The human rhombencephalon at the end of the embryonic period proper

Maternal PM2.5 exposure during gestation and offspring neurodevelopment: Findings from a prospective birth cohort study

Emerging data have suggested the potential role of prenatal PM_2.5 exposure as a neurotoxin for offspring. However, the existing results are equivocal, and no study has examined the effects of complex chemical constituents of the particular matter on offspring neurodevelopment. Therefore, in a prospective birth cohort study conducted in Jiangsu, China, we aimed to investigate the association between prenatal exposure to PM_2.5 and the neurodevelopment in infants, and further assess the effects of specific chemical constituents of PM_2.5. A total of 1531 children who had available data on daily prenatal PM_2.5 exposure and completed assessment on neurodevelopment at 1 year old were enrolled. We used the high-performance machine-learning model to estimate daily PM_2.5 exposure concentrations at 1 km × 1 km spatial resolution. The combined geospatial-statistical model was applied to evaluate average concentrations of six chemical constituents [organic matter (OM), black carbon (BC), sulfate (SO₄^2-), nitrate (NO₃^-), ammonium (NH₄⁺), and soil dust (Dust)]. The neurodevelopment of children was assessed using Bayley-III Screening Test. After adjusting for confounding factors, the risk of non-optimal gross motor development increased by 31 % for every 10 μg/m³ increase in average PM_2.5 exposure during gestation (aRR: 1.31; 95 % CI: 1.04, 1.64). Further analysis of PM_2.5 constituents showed that prenatally exposed to high SO₄^2- was associated with the risk of non-optimal gross motor development (aRR: 1.40; 95 % CI: 1.08, 1.81). Null associations were observed for the rest four neurodevelopment domains. Collectively, our study suggested that prenatal exposure to PM_2.5, particularly with high SO₄^2- concentration, was associated with children's non-optimal gross motor development at 1 year old. The short- and long-term influences of perinatal PM_2.5 exposure on children's neurodevelopment warrant further investigation.

Development of the human perihypoglossal nuclei from mid-gestation to the perinatal period: A morphological study

INTRODUCTION

Morphological data on the development of the human perihypoglossal nuclei (PHN) are scarce. This study describes the morphology of the human PHN from mid-gestation to the perinatal period.

MATERIALS AND METHODS

Ten brains were collected from infants aged 21-43 postmenstrual weeks (PW). Serial sections were cut and stained using the Klüver-Barrera method. Morphometric parameters [volume, neuronal numerical density (Nv) and total number (Nt), and neuronal profile area (PA)] were analyzed from microscopic observations.

RESULTS

Four PHN [nucleus of Roller (RO), interfascicular nucleus (IF), intercalated nucleus (IC), and prepositus nucleus (PR)] were identified at 21 PW. Medium-sized to large, oval, or polygonal neurons were concentrated in the ventral nuclei (RO and IF) and localized regions near the IC-PR transition of the dorsal nuclei (IC and PR). Small to large neurons of various shapes were scattered across the dorsal nuclei. The PR showed rostrocaudal differences in the neuronal cytoarchitecture. The volume of each nucleus increased between 21 and 43 PW, with a typical exponential increase for the dorsal nuclei. The Nv in each nucleus exponentially decreased, whereas the Nt was almost stable. The median PA linearly increased for every nucleus, and the increasing rates were greater for the ventral nuclei than those for the dorsal nuclei.

CONCLUSIONS

The dorsal and ventral PHN are identifiable at mid-gestation. The topographic relationships of the four nuclei are conserved until the perinatal period. The characteristic neuronal cytoarchitecture of each group is rapidly formed by 28-30 PW.

Development of the human accessory olivary nuclei: A morphometric and computerized 3D-reconstruction study

INTRODUCTION

This study described the prenatal development of the accessory olivary nuclei (AO) in humans.

MATERIALS/METHODS

Serial brain sections from ten pre- and full term infants aged 21-43 postmenstrual weeks (PW) were stained using the Klüver-Barrera method. A computerized 3D-reconstruction technique and morphometry were adopted for the study.

RESULTS

The medial AO (MAO) and dorsal AO (DAO) were identified at 21 PW. The dorsal cap was clearly differentiated from the main body (MB) of the MAO in neuronal cytoarchitecture. Pyknotic neurons were diffusely observed in the AO at 21 PW and were most concentrated in the MB. These neurons became infrequent from 28 PW onward. Neuronal nests existed in clusters between the AO and the medial lemniscus at 21 PW, which reduced progressively in size and number with age. The 3D-reconstructions showed that the AO are separated into caudal and rostral parts, and that this separation is achieved by mid-gestation in the DAO. Nuclear volume increased exponentially with age in the AO, although the rate of increase was half that of the principal nucleus (PO). Neuronal numerical density decreased rapidly 21-28 PW. The total neuronal number showed a weak correlation with age. The mean neuronal profile area increased linearly with age.

CONCLUSION

The human AO are separated into caudal and rostral parts in the fetal period. The nuclear volume and neuronal profile areas increase with age, although the rate of this increase is lower than in the PO. Natural neuronal death may occur at mid-gestation in the AO.

Correlation between human nervous system development and acquisition of fetal skills: An overview

Understanding the association between fetal nervous system structure and functioning should be an important goal in neurodevelopmental sciences, especially when considering the emerging knowledge regarding the importance of prenatal onset. Intrauterine development of the human central nervous system consists of specific processes: neurogenesis, neuronal migration, synaptogenesis, and myelination. However, as extensively shown by the neurobehavioral studies in the last century, the development of the central nervous system involves both structure and functioning. It is now recognised that the developing motor and sensory systems are able to function long before they have completed their neural maturation and that the intrauterine experience contributes to neurobehavioral development. This review analyzes the recent literature, looking at the association between the human nervous system maturation and fetal behavior. This article will follow the development and skill acquisition of the anatomical nervous system across the three trimesters of the gestation period.

[Motor behavior of human fetuses during the second trimester of gestation: a longitudinal ultrasound study]

INTRODUCTION

The aim of this research is to contribute to knowledge of the normal spontaneous motor behavior of the human fetus during the second trimester of pregnancy. This study focuses on five patterns of spontaneous fetal movement: startle (S), axo-rhizomelic rhythmia (ARR), axial stretching (AS), general movement (GM), and diaphragmatic contraction (DC).

METHODS

A cohort of 13 subjects was followed up using 2D obstetrical ultrasound images at 12, 16, 20, and 24 weeks of gestation. As inclusion criteria, neonatal neurological examination and general movements after eutocic delivery at term were normal in all of the subjects, and their neuromotor and cognitive development until the end of pre-school age were also normal.

RESULTS

All these five motor patterns are present at the beginning of the 2(nd) gestational trimester, but their quantitative and qualitative traits are diverse according to gestational ages. The phasic, isolated or rhythmically repeated movements, S and ARR, are prominent at 12 and 16 weeks of gestation, and then their presence gradually diminishes. By contrast, tonic and complex AS and GM movements increase their presence and quality at 20 and 24 weeks. RAR constitute a particular periodic motor pattern not described in previous literature. Moreover, the incidence of DC is progressive throughout the trimester, in clusters of 2-6 arrhythmic and irregular beats. Fetal heart rate increases during fetal motor active periods.

CONCLUSIONS

All five normal behavioral patterns observed in the ultrasounds reflect the progressive tuning of motor generators in human nervous system during mid-pregnancy.

Development of the pons in human fetuses

Morphometric and histological studies of the pons were performed by light microscopy in 28 cases of externally normal human fetuses ranging from 90 to 246 mm in crown-rump length (CRL) and from 13 to 28 weeks of gestation. The brainstems of fetuses were embedded in celloidin or paraffin, and transverse sections were prepared. The pons was divided into two regions at the most ventral margin of the medial lemniscus at the level of the motor trigeminal nucleus. The relationships between the total dorsoventral length, ventral length, and dorsal length of the pons versus CRL and gestational ages were calculated, and empiric formulas were fitted. It was found that the ventral portion increased in size more rapidly than the dorsal portion. The proportion of the ventral portion in the total dorsoventral length was constitutively higher than that of the dorsal portion in the present range of CRL. In the pontine nuclei, from 235 mm in the CRL, some large cells with rich cytoplasm, pale nuclei, and a distinct nucleolus appeared on the dorsal side of the pyramidal tract. According to Weigert stained preparations, the first myelinated fibers in each motor root of the trigeminal, abducent, and facial nerves were recognized at 130-140 mm in CRL and the medial lemniscus at 230-235 mm.

Morphogenesis of the posterior inferior cerebellar artery with three-dimensional reconstruction of the late embryonic vertebrobasilar system

The posterior inferior cerebellar artery (PICA) shows the most variable course among the cerebellar arteries, mainly at the level of the lateral medullary segment. Based on the correlation between the level of origin and the characteristics of the lateromedullary segment, we have proposed three patterns of course of the PICA. With the aim of understanding their embryological basis we review the interrelations between the developing cerebellum and the primitive hindbrain arterial plexus through the analysis of transverse serial sections of human embryos of 22.5 and 23 mm crown-rump length. Three-dimensional reconstruction of the vertebrobasilar system has been performed to study the morphology of the vascular networks. The cerebellar primordium is vascularized by the metencephalic plexus that will form the basilar artery and the superior cerebellar arteries. Due to the development of the pontine flexure the rhomboid lips approach and the cerebellum comes into contact with the myelencephalon; thus the myelencephalic plexus represents an acquired source of vascularization for the cerebellum with respect to the metencephalic plexus. The examination of the transverse sections shows that the vertebral and basilar arteries, superior cerebellar arteries, anterior inferior cerebellar arteries, and primitive lateral vertebrobasilar anastomoses are well recognizable. The three-dimensional reconstruction of the vessels shows that the PICA is not yet defined due to the persistence of a plexus of many thin vessels at the level of the lateral aspect of the myelencephalon, indicating that its origin and course are established at the end of the embryonic period. Based on the evolution of the primitive hindbrain plexus, we suggest that in synchrony with the progressive descent of the cerebellum the branches of the myelencephalic plexus succeed with a rostrocaudal progression in feeding the cerebellum and the morphogenesis of the PICA results from the selection of portions of this plexus. The high origin of the PICA from the basilar artery could be ascribed to its development from a rostral collateral of the plexus due to an early development of the vessel at the beginning of its embryonic lifetime. Moreover, the three patterns of course of the PICA could reflect the variable retention of the primitive lateral vertebrobasilar anastomosis in the trunk of the definitive PICA, which may be related to its level of origin.

Development and malformations of the human pyramidal tract

The corticospinal tract develops over a rather long period of time, during which malformations involving this main central motor pathway may occur. In rodents, the spinal outgrowth of the corticospinal tract occurs entirely postnatally, but in primates largely prenatally. In mice, an increasing number of genes have been found to play a role during the development of the pyramidal tract. In experimentally studied mammals, initially a much larger part of the cerebral cortex sends axons to the spinal cord, and the site of termination of corticospinal fibers in the spinal grey matter is much more extensive than in adult animals. Selective elimination of the transient corticospinal projections yields the mature projections functionally appropriate for the pyramidal tract. Direct corticomotoneuronal projections arise as the latest components of the corticospinal system. The subsequent myelination of the pyramidal tract is a slow process, taking place over a considerable period of time. Available data suggest that in man the pyramidal tract develops in a similar way. Several variations in the funicular trajectory of the human pyramidal tract have been described in otherwise normally developed cases, the most obvious being those with uncrossed pyramidal tracts. A survey of the neuropathological and clinical literature, illustrated with autopsy cases, reveals that the pyramidal tract may be involved in a large number of developmental disorders. Most of these malformations form part of a broad spectrum, ranging from disorders of patterning, neurogenesis and neuronal migration of the cerebral cortex to hypoxic-ischemic injury of the white matter. In some cases, pyramidal tract malformations may be due to abnormal axon guidance mechanisms. The molecular nature of such disorders is only beginning to be revealed.

Fetal alcohol syndrome: craniofacial and central nervous system manifestations

Magnetic resonance imaging (MRI) is undertaken on fetal alcohol syndrome (FAS) subjects to document central nervous system (CNS) anomalies. The abnormalities found include agenesis and hypoplasia of the corpus callosum, cavum septi pellucidi, cavum vergae, ventriculomegaly, hypoplasia of inferior olivary eminences, small brain stem, and micrencephaly. Craniofacial anomalies range from the well-recognized FAS physiognomy to the more severe frontonasal "dysplasia" (median cleft face). CNS and craniofacial abnormalities are predominantly symmetric and central or midline. The association of these anomalies becomes self-evident with recognition of the concept of the midline as a special developmental field, vulnerable to adverse factors during embryogenesis and fetal growth and development.