12.1 Essential ideas
12.1.1 Neural development
- During early embryogenesis three germ layers, called ectoderm, mesoderm and endoderm, are formed.
- Part of the ectoderm develops into the neural tube, and eventually the brain and nervous system.
The neural tube in chordates is formed from infolding and elongation of ectoderm
- The neural plate is a region of differentiated cells located on the dorsal end of the ectoderm.
Figure 12.1.1a – Neurulation of a generalised chordate embryo shown in transverse section
- During the process of neurulation, the neural plate gradually folds inwards forming a neural groove.
- As the neural groove deepens it elongates bidirectionally from the centre outwards.
- Eventually the fold is enclosed and separates from the rest of the ectoderm, forming a neural tube.
Neurons are initially produced by in the neural tube
- At the end of neurulation, the neural tube is closed at both ends.
- Some of the cells lining the cavity of the tube differentiate to become regions of cell proliferation.
- These regions undergo repeated mitosis to produce neuronal precursors, called neuroblasts.
Immature neurons migrate to a final location
- Neuroblasts must travel from their origin to different parts of the developing brain and spinal cord in order to ensure that the appropriate neural connections are made.
Figure 12.1.1c – Neuronal migration
- Neuronal migration involves two processes, which may occur simultaneously:
- Glial locomotion – the entire neuroblast is propelled forward along a scaffold of support cells called glia.
- Somal translocation – the cytoplasm of the cell extends to the attachment site, then pulls the nucleus and cell body towards the final location.
Axons grow from immature neurons in response to chemical stimuli
- A neuron with a single axon and many branching dendrites will develop from each neuroblast after its migration.
- The terminal end of the growing axon is the growth cone.
- The growth cone responds to chemical stimuli. A proposed mechanism is shown in Figure 12.1.1d.
Figure 12.1.1d – The axon growth cone responds to chemical stimuli
- Some axons extend beyond the neural tube to reach other parts of the body. A single axon may extend through the entire length of the organism’s body.
Each developing neuron forms multiple synapses
- Synapses connect neurons to other neurons via chemical neurotransmitters. They can be formed throughout life.
- During early brain development there is an explosion of brain connectivity as newly formed neurons interact with each other.
- As a neuron matures, it becomes part of a complex neural network by forming multiple synapses.
Synapses that are not used do not persist
- During synaptic transmission, metabolic pathways involved in the release of neurotransmitters are re-enforced.
- This means that specific neural pathways become stronger from use while others become weaker from disuse.
- Synapses that are not used are eliminated by neural pruning.
Figure 12.1.1e – Synaptic density in humans is evidence for neural pruning
neurons and synapseseliminates unused
- Neural pruning involves two related processes:
- Neural pruning makes existing neural pathways more efficient, and streamlines brain functions.
Nervous system plasticity allows it to change with experience
- Synaptic re-enforcement is an example of neural plasticity.
- The number of dendrites, the length of axons and the strength of neural pathways respond to environmental stimuli.
- For example, learning and practising a new skill influences brain chemistry so that a neural pathway is re-enforced.
- New neural pathways can be forged throughout life, but neural plasticity is greatest until the age of 6.
- Neural stem cells are found in some parts of the brain – neurogenesis (the formation of new neurons) also plays a role in nervous system plasticity.
Figure 12.1.1f – Time-lapse of migrating neurons
Modification of neurons starts in the earliest stages of embryogenesis and continues to the final years of life.
Figure 12.1.1g – Rita Levi-Montalcini (1909–2012)
Mrs Levi-Montalcini won a Nobel Prize for discovering nerve growth factor, a protein that acts as a positive chemical stimulus during neurogenesis.
Food for thought
- In humans, neurulation begins at about week 3 of pregnancy.
- Neuroblasts are not stem cells – they can only become neurons. However, structural and functional differences can develop depending on where they migrate to.
- Neural pruning occurs during development and should not be confused with neuroplasticity.