Early Brain Development: Weeks 1-3

Early Brain Development: Weeks 1-3

• Brain development begins at conception.
• Within the first few days, a self-organizing inner cell mass (100-150 cells) forms, developing into organs, nervous system, and bones.
• Three germ layers—endoderm (organs), mesoderm (muscles/bones), and ectoderm (nervous system/skin)—are crucial.
• A blastocyst (150-cell mass) attaches to the uterine wall a week after conception.
• The inner cell mass forms an embryonic disk, comprising these three germ layers.
• The human central nervous system forms by week 2 or 3.

Neural Tube Formation

• The neural plate, an outer layer of embryonic cells, folds to create a neural groove, then curls into a neural tube.
• This tube encloses a fluid-filled cavity, eventually developing into the hindbrain, midbrain, and forebrain.
• The fluid cavity becomes the central canal and the four ventricles of the brain.
• The folded brain structure allows for a large surface area, accommodating many neurons within a compact size.

Maturation of the Brain: Growth and Development of Neurons

• Proliferation: Cells in the neural tube divide and become stem cells, glia, or neurons.
• Migration: These cells migrate outward from the neural tube, guided by chemo-attractants and chemo-repellents—immunoglobins & chemokines.
• Differentiation: During migration, cells develop axons and dendrites.
• Myelination: Glial cells (Schwann cells in the peripheral NS) wrap axons to form myelin sheaths, allowing for rapid signal transmission, which forms the white matter.
• Synaptogenesis: Neurons connect, forming synapses to communicate. This number increases with age and experience.
• Importance of Connections: Connections are crucial for neural survival; neurotrophins (like NGF) promote survival and growth. Neurons that don't connect may die.
• Synaptic Pruning: The brain loses excessive synaptic connections; the most used ones remain.

Neuron Lifespan

• Different cell types have varied lifespans.
• New neurons do form in regions like the adult hippocampus, important for learning.
• Olfactory receptor replacement occurs throughout life.

Sperry's Experiment: Axon Pathfinding

• Axons have growth cones that aid in navigating to correct targets.
• Sperry showed that axons reconnect to their proper targets even after being experimentally rearranged.
• Chemical gradients within the brain guide axon pathfinding.

Determinants of Neural Survival: Levi-Montalcini

• Levi-Montalcini's research showed that neurotrophins are crucial for neuron survival and growth.
• Neurotrophins like NGF were found to originate from muscle cells, promoting axon survival beyond early development.

Vulnerable Developing Brain: Intrinsic and Extrinsic Factors

• Intrinsic factors (genes): Genetic factors cause similar brain development (e.g., in twins), and variations can result in cognitive differences.
• Extrinsic factors (experience): The mother's experiences (e.g., stress) and environmental factors (e.g., malnutrition, infections, toxins) have profound impacts on brain development.

Extrinsic Factors: Drugs and Toxins (Alcohol)

• Alcohol during pregnancy can cause Fetal Alcohol Syndrome (FAS), affecting brain development and resulting in symptoms like facial abnormalities, mental retardation, and motor problems.
• Alcohol suppresses glutamate release, reducing excitation and potentially inducing apoptosis.
• Alcohol consumption affects cortical thickness and dendritic structure.

Cortical Plasticity and Experience

• The brain can reorganize itself in response to experience through structural changes in axons and dendrites.
• Experience, particularly early experiences, plays a significant role in neuron branching and synaptic development (as in enriched environments).
• Blind individuals' brains demonstrate reassignment of visual cortex functions to other senses.
• Musicians experience changes in brain structure and function due to training.

Loss of Input and Reorganization

• Amputation and blindness demonstrate the brain's reorganization in response to decreased sensory input.
• Phantom limb sensations are a consequence of this reorganization.
• Focal hand dystonia in musicians reveals extreme reorganizations.

Explore the critical stages of brain development during the first three weeks after conception. This quiz covers the formation of the neural tube and the establishment of the three germ layers essential for organ development. Test your knowledge on the processes that lead to the formation of the central nervous system.