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.