Neural Development

Neurodevelopment & Plasticity

Overview

  • Course: PSYC 304

  • Date: 02/25

Main Topics of the Course

  • Brain Development: Gastrulation, neurulation & segmentation

  • Cell Proliferation and Differentiation: Migration and maturation

  • Neurite Outgrowth: Formation of neuronal connections via synaptogenesis

  • Developmental Sensitive Periods: Key stages where development is affected by external stimuli

  • Neurodegeneration: Disorders associated with degeneration of the nervous system

Neural Development

  • Defined as the growth and maturation of the nervous system from conception onward.

  • Key developmental milestones occur at:

    • 10 weeks: Early formation stages

    • 15 weeks: Progress in structure

    • 24 weeks: Further differentiation

    • 30 weeks: Refinement continues

    • 41 weeks: Near completion of development

Early Embryonic Development

  • Begins when sperm and egg fuse (fertilization) leading to zygote formation.

  • Cleavage (cell division) creates a blastocyst within 5-6 days post-fertilization.

Gastrulation and Germ Layers

  • Gastrulation: Reorganization of cells into three germ layers:

    • Ectoderm: Develops into the Central Nervous System (CNS), Peripheral Nervous System (PNS), and skin.

    • Mesoderm: Forms muscle and immune cells.

    • Endoderm: Becomes internal organs (digestive & respiratory systems).

Formation of the Neural Plate and Neural Tube

  • At around 18 days, the notochord forms and signals the ectoderm to turn into the neural plate.

  • By about 20 days, the neural plate deepens into the neural groove and by 22 days, it closes to form the neural tube (neurulation).

  • Inside the tube will become the brain’s ventricles, where stem cells proliferate.

Neural Tube Defects

  • Common defect: Spina bifida caused by improper closure of the neural tube (occurs in ~1300 births/year in the US).

Segmentation and Brain Region Differentiation

  • By 35 days, the neural tube begins forming distinct brain regions through a process called segmentation by about 6 weeks.

  • Rapid growth of the cerebrum (telencephalon) occurs throughout gestation.

Summary of Early Neurodevelopment

  • Around 18 days, gastrulation creates the three germ layers.

  • The neural plate closes into the neural tube by 21 days.

  • Brain region differentiation happens by day 40 through segmentation.

Neural Stem Cells

  • Neural stem cells (NSCs) proliferate in the ventricular zone and can differentiate into neurons (neurogenesis) or glial cells (gliogenesis).

    • NSCs can self-renew or produce more specialized progenitor cells.

    • Neurogenesis occurs approximately from weeks 8-25, with gliogenesis starting around week 16.

Adult Neurogenesis

  • Controversial topic: Do new neurons form in adulthood? Some studies suggest minimal neurogenesis can occur in adults, especially in the hippocampus.

Microglia

  • Microglia: Immune cells derived from yolk sac precursors that colonize the brain and share time during development (around day 9).

Neural Migration

  • New neurons must migrate from the ventricular zone to their final locations, typically occurring between 12-24 weeks.

Abnormal Cell Proliferation & Migration

  • Abnormalities can result in conditions like:

    • Megalencephaly: Enlarged brains with associated seizures.

    • Microcephaly: Smaller brain size linked to intellectual disability.

Summary of Proliferation & Migration

  • NSCs in the neural tube give rise to most brain cells, excluding microglia.

  • Microglia have unique origins and function in the brain.

  • Neurons migrate to their designated areas for proper function.

Neurite Outgrowth & Synaptogenesis

  • Neurites: Extensions from the cell body developed into dendrites and axons.

  • Growth cones with lamellipodia and filopodia guide neurite directionality via environmental signals.

Synaptogenesis Process

  • Occurs once an axon identifies a potential postsynaptic target. This includes forming synapses through synaptogenesis.

Myelination

  • Myelination occurs as axons receive myelin insulation, a crucial step extending throughout childhood and adolescence.

Summary of Neurite Outgrowth, Synaptogenesis & Myelination

  • Neurites develop into axons & dendrites, facilitated by growth cones.

  • Synaptogenesis is vital for neuronal connections.

  • Myelination is a late phase of neurodevelopment.

Brain Refinement

  • Development involves an initial overproduction of cells & synapses.

  • Apoptosis: Programmed cell death occurs, and microglial phagocytosis contributes to this refinement.

  • Synaptic pruning eliminates weak synapses to enhance efficacy.

Neuroplasticity

  • Defined as the brain’s capacity to change structurally/functionally in response to environmental factors (experience-dependent).

  • Key developmental concepts:

    • Critical periods: Narrow time windows for appropriate neurological stimuli.

    • Sensitive periods: Broader windows allowing lasting changes to neural function.

Perinatal and Adolescent Plasticity

  • High plasticity phases occur during the perinatal period (pregnancy to early childhood) affecting brain development positively and negatively, such as through alcohol exposure leading to Fetal Alcohol Syndrome (FAS).

  • Adolescence is marked by reorganization and refinement, critical for cognitive development.

Neurodegenerative Disorders

  • Neurodegenerative disorders cause the progressive breakdown of the nervous system, often due to abnormal protein accumulation (proteinopathy).

  • Key examples include:

    • Alzheimer’s Disease: Characterized by memory loss and cognitive impairment.

    • Frontotemporal Dementia: Alters personality and emotional regulation.

    • Chronic Traumatic Encephalopathy (CTE): Associated with repeated head trauma, leading to long-term effects on mood and cognition.

Summary of Neurodegenerative Disorders

  • Alzheimer’s disease affects the hippocampus significantly impacting memory.

  • FTD and CTE present with various symptoms relating to personality and cognitive abilities.

Final Reminders

  • A PDF paper for review is posted and will be referenced in the upcoming quiz.

  • Attend group journal club on 3/8 to discuss the reading material and prepare for Exam 2.