Brain Development Overview

Early Brain Development

• First 5 Years: Brain development during this period accounts for 90% of growth, reaching significant milestones at different ages.

  • At Birth: Brain weighs approximately 300 grams.

  • At 1 Year: Brain weighs about 1000 grams.

  • Adult Brain Weight: Ranges from 1200 to 1400 grams.

  • Neurons: An adult brain contains 80 billion neurons and forms approximately 100 trillion connections.

Human Development Stages

• Embryonic Development:

  • Formation starts with single cell (zygote) post-fertilization.

  • Cell division begins shortly after.

Stages of In Utero Development

• Embryo: Defined from fertilization to the 9th week of development.

• Fetus: Fetal stage starts from 10 weeks until birth.

Nervous System Development

• Origin: The nervous system develops from the ectoderm (outer layer).

  • Neural Plate Formation:

    • At around 18 days, the ectoderm thickens to form a neural plate.

  • Neural Groove: The plate folds to create a neural groove, which eventually closes to form the neural tube (source of the brain and spinal cord).

Subdivisions of the Brain

• Formation of three subdivisions: Forebrain, Midbrain, and Hindbrain.

  • Critical Point: Development begins at the anterior end of the neural tube.

Cellular Level Development Stages

1. Neurogenesis

• Process of neuron production through division of non-neuronal cells in the ventricular zone.

  • Cells move away, some stay and proliferate.

2. Cell Migration

• Newly created neurons migrate from the ventricular zone to their functional locations.

  • Guidance Chemicals: Immunoglobins and chemokines assist in directing the cells.

3. Cell Differentiation

• Post-migration, cells acquire specific characteristics, becoming either neurons or glial cells.

  • The expression of distinct genes leads to new protein production crucial for neuron development.

  • As new cells move to new areas they do what neighboring cells are doing. If neighboring cells differentiate to form neurons the new cells also become neurons.

  • Some cells remain undifferentiated (do not take a form or shape). These are called stem cells and they can be transformed into any type of cell when put in any part of the brain.

4. Synaptogenesis

• Formation of synaptic connections. Initially, a high number of synapses are produced; only the most successful persist, while others are eliminated.

5. Neuronal Cell Death (Apoptosis or programmed cell death)

• A natural process post-birth where unnecessary neurons die off.

  • Neurotrophic Factors: Determines which neurons survive, providing nourishment. If no signal or neurotropic factors received from the target neuron the new neurons die.

  • At birth we have the most neurons, many die directly after in the CNS.

6. Synapse Rearrangement (Synaptic Remodeling)

• Rearranging of synaptic connections; some original synapses are retracting, while new connections are formed.

  • Increased synaptic connections contribute to brain growth immediately after birth.

  • Intellectual Stimulation: Plays a vital role in the quality and quantity of synaptic connections formed during this stage.

  • It is seen in the cerebellum, brain stem, and visual cortex.