Hotspot analysis of COVID-19 infection using mobile-phone location data.

Restrictions on outdoor activities are required to suppress the COVID-19 pandemic. To monitor social risks and control the pandemic through sustainable restrictions, we focus on the relationship between the number of people going out and the effective reproduction number. The novelty of this study is that we have considered influx population instead of staying-population, as the data represent congestion. This enables us to apply our analysis method to all meshes because the influx population may always represent the congestion of specific areas, which include the residential areas as well. In this study, we report the correlation between the influx population in downtown areas and business districts in Tokyo during the pandemic considering the effective reproduction number and associated time delay. Moreover, we validate our method and the influx population data by confirming the consistency of the results with those of the previous research and epidemiological studies. As a result, it is confirmed that the social risk with regard to the spread of COVID-19 infection when people travel to downtown areas and business districts is high, and the risk when people visit only residential areas is low.

The polarity and properties of radial glia-like neural stem cells are altered by seizures with status epilepticus: Study using an improved mouse pilocarpine model of epilepsy.

In the adult mouse hippocampus, new neurons are produced by radial glia-like (RGL) neural stem cells in the subgranular zone, which extend their apical processes toward the molecular layer, and express the astrocyte marker glial fibrillary acidic protein, but not the astrocyte marker S100ß. In rodent models of epilepsy, adult hippocampal neurogenesis was reported to be increased after acute and mild seizures, but to be decreased by chronic and severe epilepsy. In the present study, we investigated how the severity of seizures affects neurogenesis and RGL neural stem cells in acute stages of epilepsy, using an improved mouse pilocarpine model in which pilocarpine-induced hypothermia was prevented by maintaining body temperature, resulting in a high incidence rate of epileptic seizures and low rate of mortality. In mice that experienced seizures without status epilepticus (SE), the number of proliferating progenitors and immature neurons were significantly increased, whereas no changes were observed in RGL cells. In mice that experienced seizures with SE, the number of proliferating progenitors and immature neurons were unchanged, but the number of RGL cells with an apical process was significantly reduced. Furthermore, the processes of the majority of RGL cells extended inversely toward the hilus, and about half of the aberrant RGL cells expressed S100ß. These results suggest that seizures with SE lead to changes in the polarity and properties of RGL neural stem cells, which may direct them toward astrocyte differentiation, resulting in the reduction of neural stem cells producing new granule cells. This also suggests the possibility that cell polarity of RGL stem cells is important for maintaining the stemness of adult neural stem cells.

Seizure severity-dependent selective vulnerability of the granule cell layer and aberrant neurogenesis in the rat hippocampus.

The pilocarpine-induced status epilepticus rodent model has been commonly used to analyze the mechanisms of human temporal lobe epilepsy. Recent studies using this model have demonstrated that epileptic seizures lead to increased adult neurogenesis of the dentate granule cells, and cause abnormal cellular organization in dentate neuronal circuits. In this study, we examined these structural changes in rats with seizures of varying severity. In rats with frequent severe seizures, we found a clear loss of Prox1 and NeuN expression in the dentate granule cell layer (GCL), which was confined mainly to the suprapyramidal blade of the GCL at the septal and middle regions of the septotemporal axis of the hippocampus. In the damaged suprapyramidal region, the number of immature neurons in the subgranular zone was markedly reduced. In contrast, in rats with less frequent severe seizures, there was almost no loss of Prox1 and NeuN expression, and the number of immature neurons was increased. In rats with no or slight loss of Prox1 expression in the GCL, ectopic immature neurons were detected in the molecular layer of the suprapyramidal blade in addition to the hilus, and formed chainlike aggregated structures along the blood vessels up to the hippocampal fissure, suggesting that newly generated neurons migrate at least partially along blood vessels to the hippocampal fissure. These results suggest that seizures of different severity cause different effects on GCL damage, neurogenesis, and the migration of new neurons, and that these structural changes are selective to subdivisions of the GCL and the septotemporal axis of the hippocampus.

Neuraminidase inhibition promotes the collective migration of neurons and recovery of brain function.

In the injured brain, new neurons produced from endogenous neural stem cells form chains and migrate to injured areas and contribute to the regeneration of lost neurons. However, this endogenous regenerative capacity of the brain has not yet been leveraged for the treatment of brain injury. Here, we show that in healthy brain chains of migrating new neurons maintain unexpectedly large non-adherent areas between neighboring cells, allowing for efficient migration. In instances of brain injury, neuraminidase reduces polysialic acid levels, which negatively regulates adhesion, leading to increased cell-cell adhesion and reduced migration efficiency. The administration of zanamivir, a neuraminidase inhibitor used for influenza treatment, promotes neuronal migration toward damaged regions, fosters neuronal regeneration, and facilitates functional recovery. Together, these findings shed light on a new mechanism governing efficient neuronal migration in the adult brain under physiological conditions, pinpoint the disruption of this mechanism during brain injury, and propose a promising therapeutic avenue for brain injury through drug repositioning.

The PSA-NCAM-Positive "Immature" Neurons: An Old Discovery Providing New Vistas on Brain Structural Plasticity.

Studies on brain plasticity have undertaken different roads, tackling a wide range of biological processes: from small synaptic changes affecting the contacts among neurons at the very tip of their processes, to birth, differentiation, and integration of new neurons (adult neurogenesis). Stem cell-driven adult neurogenesis is an exception in the substantially static mammalian brain, yet, it has dominated the research in neurodevelopmental biology during the last thirty years. Studies of comparative neuroplasticity have revealed that neurogenic processes are reduced in large-brained mammals, including humans. On the other hand, large-brained mammals, with respect to rodents, host large populations of special "immature" neurons that are generated prenatally but express immature markers in adulthood. The history of these "immature" neurons started from studies on adhesion molecules carried out at the beginning of the nineties. The identity of these neurons as "stand by" cells "frozen" in a state of immaturity remained un-detected for long time, because of their ill-defined features and because clouded by research ef-forts focused on adult neurogenesis. In this review article, the history of these cells will be reconstructed, and a series of nuances and confounding factors that have hindered the distinction between newly generated and "immature" neurons will be addressed.

The long-term prognosis of hippocampal neurogenesis and behavioral changes of offspring from rats exposed to valproic acid during pregnancy.

AIM: In pregnant women with epilepsy, it is essential to balance maternal safety and the potential teratogenicity of anticonvulsants. Recently, growing evidence has indicated that valproic acid (VPA) can produce postnatal congenital malformations and impair cognitive function. However, the mechanisms underlying cognitive dysfunction in long-term prognoses remain unclear. METHODS: Pregnant Wistar rats received daily intraperitoneal injections of VPA (200 mg/kg/day) from embryonic day 12.5 until birth. On postnatal day (PD) 149, the rats received an injection of bromodeoxyuridine (BrdU). On PD 150, the rats were subjected to the open field (OF), elevated plus-maze (EPM), and Y-maze tests. After behavioral testing, perfusion fixation was performed and the brain was dissected for immunohistochemistry. RESULTS: A significant marked decrease was seen in the number of BrdU-positive cells in the dentate gyrus of offspring of VPA-treated dams compared to those of control. However, no significant differences in hyperactivity were found based on the results of the OF test among the offspring on PD 150 of 200 VPA-treated dams. In addition, no significant differences were seen in the EPM test. CONCLUSION: The behavioral abnormality observed in young offspring of VPA-treated dams was not significantly different from that of controls in adult offspring on PD 150. However, compared with controls, the number of BrdU-positive cells in VPA-treated rats was halved. The findings suggest that the behavioral abnormality seems to improve as they grow, even if some structural abnormalities may remain in the central nervous system.

Characterization and Stage-Dependent Lineage Analysis of Intermediate Progenitors of Cortical GABAergic Interneurons.

Intermediate progenitors of both excitatory and inhibitory neurons, which can replenish neurons in the adult brain, were recently identified. However, the generation of intermediate progenitors of GABAergic inhibitory neurons (IPGNs) has not been studied in detail. Here, we characterized the spatiotemporal distribution of IPGNs in mouse cerebral cortex. IPGNs generated neurons during both embryonic and postnatal stages, but the embryonic IPGNs were more proliferative. Our lineage tracing analyses showed that the embryonically proliferating IPGNs tended to localize to the superficial layers rather than the deep cortical layers at 3 weeks after birth. We also found that embryonic IPGNs derived from the medial and caudal ganglionic eminence (CGE) but more than half of the embryonic IPGNs were derived from the CGE and broadly distributed in the cerebral cortex. Taken together, our data indicate that the broadly located IPGNs during embryonic and postnatal stages exhibit a different proliferative property and layer distribution.

Effects of repeated electroconvulsive seizure on cell proliferation in the rat hippocampus.

Electroconvulsive therapy (ECT) is known as a successful treatment for severe depression. Despite great efforts, the biological mechanisms underlying the beneficial effects of ECT remain largely unclear. In this study, animals received a single, 10, or 20 applications of electroconvulsive seizure (ECS), and then cell proliferation and apoptosis were investigated in the subgranular zone (SGZ) of the dentate gyrus. We analyzed whether a series of ECSs could induce changes in the dentate gyrus in a dose-response fashion. A single-ECS seizure significantly increased cell proliferation in the SGZ by ∼2.3-fold compared to sham treatment. After 10 ECSs, a significant increase in cell proliferation was observed in the SGZ by ∼2.4-fold compared to sham treatment. Moreover, 10 ECSs induced a significant increase in cell proliferation by 1.3-fold compared to a single-ECS group. However, cell proliferation did not differ between the group with 20 ECSs and sham group. In addition, a significant increase in the number of apoptotic cells was found in the group with 10 ECSs, whereas no significant change in it was found in either a single ECS or 20 ECSs group compared to sham treatment. These findings indicate that the optimal number of treatments and duration of stimulation requires investigation. Further studies are needed to elucidate the intracellular mechanisms underlying both effective and excessive ECT.

Understanding the Real State of Human Adult Hippocampal Neurogenesis From Studies of Rodents and Non-human Primates.

The concept of adult hippocampal neurogenesis (AHN) has been widely accepted, and a large number of studies have been performed in rodents using modern experimental techniques, which have clarified the nature and developmental processes of adult neural stem/progenitor cells, the functions of AHN, such as memory and learning, and its association with neural diseases. However, a fundamental problem is that it remains unclear as to what extent AHN actually occurs in humans. The answer to this is indispensable when physiological and pathological functions of human AHN are deduced from studies of rodent AHN, but there are controversial data on the extent of human AHN. In this review, studies on AHN performed in rodents and humans will be briefly reviewed, followed by a discussion of the studies in non-human primates. Then, how data of rodent and non-human primate AHN should be applied for understanding human AHN will be discussed.

GABAergic excitation promotes neuronal differentiation in adult hippocampal progenitor cells.

Hippocampal activity influences neurogenesis in the adult dentate gyrus; however, little is known about the involvement of the hippocampal circuitry in this process. In the subgranular zone of the adult dentate gyrus, neurogenesis involves a series of differentiation steps from radial glia-like stem/progenitor (type-1) cells, to transiently amplifying neuronal progenitor (type-2) cells, to postmitotic neurons. In this study, we conducted GFP-targeted recordings of progenitor cells in fresh hippocampal slices from nestin-GFP mice and found that neuronal progenitor (type-2) cells receive active direct neural inputs from the hippocampal circuitry. This input was GABAergic but not glutamatergic. The GABAergic inputs depolarized type-2 cells because of their elevated [Cl(-)](i). This excitation initiated an increase of [Ca(2+)](i) and the expression of NeuroD. A BrdU-pulse labeling study with GABA(A)-R agonists demonstrated the promotion of neuronal differentiation via this GABAergic excitation. Thus, it appears that GABAergic inputs to hippocampal progenitor cells promote activity-dependent neuronal differentiation.

Role of polysialylated neural cell adhesion molecule in rapid eye movement sleep regulation in rats.

Recent evidence suggests that synaptic plasticity occurs during homeostatic processes, including sleep-wakefulness regulation, although the underlying mechanisms are not well understood. Polysialylated neural cell adhesion molecule (PSA NCAM) is a transmembrane protein that has been implicated in various forms of plasticity. To investigate whether PSA NCAM is involved in the neuronal plasticity associated with spontaneous sleep-wakefulness regulation and sleep homeostasis, four studies were conducted using rats. First, we showed that PSA NCAM immunoreactivity is present in close proximity to key neurons in several nuclei of the sleep-wakefulness system, including the tuberomammillary hypothalamic nucleus, dorsal raphe nucleus, and locus coeruleus. Second, using western blot analysis and densitometric image analysis of immunoreactivity, we found that 6 h of sleep deprivation changed neither the levels nor the general location of PSA NCAM in the sleep-wakefulness system. Finally, we injected endoneuraminidase (Endo N) intracerebroventricularly to examine the effects of polysialic acid removal on sleep-wakefulness states and electroencephalogram (EEG) slow waves at both baseline and during recovery from 6 h of sleep deprivation. Endo N-treated rats showed a small but significant decrease in baseline rapid eye movement (REM) sleep selectively in the late light phase, and a facilitated REM sleep rebound after sleep deprivation, as compared with saline-injected controls. Non-REM sleep and wakefulness were unaffected by Endo N. These results suggest that PSA NCAM is not particularly involved in the regulation of wakefulness or non-REM sleep, but plays a role in the diurnal pattern of REM sleep as well as in some aspects of REM sleep homeostasis.

Prenatal phencyclidine exposure alters hippocampal cell proliferation in offspring rats.

Multiple case reports have described pregnancy in phencyclidine hydrochloride (PCP) abusers. Characteristic clinical symptoms of PCP-exposed infants have revealed neurobehavioral or physical abnormalities. We designed this study to evaluate whether chronic prenatal exposure to PCP during the last 2 weeks of gestation in rats produces alterations of hippocampal neurogenesis in offspring. Rats received repeated subcutaneous injection of PCP (5 mg/kg) once daily during the last 2 weeks of gestation. Control animals received subcutaneous injection of physiological saline during gestation. Dams receiving repeated PCP administrations showed markedly increased locomotor activities on days 1, 5, and 10 during the last 2 weeks of gestation. At 21 days after birth, 5-bromo-2'-deoxyuridine (BrdU)-positive cells of offspring were counted in the granule cell layer (GCL) and subgranular zone of the dentate gyrus. The numbers of BrdU-positive cells in the GCL in male and female offspring of the PCP-treated group were significantly increased by approximately 77% compared with those from the control group. At 56 days, the number of surviving BrdU-positive cells also remained to be increased by 74% in the GCL in PCP-treated group. At 21 days, locomotor activities of offspring in the PCP-treated group were significantly decreased by approximately 30% compared with those in the control group. However, neuronal differentiation of newly formed cells and cell survival were not influenced at 5 weeks after BrdU injections. Some altered biochemical or physiological conditions of offspring from dams receiving repeated PCP injections during pregnancy could influence changes in cell proliferation in the GCL of offspring during early development. Changes to cell proliferation in the hippocampus may affect behavioral abnormalities during infancy in offspring.

Immature enteric neurons in Ncx/Hox11L.1 deficient intestinal neuronal dysplasia model mice.

AIM: The Ncx/Hox11L.1 gene is required for adequate development of enteric neurons in mice and Ncx/Hox11L.1 deficient (Ncx-/-) mice are used as a model for human intestinal neuronal dysplasia (IND) because of similar histopathology (hyperganglionosis), however, some 50% of Ncx-/- mice develop megacolon with a caliber change in the proximal colon, and die when 21-35 days old. We used polysialylated neural cell adhesion molecule (PSA-NCAM) to examine the maturity of enteric neurons in Ncx-/- mice to further understand the etiology of IND. METHODS: PSA-NCAM immunoreactivity was measured in specimens taken 1 cm proximal to the ileocecal valve (ileum), 1 cm distal to the ileocecal valve (proximal colon), and 1 cm proximal to the anus (distal colon) from 63 mice (Ncx-/-: n = 14, Ncx+/-: n = 30, and Ncx+/+: n = 19) on days 14 (D14), 21 (D21), and 27 or later (>D27). RESULTS: PSA-NCAM was positive (indicating immaturity) in proximal colon (submucosal and myenteric plexuses) from 8/14 (57%) Ncx-/- mice (2/4 on D14, 4/6 on D21, and 2/4 on >D27) and from 5/30 (17%) Ncx+/- mice (0/2 in D14, 2/13 in D21, and 3/15 in >D27). PSA-NCAM was negative (indicating maturity) in all other specimens. The incidence of PSA-NCAM positive neurons in Ncx-/- appeared to be correlated with the mortality rate seen in IND mice. CONCLUSIONS: Our data suggest that colonic dysmotility and pathology seen in Ncx-/- mice may be due to persistence of immature neurons in the proximal colon, which could also be the case in human IND and warrants further investigation.

Non-radial tortuous migration with cell polarity alterations of newly generated granule neurons in the neonatal rat dentate gyrus.

To establish functional neuronal circuits, newborn neurons generally migrate from the ventricular germinal zones to their final positions during embryonic periods. However, most excitatory neurons of the hippocampal dentate gyrus are born postnatally in the hilus, far from the lateral ventricle. Newly generated granule neurons must then migrate to the surrounding granule cell layer (GCL), which suggests that newborn granule cells may migrate by unique cellular mechanisms. In the present study, we describe the migratory behaviors of postnatally generated granule neurons using combined retroviral labeling and time-lapse imaging analysis. Our results show that whereas half of the newly generated neurons undergo radial migration, the remainder engages in more complex migratory patterns with veering and turning movements accompanied by process formation and cell polarity alterations. These data reveal a previously unappreciated diversity of mechanisms by which granule neurons distribute throughout the GCL to contribute to hippocampal circuitry.

Analysis of proliferating neuronal progenitors and immature neurons in the human hippocampus surgically removed from control and epileptic patients.

Adult neurogenesis in the mammalian hippocampus is a well-known phenomenon. However, it remains controversial as to what extent adult neurogenesis actually occurs in the adult human hippocampus, and how brain diseases, such as epilepsy, affect human adult neurogenesis. To address these questions, we analyzed immature neuronal marker-expressing (PSA-NCAM+) cells and proliferating neuronal progenitor (Ki67+/HuB+/DCX+) cells in the surgically removed hippocampus of epileptic patients. In control patients, a substantial number of PSA-NCAM+ cells were distributed densely below the granule cell layer. In epileptic patients with granule cell dispersion, the number of PSA-NCAM+ cells was reduced, and aberrant PSA-NCAM+ cells were found. However, the numbers of Ki67+/HuB+/DCX+ cells were very low in both control and epileptic patients. The large number of PSA-NCAM+ cells and few DCX+/HuB+/Ki-67+ cells observed in the controls suggest that immature-type neurons are not recently generated neurons, and that the level of hippocampal neuronal production in adult humans is low. These results also suggest that PSA-NCAM is a useful marker for analyzing the pathology of epilepsy, but different interpretations of the immunohistochemical results between humans and rodents are required.

Prenatal exposure to valproic acid is associated with altered neurocognitive function and neurogenesis in the dentate gyrus of male offspring rats.

In pregnant women with epilepsy, it is imperative to balance the safety of the mother and the potential teratogenicity of anticonvulsants, which could cause impairments such as intellectual disability and cleft lip. In this study, we examined behavioral and hippocampal neurogenesis alterations in male offspring of rats exposed to valproic acid (VPA) during pregnancy. Pregnant Wistar rats received daily intraperitoneal injections of VPA (100 mg/kg/day or 200 mg/kg/day) from embryonic day 12.5 until birth. At postnatal day 29, animals received an injection of bromodeoxyuridine (BrdU). At postnatal day 30, animals underwent the open field (OF), elevated plus-maze, and Y-maze tests. After behavioral testing, animals were decapitated, and their brains were dissected for immunohistochemistry. Of the offspring of the VPA200 mothers, 66.6% showed a malformation. In the OF test, these animals showed locomotor hyperactivity. In the elevated plus-maze, offspring of VPA-treated mothers spent significantly more time in the open arms, irrespective of the treatment dose. The number of BrdU-positive cells in the dentate gyrus of the offspring of VPA-treated mothers increased significantly in a dose-dependent manner compared with the control. A significant positive correlation between spontaneous locomotor activity in the OF and BrdU-positive cell counts was observed across groups. In conclusion, VPA administration during pregnancy results in malformations and attention-deficit/hyperactivity disorder-like behavioral abnormalities in the offspring. An increase in cell proliferation in the hippocampus may underlie the behavioral changes observed. Repeated use of high doses of VPA during pregnancy may increase the risk of neurodevelopmental abnormalities dose dependently and should be carefully considered.

Alteration in the differentiation-related molecular expression in the subventricular zone in a mouse model of Parkinson's disease.

Enhancement of neurogenesis could be a suitable treatment approach to up-regulate dopaminergic neurons in Parkinson's disease (PD). In the present study, we focused on the kinetics of the subventricular zone (SVZ) in a mouse model of PD induced by MPTP injection. We showed recently the proliferation potential of neuronal stem cells (NSCs) prepared from the olfactory bulb of an animal model of PD [Hayakawa, H., Hayashita-Kinoh, H., Nihira, T., Seki, T., Mizuno, Y., Mochizuki, H., 2007. The isolation of neural stem cells from the OB of Parkinson's disease model. Neurosci. Res.]. In this study, we examined the relationship between proliferation and differentiation of NSCs in SVZ of both acute and chronic PD models. Only acute MPTP treatment significantly increased the areas of glial fibrillary acidic protein (GFAP)-expressing cells and decreased the areas of polysialylated neural cell adhesion molecule (PSA-NCAM)-expressing cells in the SVZ. In the case of caspase-11 knockout mice, MPTP did not induce alteration in the areas of GFAP-expressing cells and PSA-NCAM-expressing cells. Our results suggest that neuroinflammation related to the caspase-11 cascade in the striatum regulates differentiation of neural stem cells in the SVZ of our mouse model of PD.

Dentate granule progenitor cell properties are rapidly altered soon after birth.

Neurogenesis occurs during the embryonic period and ceases soon after birth in the neocortex, but continues to occur in the hippocampus even in the adult. The embryonic neocortex has radial glia or progenitor cells expressing brain lipid-binding protein (BLBP), whereas the adult hippocampus has radial granule progenitor cells expressing BLBP and glial fibrillary acidic protein (GFAP) in the subgranular zone. We previously found that embryonic hippocampal granule progenitor cells express GFAP, but not BLBP, indicating that these cells are different from both embryonic neocortical and adult granule progenitor cells. In the present study, as the first step towards understanding the mechanism of persistent hippocampal neurogenesis, we aimed to determine the stage at which embryonic-type granule progenitors become adult-type progenitors using mouse Gfap-GFP transgenic mice. During the embryonic stages, Gfap-GFP-positive (Gfap-GFP+) cells were distributed in the entire developing dentate gyrus (DG), whereas BLBP-positive (BLBP+) cells were mainly present in the fimbria and subpial region, and to some extent in the DG. Up to postnatal day 0 (P0), double-positive cells were scarcely detected. However, at P1, one-third of the Gfap-GFP+ cells in the DG suddenly began to weakly express BLBP. Thereafter, Gfap-GFP+/BLBP+ cells rapidly increased in number, and extended their radial processes in the inner granular cell layer. At P14 and in the adult, two-thirds of the Gfap-GFP+ cells in the subgranular zone showed BLBP immunoreactivity. These results suggest that the properties of hippocampal granule progenitor cells are rapidly altered from an embryonic to adult type soon after birth.

Correction to: Dentate granule progenitor cell properties are rapidly altered soon after birth.

In the original publication figure parts 8c, 8f, and 8i were mixed up and thus incorrectly labeled. Here is a corrected version with the parts properly labeled.

Clustering, migration, and neurite formation of neural precursor cells in the adult rat hippocampus.

Adult neurogenesis occurs in the subgranular zone and innermost part of the dentate granule cell layer. To examine how neural precursor cells proliferate, migrate, and extend their neurites, we performed BrdU- and improved retrovirus-green fluorescence protein (GFP)-labeling analyses. Soon after labeling the majority of BrdU+ cells and GFP+ cells expressed Ki67, a cell cycle marker, and formed clusters together with PSA+ neuroblasts. Most of the Ki67+ proliferating cells expressed Hu, an immature and mature neuronal marker, and the subpopulation expressed both Hu+ and GFAP+. In the clusters, Ki67+ and PSA+ cells strongly expressed beta-catenin and N-cadherin, but PSA+ cells outside the clusters did not. Therefore, it was mainly Hu+ neuronal precursor cells that proliferated within clusters in which the cluster cells are closely associated via cell adhesion molecules, such as N-cadherin/beta-cateninIn and PSA. The newly generated cells appeared to stay in the clusters for a few days and then disperse around the clusters. The findings of this in vivo analysis and in vitro time-lapse imaging of early postnatal hippocampal slices support the notion that most postmitotic neuroblasts migrate tangentially from clusters, extending tangentially oriented processes, one of which often retains close contact with the clusters, and finally extend radial processes, or prospective apical dendrites. These results suggest that the clustering cells and tangentially migrating cells have a systematic cellular arrangement and intercellular interaction.