Development of Structural and Functional Connectivity

Abstract

Elsevier Inc. All rights reserved. GlossaryFiber myelination brain maturational mechanism corresponding to the ensheathment around neuronal axons of oligodendrocytes' processes that form the myelin sheaths. It leads to an increase in the conduction speed of the nerve impulse and progresses from the second part of pregnancy to the end of adolescence with a specific calendar across white matter bundles and cerebral regions.Subplate transient cerebral compartment containing the most differentiated postmigratory neurons, observed between the intermediate zone (fetal white matter) and the cortical plate between 13 PCW and around term age (still observed in the first postnatal months in some frontal association regions). AbbreviationsAF Arcuate fasciculus ALIC Anterior limb of the internal capsule CC Corpus callosum (g/b/s, genu/body/splenium) CG Cingulum (inf/sup, inferior/superior parts) CST Corticospinal tract (inf/mid/sup, inferior/middle/ superior portions) DSI Diffusion spectrum imaging DTI Diffusion tensor imaging DWI Diffusion-weighted imaging EC External capsule EEG Electroencephalography ERP Event-related potentials FA Fractional anisotropy fcMRI Functional connectivity MRI FX Fornix GA Gestational age GM Gray matter iFOF Inferior fronto-occipital fasciculus ILF Inferior longitudinal fasciculus MD Mean diffusivity MRI Magnetic resonance imaging OR Optic radiations PCW Postconceptional weeks SLF Superior longitudinal fasciculus STT Spinothalamic tract T 1 Longitudinal relaxation time T 1 w T 1 -weighted T 2 Transverse relaxation time T 2 w T 2 -weighted UF Uncinate fasciculus w GA Weeks of gestational age IntroductionDuring the early fetal period, there are few axonal pathways and synaptic connections already established in the human brain. Thus, starting with this period, we have a unique opportunity to follow the step-by-step, complex, but sequential development of cerebral connectivity, until all major connections become established in the late fetal (preterm) period. Even though the pathways' connectivity and the cortical circuitry are not fully established yet in preterm newborns, brain regions are organized early on into networks specialized to process sensorimotor and cognitive functions (Mahmoudzadeh et al., 2013). These functional networks further develop, mature, and refine until the end of adolescence. In this article, we aim to describe the early development of connectivity, from the fetal to the postnatal periods, as mapped in postmortem histological studies and in vivo MRI studies of fetuses, newborns, and infants. We successively detail (1) how the structural connectivity first develops, (2) how the white matter pathways then become myelinated and functionally mature, and (3) how functional connectivity emerges in the course of development.

White Matter Dissection of the Fetal Brain

“…Agenesis of corpus callosum is often associated with smaller dimensions of the cingulum (Nakata et al, 2009), linking the cingulate cortex in their development. Interestingly, cortical-cortical connections (superior and inferior longitudinal fasciculi) were speculated to appear later (Dubois et al, 2015), but histology studies regarding neocortical pioneering axons are lacking.…”

“…The internal capsule fulfilled our tract identification criteria at 17 weeks of gestation, accounting for the third wave. Although some vertical fibers were identified using Klingler's dissection at 13 weeks, they did not meet two of our three criteria (i.e., traceable course for at least 1 cm and fiber consistency), so we considered them to belong to the radial glia, which is known to have the same position (Dubois et al, 2015). DTI studies suggested the same development of the internal capsule starting from 13 weeks (Huang et al, 2009;Dubois et al, 2015).…”

Physiology of Human Birth and Mental Disease

“…The latter is still partly unknown and we do not intend to deny nor underestimate their role believing that they should be considered object of further investigation. We should remember that brain neural plasticity reaches a peak during the first year of life and continues developing until the age of 18-30 [10,12]. However, we must point out the limits founded in the approaches previously mentioned.…”

“…In the darkness of this "before birth" condition there is no light stimulation. Although around the 22nd post conceptional week (PCW) visual connections are already partially formed, the sight function has not developed yet [9,10]. Clinical experience and publications have shown that from the 23rd to 24th PCW, the fetus is capable of surviving without any developmental harm if adequately assisted with medical intensive treatment.…”

“…These develop in the subplate zone, creating temporary synaptic circuits and normally wait months before being relocated in the cerebral cortex, their final target [10,12,13]. In the occipital zone, the subplate'smainfunctionistoorganize the optical orientation columns.…”