What are the main characteristics of brain development? - Chapter 2

Brain development is not a linear process. Instead, it is a complex developmental process with many different developmental mechanisms occurring both in sequence and simultaneously. Roughly speaking, brain development can be divided into two stages: prenatal and post-natal. The first stage, prenatal brain development is mainly concerned with the development of the central nervous system (CNS). In doing so, there is are various dynamic, developmental processes involved: hierarchical progression; regressive and additive processes, and; growth spurts in neurological processes. The second stage, post-natal development, is mainly characterised by elaboration of the brain, in particular dendritic aborisation, myelination, and synaptogenesis.

Which two stages can be distinguished in brain development? 

Cerebral development is an ongoing process, beginning early in gestation and continuing into childhood through early adulthood. Brain development can be divided into two phases in which birth is a rough marker for the transition between these two qualitatively distinct stages. The first stage, prenatal development, is mainly concerned with the structural formation of the central nervous system (CNS). This stage is thought to be largely determined by genes. The quickest rate of brain growth occurs during the prenatal stage, when it is estimated that 250,000 brain cells are formed each minute through continuous rapid cell division. The second stage, post-natal development, is mainly characterised by elaboration of the brain, in particular dendritic aborisation, myelination, and synaptogenesis. While these processes are still largely determined genetically, they are more susceptible to the impact of neuronal activity and they are thus more susceptible to environmental and experimental influences.  

Which three developmental processes characterise cerebral development?

Cerebral development is not a simple linear process of development. Instead, there is a range of developmental processes, with many occurring simultaneously, reflecting differential developmental timing for various brain regions. These developmental brain processes are:

• Hierarchical progression: first the brainstem and cerebral regions, then the posterior areas, lastly the anterior regions.
• Additive and regressive events: 
  • Additive development refers to the ongoing accumulation of growth processes, for example myelination, shown by the increasing connectivity of myelinated white matter from birth to eighteen years. 
  • Regressive development refers to initial overproduction, followed by selective elimination of redundant element. For example, excess synapses are overproduced in infancy and those that do not form functional networks become obsolete connections and are pruned. 
• Growth spurts in neurological process: brain maturation is not linear, but is characterized by a series of growth spurts. An initial growth spurt is recorded between 1.5 and 5 years, a second growth spurt between 5 and 10 years, and a final growth spurt between 10 and 16 years. These accompany critical periods for development. However, many questions regarding these periods remain. For example, are there different critical periods for different neurobehavioural domains? Do some skills have shorter critical periods than others? 

Which two classes of cells form the basis for brain development?

Brain development progresses via the rapid generation of two main classes of cells: neurons and glial cells.

Neurons form the basic functional unit of the CNS and are responsible for neural transmission within the brain. Neurons consists of four primary components:

1. Cell body; important for the metabolic functions of the neurons. The cell body holds the RNA and DNA.
2. Axon; conducting neural impulses away from the cell body; progressively sheated with myelin throughout childhood.
3. Dendrites; branch off the cell body and receive impulses from other neurons, conducting them towards the cell body. Dendrite spines are the locus of the synapse; they form the location where information is transmitted from one neuron to another.
4. The presynaptic terminals; where neurotransmitters are stored and released; activate the neurons at the post-synapse. 

Glial cells play a supportive and nutrient role within the CNS enabling regeneration of damaged neurons. They produce scar tissue to occupy damaged sites and transport nutrients from nerve cells. There are different types of glial cells. Astrocytes form the blood - brain barrier. They support the cellular structure of the brain and contribute to the migration of neurons and cleaning of plug injury sites. Oligodendrocytes are responsible for speeding up the transmission of neural impulses throughout the nervous system by coating axons with myelin. And lastly, microglia clean up injury sites, mainly in the grey matter. 

What is the impact of prenatal brain dysfunction?

Prenatal brain dysfunction has different consequences, depending on the timing of the insults. A brief summary is provided in the table below.

Which factors influence brain development?

Brain development is complex with many different developmental mechanisms occurring both in sequence and simultaneously. In addition, a number of factors can interfere with this process, potentially causing irreversible changes to these developmental processes and to the final outcome. 

Prenatal risk factors include:

• Maternal stress and age.
• Maternal health (such as: history of infection, rubella, AIDS, herpes simplex).
• Nutrition (diet, malnutrition).
• Maternal drug and alcohol addiction.
• Environmental toxins (such as: lead, radiation, trauma).

Post-natal risk factors include:

• Birth complications.
• Nutrition.
• Environmental toxins (such as: lead, radiation, trauma).
• Cerebral infection.
• Environment / experience.