into astrocytes or neurons. somal translocation can occur at any time during development. subsequent waves of neurons split the preplate by migrating along radial glial fibres to form the cortical plate. each wave of migrating cells travel past their predecessors forming layers in an inside-out manner, meaning that the youngest neurons are the closest to the surface. it is estimated that glial guided migration represents 80-90% of migrating neurons. axophilic migration many neurons migrating along the anterior-posterior axis of the body use existing axon tracts to migrate along in a process called axophilic migration. an example of this mode of migration is in gnrh-expressing neurons, which make a long journey from their birthplace in the nose, through the forebrain, and into the hypothalamus. many of the mechanisms of this migration have been worked out, starting with the extracellular guidance cues that trigger intracellular signaling. these intracellular signals, such as calcium signaling, lead to actin and microtubule cytoskeletal dynamics, which produce cellular forces that interact with the extracellular environment through cell adhesion proteins to cause the movement of these cells. neurophilic migration refers to the migration of neurons along an axon belonging to a different nerve. gliophilic migration is the migration of glia along glial fibres. tangential migration most interneurons migrate tangentially through multiple modes of migration to reach their appropriate location in the cortex. an example of tangential migration is the movement of cajal–retzius cells within the marginal zone of the cortical neuroepithelium. others there is also a method of neuronal migration called multipolar migration. this is seen in multipolar cells, which are abundantly present in the cortical intermediate zone. they do not resemble the cells migrating by locomotion or somal translocation. instead these multipolar cells express neuronal markers and extend multiple thin processes in various directions independently of the radial glial fibers. neurotrophic factors neurotrophic factors are molecules which promote and regulate neuronal survival in the developing nervous system. they are distinguished from ubiquitous metabolites necessary for cellular maintenance and growth by their specificity; each neurotrophic factor promotes the survival of only certain kinds of neurons during a particular stage of their development. in addition, it has been argued that neurotrophic factors are involved in many other aspects of neuronal development ranging from axonal guidance to regulation of neurotransmitter synthesis. adult neural development main articles: neuroregeneration and adult neurogenesis neurodevelopment in the adult nervous system includes mechanisms such as remyelination, generation of new neurons, glia, axons, myelin or synapses. neuroregeneration differs between the peripheral nervous system (pns) and the central nervous system (cns) by the functional mechanisms and especially, the extent and speed. the nervous system continues to develop during adulthood until brain death. for example: physical exercise has neurobiological effects the consumption of foods (or nutrients), obesity, alterations of the microbiome, drinks, dietary supplements, recreational drugs and medications may possibly also have effects on the development of the nervous system various diseases, such as covid-19, have effects on the development of the nervous system for example, several genes have been identified as to being associated with changes in brain structure over lifetime and are potential alzheimer's disease therapy-targets. psychological events such as mental trauma and resilience-building exposure to environmental pollution and toxins such as air pollution may have effects on the further development of the nervous system other activities may also have effects on the development of the nervous system, such as lifelong learning, retraining, and types of media- and economic activities broadly, brain aging research, treatments and policies often distinguish between "mature" brains and "developing" brains while scientists have pointed out that "the complex nature of neurodevelopment itself poses challenges to establishing a point of reference that would indicate when a brain is mature" and that various structural brain measures change constantly throughout the adult phase of life, albeit childhood neuroplasticity-levels may not be reached again and it is thought that there are various critical and sensitive periods of brain development. differences to children's learning this section is an excerpt from learning § adult learning vs children's learning. learning is often more efficient in children and takes longer or is more difficult with age. a study using neuroimaging identified rapid gaba boosting as a major potential explanation-component for why that is. children's brains contain more "silent synapses" that are inactive until recruited as part of neural plasticity and flexible learning or memories. neuroplasticity is heightened during critical or sensitive periods of brain development, mainly referring to brain development during child development. research spatio-temporal modeling of brain development in early development (before birth and during the first few months), the brain undergoes more changes in size, shape and structure than at any other time in life. improved understanding of cerebral development during this critical period is important for mapping normal growth, and for investigating mechanisms of injury associated with risk factors for maldevelopment such as premature birth. hence, there is a need for dense coverage of this age range with a time-varying, age-dependent atlas. such a spatio-temporal atlases can accurately represent the dynamic changes occurring during early brain development, and can be used as a normative reference space. furthermore, large scale gene expression studies of different brain regions from early gestation to aging have been performed. this kind of data provides a unique insight into changes that happen in brain during this long period. this approach showed that 86 per cent of the genes were expressed, and that 90 per cent of these were differentially regulated at the whole-transcript or exon level across brain regions and/or time. the majority of these spatio-temporal differences were detected before birth, with subsequent increases in the similarity among regional transcriptomes. furthermore, interareal differences exhibit a temporal hourglass pattern, dividing the human neocortical development into three major phases. during the first phase, in the first six months after conception, general architecture of brain regions is largely formed by a burst of genetic activity, which is distinct for specific regions of the neocortex. this rush is followed by a sort of intermission beginning in the third trimester of pregnancy. during this period, most genes that are active in specific brain regions are quieted — except for genes that spur connections between all neocortex regions. then in late childhood and early adolescence, the genetic orchestra begins again and helps subtly shape neocortex regions that progressively perform more specialized tasks, a process that continues into adulthood. embryonic brain development research approaches to investigate the organogenesis and early development of the human brain or nervous system include: brain organoids and 'assembloids' (see also: genetic factors of recent brain evolution) synthetic embryos/embryo models model animals post-mortem studies non-invasive in vivo imaging as of 2014 imaging in utero is not commonly done without strong medical arguments; in 2019 a study reported that "neuroimaging approaches have contributed significantly to our understanding of early brain development" human tissue inaccessibility has impeded molecular understanding of the formation of cognitive capacities. the placenta is researched as well. better understanding of the development may potentially enable insights into nervous system diseases, improving intelligence, and better protection against harmful impacts from identified factors of fetal development (potentially including from diseases of the mother, various events and xenobiotics). specific regions research has been able to make new discoveries for various parts of the brain thanks to the noninvasive imaging available. medial prefrontal cortex (mpfc) in this region, more activity is noted in adolescents than in adults when faced with tests on mentalising tasks as well as communicative and personal intent. decreased activity from adolescence to adulthood. in a mentalising task employing animation, the dmpfc was more stimulated in adults while the ventral mpfc was more stimulated in children. they can be attributed to the use of objective strategy associated with the dmpfc. theories for decrease in activity from adolescence to adulthood vary. one theory is that cognitive strategy becomes more automatic with age and another is that functional change occurs parallel to neuroanatomical change which is characterized by synaptogenesis and pruning. the mpfc is an example of one specific region that has become better understood using current imaging techniques. current research provides many more findings like this. early life stress early life stress is defined as exposure to circumstances during childhood that overwhelm a child’s coping resources and lead to sustained periods of stress. results from multiple studies indicate that the effects of early life stress on the developing brain are significant and include, but are not limited to the following: increased amygdala volume, decreased activity in frontal cortical and limbic brain structures, and altered white matter structures. early life stress is believed to produce changes in brain development by interfering with neurogenesis, synaptic production, and pruning of synapses and receptors. interference with these processes could result in increased or decreased brain region volumes, potentially explaining the findings that early life stress is associated with increased amygdala volume and decreased anterior cingulate cortex volume. from the literature, several important conclusions have been drawn. brain areas that undergo significant post-natal development, such as those involved in memory and emotion are more vulnerable to effects of early life stress. for example, the hippocampus continues to develop after birth and is a structure that is affected by childhood maltreatment. early life stress seems to interfere with the overproduction of synapses that is typical in childhood, but does not interfere with synaptic pruning in adolescence. this results in smaller hippocampal volumes, potentially explaining the association between early life stress and reduced hippocampal volume. this volume reduction may be associated with the emotion regulation deficits seen in those exposed to early life stress. the amygdala is particularly vulnerable to early life stress. the amygdala also undergoes significant development during childhood, is structurally and functionally altered in individuals that have experienced early life stress, and is associated with the socioemotional difficulties linked with early life stress. receptor type is another consideration when determining whether or not a brain region is sensitive to the effects of early life stress. brain regions with a high density of glucocorticoid receptors are especially vulnerable to the effects of early life stress, likely because glucocorticoids bind to these receptors during stress exposure, facilitating the development of survival responses at the cost of other important neural pathways. some examples of brain regions with high glucocorticoid receptor density are the hippocampus and cerebellar vermis. stress activates the hpa axis, and results in the production of glucocorticoids. increased glucocorticoid production results in increased activation of these brain regions, facilitating the development of certain neural pathways at the cost of others. abnormalities in brain structure and function are often associated with deficits that may persist for years after the stress is removed, and may be a risk factor for future psychopathology. the brain regions most sensitive to early life stress are those undergoing developmental changes during the stress exposure. as a result, stress alters the developmental trajectory of that brain region, producing long-lasting alterations in structure and function. common types of early life stress that are documented include maltreatment, neglect, and previous institutionalization. living in poverty has also been shown to similarly influence brain function.