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1 the Developing Brain: Experience and Development Embryogenesis The Developing Brain: Experience and Development Charles A. Nelson Institute of Child Development Department of Pediatrics University of Minnesota Human Brain Development Embryogenesis • Following conception, zygote rapidly divides, forming ball of cells (blastocyst) • About 1 week post conception, blastocyst divides into inner and outer layer • Outer layer becomes cord, sac, placenta; inner becomes embryo 1 Embryo, Umbilical Cord, Amniotic Sac Embryogeneis, Con’t Placenta • Between 1st and 2nd week post conception, embryo divides into 3 layers embryo • Inner (endoderm) becomes organs, etc. • Middle (mesoderm) becomes muscle/skeletal Amniotic sac system • Outer (ectoderm) becomes outer coverings PLUS nervous system Umbilical cord Neurulation Neural Tube Formation • Days 18-24: – dorsal region of ectodermal layer of embryo thickens and forms neural plate – plate forms a groove, followed by formation of neural tube – tube closes at rostral and caudal ends – cells trapped inside tube form CNS; those outside tube form ANS. 2 Errors in Neurulation Spina Bifida • Neural Tube Defects, including – Spina bifida (“open spine”) – Anencephaly (lack of cortex) – (next slides are graphic) 3 Anencephaly Neurogenesis (note: there are multiple anomalies here in addition to anencephaly) • Precursors to neurons and supportive tissue (glia) form; this continues postnatally. • Regarding neurogenesis of neurons, previously believed was complete at birth. Now know there is postnatal neurogenesis through at least middle age in dentate gyrus (region of hippocampus), and possibly in other areas. Neurogenesis, Con’t Cell Migration • Postnatal neurogenesis influenced by experience (positive = ↑new neurons and negative = ↓ new • Following neural tube closure, there is proliferation of neurons). a single layer of epithelial cells that line the tube. • Recent report (Gould et al., Sept., 2001) suggests • Cells are connected to each other and in some these new cells are short-lived (substantial decline by cases, to radial glial fibers. 9 weeks). • Expansion occurs between layers, and neuroblasts climb onto fibers and migrate in radial direction. Nevertheless… • Cortex forms in inside-out pattern, with earliest of 6 • Believe new cells involved in learning and memory layers formed first, followed by each subsequent layer. 4 Cell Migration in Ferret Brain Errors in Cell Migration (Courtesy of Susan McConnell, Ph.D.) • Schizophrenia (?) • Agenesis of the corpus callosum Migrating • Prematurity may be example of incomplete Neuron cell migration, depending on degree 5 The Preterm Brain Differentiation and Synaptogenesis (approximately 25 weeks gestation) Temporal • Once cells reach target destination, following Lobe events may occur: Frontal Lobe – cell body develops or – cell dies • If cell body develops: – processes (axons, dendrites) form – synapses form Note lack of gyri and sulci 6 Synaptogenesis • massive overproduction of synapses • Newborn brain has many more synapses than adult brain. • Overproduction followed by retraction to adult numbers of synapses. • In human, rate of retraction varies from brain area to brain area. Overproduction of Synapses Retraction of Synapses: • In visual cortex, is burst of synapses 3 – 4 postnatal months, with peak occurring 4 months. • In visual and auditory cortices, adult levels of • Primary auditory cortex (Heschl’s gyrus) follows synapses obtained in early childhood (2-6 similar timetable. years). • In area involved in receptive language (angular gyrus) and in area involved in language production (Broca’s area) there are slightly fewer synapses at 3 • In middle frontal gyrus, adult levels are not – 4 months than in primary auditory cortex reached until mid- to late adolescence. (i.e.,these areas lag slightly behind basic auditory areas). • In middle frontal gyrus maximum density not reached until 12 months. 7 Summary of Synaptogenesis • Synapse overproduction occurs early in life, whereas synapses elimination occurs much later, and varies greatly by area (e.g., 6 years for visual cortex, 15+ years for prefrontal cortex). Why Overproduce Synapses? Myelination • captures experience, thereby • Myelin is lipid/protein substance pruning/cultivating synapses • In CNS, myelin produced by oligodendroglia • can be adaptive for the organism (period of • In ANS, myelin produced by Schwann cells opportunity) but • Myelin wraps itself around axon as form of • can also be maladaptive (period of insulation vulnerability), depending on nature of • Myelin speeds conduction velocity experience • “Plasticity cuts both ways” (J. McVicker Hunt) 8 Myelination During Prenatal and Myelination, Con’t Early Postnatal Period • Myelination has implications for both serial • System: and parallel processing (e.g., multitasking). – motor roots followed by • Myelination occurs in “waves” beginning – sensory roots, followed by – somesthetic (touch) cortex, prenatally and ending in young adulthood – primary visual (seeing) and then (and in some regions, as “late” as middle – primary auditory (hearing) cortex. age). • First postnatal year: • Historically stained for myelin in postmortem – regions of brain stem, tissue; now use structural Magnetic – cerebellum and Resonance Imaging (MRI) and Diffusion – splenium of corpus callosum all begin Tensor Imaging (DTI). • By 1 year all regions of corpus callosum underway Myelination During Childhood to Adolescent Period Developmental Event Timeline Overview of Developmental Event Neuralation 18-24 prenatal days Cells differentiate into one of three layers: endoderm, mesoderm and ectoderm, which • Through pre-adolescent period, observe then form the various organs in the body. The neural tube (from which the CNS is derived) develops from the ectoderm cells; the neural crest (from which the ANS is derived) lies between the ectodermal wall and increase in gray matter volume (more the neural tube. Neuronal Migration 6-24 prenatal weeks Neurons migrate at the ventricular zone along radial glial cells to the cerebral cortex. dendtritic spines?) and decrease in white; this The Neurons migrate in an inside-out manner, with later generations of cells migrating through previously developed cells. reverses in adolescence, with decrease in The cortex develops into 6 layers Synaptogenesis 3rd trimester - Neurons migrate into the cortical plate and extend apical and basilar dendrites. adolescence Chemical signals guide the developing dendrites toward their final location, where gray and increase in white (retraction of synapses are formed with projections from subcortical structures. These connections are strengthened through neuronal activity, and connections with very synapses, increase in connectivity?). little activity are pruned. Postnatal Neurogenesis Birth - Adulthood The development of new cells in several brain regions, including: - dentate gyrus of the hippocampus – Changes of note: parietal, temporal, and - olfactory bulb - possibly cingulate gyrus; regions of parietal cortex occipital lobes show relatively little change, Myelination 3rd trimester – middle Neurons are enclosed in a myelin sheath, resulting in an increased speed of action whereas frontal lobe show large changes. age potentials. Gyrification 3rd trimester – adulthood The smooth tissue of the brain folds to form gyri and sulci. Structural Development of the Birth – late adulthood The prefrontal cortex is the last structure to undergo gyrification during uterine life. Prefrontal Cortex The synaptic density reaches its peak at 12 months, however, myelination of this structure continues into adulthood Neurochemical Development Uterine life - All major neurotransmitter systems undergo initial development during uterine life and are of the Prefrontal Cortex adolescence present at birth. Although it is not well studied in humans, it is thought that most neurotransmitter systems do not reach full maturity until adulthood. 9 Neural Plasticity: Two Types of Plasticity Some General Principles and Terms (from William Greenough ) Experience-expectant • Development based the expectation that appropriate • Recovery of Function: cortical function environments will provide information needed to returned following injury (e.g., return of select appropriate subsets of synaptic connections. language following stroke) Common to all members of the species (examples: visual development; language; initial formation of • Sparing: lack of loss in performance following attachment) brain damage (e.g., no impairment in Experience-dependent language despite a perinatal stroke that • Unique to each individual, it most likely involves destroys left frontal cortex). active formation of new synaptic connections throughout the life span based on each person’s • Neuronal Plasticity: hypothesized to underlie interaction with his/her environment (example: recovery of function. learning; quality of attachment?). Examples of Developmental Plasticity Examples of Developmental Plasticity, Con't • The development of normal vision depends • Prenatal exposure to teratogens alters brain on access to normal visual input during a development sensitive period of development. • Prenatal exposure to stress can cause longlasting neurobehavioral deficits in rats and monkeys • Similarly, correcting certain visual impairments (e.g., strabismus) must occur during this sensitive period. • Development of speech perception depends on exposure to normal speech sounds during a sensitive period of development. 10 Conclusions about Examples of Developmental Plasticity, Con't Developmental Plasticity • Memory
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