400 mg/kg valproic acid on gestational day 6, 7, 8, or 9, and their embryos were observed on gestational day 10. The whole-mount immunostaining using an anti-neurofilament antibody allowed us to identify spinal nerve defects, such as a loss of bundle, anastomosis among bundles arising from adjacent segment, and a disrupted segmental pattern of the dorsal root ganglia, in valproic acid-exposed embryos. The prevalence of spinal nerve defects was the highest in the embryos exposed to valproic acid on gestational day 8 among the experimental groups. Then, effects of the administration dose of valproic acid on the prevalence of spinal nerve defects were examined on gestational day 10 and found to be dose-dependently increased. It was noteworthy that all embryos exposed to 600 mg/kg of valproic acid on gestational day 8 suffered spinal nerve defects. Folic acid (3 mg/kg/day) supplementation during gestational day 6-10 suppressed the prevalence of valproic acid-induced neural tube defects, which are common malformations in offspring prenatally exposed to valproic acid, but not that of spinal nerve defects. Thus, the spinal nerve defects due to prenatal valproic acid exposure might be induced by mechanisms different from those of neural tube defects. Because spinal nerve defects were predicted to be caused by the disrupted segmental arrangement of the somites and/or that of neural crest cells, which was the origin of the dorsal root ganglia and/or abnormal polarity of the somite, this mouse model with spinal nerve defects at high incidence would be useful to examine the effects of valproic acid on the somitogenesis and morphogenesis of somite-associated structures.