Morphometric development of the human fetal auditory system: inferior collicular nucleus

Short-term cognitive loading deteriorates breathing pattern and gas exchange in adult patients with congenital central hypoventilation syndrome

Question

Human PHOX2B mutations result in life-threatening sleep-related hypoventilation (congenital central hypoventilation syndrome, CCHS). Most patients retain ventilatory activity when awake through a respiratory-related cortical network. We hypothesised that this need to mobilise cortical resources to breathe would lead to breathing-cognition interferences during cognitive loading.

Patients and methods

Seven adult CCHS patients (five women; median age 21) performed standard neuropsychological tests (paced auditory serial addition test - calculation capacity, working memory, sustained and divided attention; trail making test - visuospatial exploration capacity, cognitive processing speed, attentional flexibility; Corsi block-tapping test - visuospatial memory, short-term memory, working memory) during unassisted breathing and under ventilatory support. Ventilatory variables and transcutaneous haemoglobin oxygen saturation were recorded. Cortical connectivity changes between unassisted breathing and ventilatory support were assessed using electroencephalographic recordings (EEG).

Results

Baseline performances were lower than expected in individuals of this age. During unassisted breathing, cognitive loading coincided with increased breathing variability, and decreases in oxygen saturation inversely correlated with an increasing number of apnoeic cycles per minute (rho -0.46, 95% CI -0.76 to -0.06, p=0.01). During ventilatory support, cognitive tasks did not disrupt breathing pattern and were not associated with decreased oxygen saturation. Ventilatory support was associated with changes in EEG cortical connectivity but not with improved test performances.

Conclusions

Acute cognitive loads induce oxygen desaturation in adult CCHS patients during unassisted breathing, but not under ventilatory support. This justifies considering the use of ventilatory support during mental tasks in CCHS patients to avoid repeated episodes of hypoxia.

The prenatal brain readiness for speech processing: A review on foetal development of auditory and primordial language networks

Despite consolidated evidence for the prenatal ability to elaborate and respond to sounds and speech stimuli, the ontogenetic functional brain maturation of language responsiveness in the foetus is still poorly understood. Recent advances in in-vivo foetal neuroimaging have contributed to a finely detailed picture of the anatomo-functional hallmarks that define the prenatal neurodevelopment of auditory and language-related networks. Here, we first outline available evidence for the prenatal development of auditory and language-related brain structures and of their anatomical connections. Second, we focus on functional connectivity data showing the emergence of auditory and primordial language networks in the foetal brain. Third, we recapitulate functional neuroimaging studies assessing the prenatal readiness for sound processing, as a crucial prerequisite for the foetus to experientially respond to spoken language. In conclusion, we suggest that the state of the art has reached sufficient maturity to directly assess the neural mechanisms underlying the prenatal readiness for speech processing and to evaluate whether foetal neuromarkers can predict the postnatal development of language acquisition abilities and disabilities.

In vivo Probabilistic Structural Atlas of the Inferior and Superior Colliculi, Medial and Lateral Geniculate Nuclei and Superior Olivary Complex in Humans Based on 7 Tesla MRI

Despite extensive neuroimaging research of primary sensory cortices involved in auditory and visual functions, subcortical structures within these domains, such as the inferior and superior colliculi, the medial and lateral geniculate nuclei and the superior olivary complex, are currently understudied with magnetic resonance imaging (MRI) in living humans. This is because a precise localization of these nuclei is hampered by the limited contrast and sensitivity of conventional neuroimaging methods for deep brain nuclei. In this work, we used 7 Tesla multi-modal (T₂-weighted and diffusion fractional anisotropy) 1.1 mm isotropic resolution MRI to achieve high sensitivity and contrast for single-subject brainstem and thalamic nuclei delineation. After precise coregistration to stereotactic space, we generated an in vivo human probabilistic atlas of auditory (medial geniculate nucleus, inferior colliculus, and superior olivary complex) and visual (lateral geniculate nucleus and superior colliculus) subcortical nuclei. We foresee the use of this atlas as a tool to precisely identify the location and shape of auditory/visual deep nuclei in research as well as clinical human studies.

Morphological and morphometrical maturation of ventral cochlear nucleus in human foetus

Auditory impulses perceived by the hair cells of the organ of corti are relayed in the cochlear nucleus, the first relay station in the brainstem, by the cochlear nerve. The human foetus is well known to respond to sound during the last trimester of gestation. On the contrary, studies conducted in rat, cat and mouse have shown that these mammals have an immature auditory system at the time of birth. There are very few reports available regarding the morphological and functional maturation of the cochlear nucleus in human. Although the human cochlear nucleus neurons attain adult morphological characters by mid-gestation, there are hardly any studies discussing the functional maturation of the cochlear nucleus. Hence the present study was aimed at observing the morphological as well as functional maturation of the human foetal cochlear nuclei at various gestational ages. Morphological maturation was observed qualitatively while stereological estimation of the volume of well defined ventral cochlear nucleus (VCN) was calculated by the Cavalieri principle; neuronal count and density was estimated by dissector principle. The functional maturation was assessed by observing the expression of synaptophysin, a synaptic marker, at different gestational ages and by the presence of parvalbumin, a calcium binding functional neuronal marker by immunohistochemistry. Neurons showed coarse Nissl's substance and well developed cell processes and gradual increase in cell size by the 24th-30th gestational week. Synaptophysin labeling in the complete cochlear nucleus was observed at 20 weeks of gestation. Adult pattern of synaptophysin labeling was observed finally at37weeks of gestation. Earliest presence of parvalbumin expression was detected at 16 weeks of gestation and a distinct adult pattern was seen at 37 weeks of gestation. This study concluded that morphological and functional maturation of the human cochlear nuclei occurs simultaneously during mid-gestation which represents the critical period of development and continues up to term.

The human auditory system: A timeline of development

This review traces the structural maturation of the human auditory system, and compares the timeline of anatomical development with cotemporaneous physiological and behavioral events. During the embryonic period, there is formation of basic structure at all levels of the system, i.e. the inner ear, the brainstem pathway, and the cortex. The second trimester is a time of rapid growth and development, and by the end of this period, the cochlea has acquired a very adult-like configuration. During the perinatal period, the brainstem reaches a mature state, and brainstem activity is reflected in behavioral responses to sound, including phonetic discrimination, and in evoked brainstem and early middle latency responses. The perinatal period is also the time of peak development of brainstem input to the cortex through the marginal layer, and of the long latency cortical potentials, the N(2) and mismatch negativity. In early childhood, from the sixth post-natal month to age five, there is progressive maturation of the thalamic projections to the cortex and of the longer latency Pa and P(1) evoked potentials. Later childhood, from six to twelve years, is the time of maturation of the superficial cortical layers and their intracortical connections, accompanied by appearance of the N(1) potential and improved linguistic discriminative abilities. Some consideration is given to the potential negative effects of deafness-induced sound deprivation during the perinatal period and childhood.

Stereological investigation and expression of calcium-binding proteins in developing human inferior colliculus

The mammalian inferior colliculus (IC) is a major relay nucleus in the auditory pathway. Prenatal development of the human IC has been inadequately studied. The present study reports the morphometric development and maturation of the human IC using unbiased stereology, in 18 aborted fetuses of various gestational ages (12-29 weeks) and two babies aged 40 postnatal days (PND) and 5 months (that died of postoperative complications). It also demonstrates the functional maturation of the IC by examining the expression of calcium-binding proteins--parvalbumin (PV) and calbindin (CB). There was a significant increase in the total number of neurons and glia from 18 weeks of gestation (WG). The glia and neuron volume increased significantly from 16 WG to 22 WG, respectively. The total volume of IC also increased significantly from 18 WG onwards. On the other hand, the number and volume of undifferentiated cell bodies across all ages decreased significantly. Expression of CB was concentrated in the dorsal cortex while that of PV was mainly confined to the central nucleus of the IC, possibly indicating an early segregation of parallel processing of information in the auditory pathways. Intense staining for CB in the soma and dendrites appeared earlier than that of the PV. The morphological maturation appeared to overlap the onset of functional maturation suggesting an activity-dependent mechanism in the development of IC.

Morphometric evaluations of the human nervous system

It is very important to evaluate and accurately understand the various conditions of the human nervous system. In this review article, we introduce several morphometric reports that are proven to be accurate from the view point of various errors (range of tissue shrinkage ratios, microscopic multiple counting, artifacts of microscopic structures, etc.). We review the following aspects of the selected reports: methodology, developmental research, neuronal differences, gender differences, aging process and miscellaneous (nerve fibers, unmyelinated fibers, in relation to neuropathology, clinical image-analysis and immunohistochemistry).

MRI of normal fetal brain development

Normal fetal brain maturation can be studied by in vivo magnetic resonance imaging (MRI) from the 18th gestational week (GW) to term, and relies primarily on T2-weighted and diffusion-weighted (DW) sequences. These maturational changes must be interpreted with a knowledge of the histological background and the temporal course of the respective developmental steps. In addition, MR presentation of developing and transient structures must be considered. Signal changes associated with maturational processes can mainly be ascribed to the following changes in tissue composition and organization, which occur at the histological level: (1) a decrease in water content and increasing cell-density can be recognized as a shortening of T1- and T2-relaxation times, leading to increased T1-weighted and decreased T2-weighted intensity, respectively; (2) the arrangement of microanatomical structures to create a symmetrical or asymmetrical environment, leading to structural differences that may be demonstrated by DW-anisotropy; (3) changes in non-structural qualities, such as the onset of a membrane potential in premyelinating axons. The latter process also influences the appearance of a structure on DW sequences. Thus, we will review the in vivo MR appearance of different maturational states of the fetal brain and relate these maturational states to anatomical, histological, and in vitro MRI data. Then, the development of the cerebral cortex, white matter, temporal lobe, and cerebellum will be reviewed, and the MR appearance of transient structures of the fetal brain will be shown. Emphasis will be placed on the appearance of the different structures with the various sequences. In addition, the possible utility of dynamic fetal sequences in assessing spontaneous fetal movements is discussed.

Development of the human cuneatus medialis nucleus: a morphometric evaluation

The morphometric features of the development of the human cuneatus medialis nucleus (CMN) were examined using complete serial celloidin sections of 11 brains: nine fetal brains, the brain of a 2-month-old infant and that of a 63-year-old adult. The morphometric analysis revealed at least four stages in the development of the CMN characterized by the maturation of neurons: (1) immature neuronal stage, (2) preparatory stage, (3) accelerated development stage and (4) postnatal stage. The development of the human CMN accelerates after 32 weeks of gestation (WG), and postnatal qualitative maturation also occurs. From the viewpoint of morphometric evaluation, the CMN gradually matures between 18 and 40 WG without a rapid growth phase as far as we have observed.

Development of the human lateral cuneate nucleus: a morphometric evaluation

The morphometric development of the human lateral cuneate nucleus was examined in nine fetuses and neonates at 18-40 weeks of gestation, a 2-month-old infant and a 63-year-old adult. The authors observed serial sections of the brain containing lateral cuneate nucleus and measured the volume of nuclear column and the number, cell body area and perimeter of neurons using a microscope with a drawing tube and an image-analyzing computer system. A morphometric evaluation revealed that a turning point in the development of the human lateral cuneate nucleus occurring at 30 WG, as this marks the beginning of a gradual increase in the average area and average perimeter; And it is the point at which the neuropil index suddenly increases sharply, to fall back down to lower levels afterwards. The establishment of a neuronal connection between the lateral cuneate nucleus and the cerebellum, which is necessary for proper performance of movements in the upper part of the body.

Development of the human principal sensory trigeminal nucleus: a morphometric analysis

The anatomical development of the principal sensory trigeminal nucleus was assessed with morphometric features using serial celloidin sections of 15 human brains, including 12 brains obtained from fetuses and neonates. A microscope and an optical electronic planimeter combined with a computer were employed for morphometric measurements of columnar areas, neuronal numbers, neuronal areas and neuronal perimeters to statistically analyze and evaluate the development of neuronal densities, neuropil indexes and circularity ratios. We could not detect the principal sensory trigeminal nucleus in the specimen of 12 gestational weeks (GW). Microscopic observation disclosed that the fetal principal sensory trigeminal neurons approached those of the adult around 33 GW in terms of cell arrangements, amounts of Nissl bodies and morphology of neurons. Our morphometric analysis showed that the columnar volume, the neuronal area and the neuropil index increased with gestational age. The neuronal density decreased with gestational age, especially from 16 to 32 GW. Comparing the neuronal area with the columnar volume, it developed before the columnar volume during the fetal period. The neurons of the principal sensory trigeminal nucleus matured around 33 GW under microscopic observation and in terms of the distribution of neuronal areas. The development of the neuropil may accelerate after individual neurons of principal sensory trigeminal nucleus mature.