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Antenatal Period
This period is the time prior to birth, in which the processes proliferation and migration occurs. Can be thought of the time period in which the "hardware" for the brain is developed. -
Period: to
Brain Development of a Child from Conception to 2 Years Old
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Brain Development During Conception
Before a person is even born, neurogenesis and intense growth and expansion occurs in the brain already. New neurons are created rapidly. Rapid growth occurs in the womb. By the last trimester of pregnancy, a fetus's brain undergoes "major increases in size, folding of the brain's outer layer (cortex)", and the connections between neurons are already developing as well. -
Brain Development During Conception
The brain also begins to already develop its wrinkled outer surface, creating more space for connections to be made. Major long-range signaling pathways made up of white matter also begin to form only several months into gestation and by birth, these pathways have already established themselves. -
Birth
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During Conception
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Differentiation
During the first two weeks of conception, in the gastrula stage, neurons differentiate into a mature neuron, or it dies through apoptosis (cell death). The development of axons is facilitated by growth cones, which form at the edge of an axon. Dendrite formation, however, is formed by "genes controlling calcium-regulated transcription factors (Aizawa et al., 2004)". With contact of the dendrites to the axons, connections are formed which form the basis for synaptic connections. -
Neurulation
About two weeks after conception occurs, the embryo has organized itself already into a "three-layered, spherical structure". In one of the areas of this sphere, the cells thicken to form what is called the neural plate. This plate then folds over onto itself, which forms a tube that resembles a zipper. This creates the neural tube, the "inner cells of which will lead to the formation of the central nervous system", while the outer cells will help shape the autonomic nervous system. -
Neurulation 2
The different regions of tissues around the ventricles then become the distinct brain structures. The anterior portion of the brain becomes the forebrain, containing the cerebral hemispheres such as the hypothalamus. The cells around the middle vesicle become the midbrain, the structure that connects the diencephalon to the hindbrain. The rear-most portion becomes the hindbrain, which contains areas of the brain such as the cerebellum. The rest of the cells make up the spinal cord. -
Proliferation
Proliferation occurs for 40 weeks in gestation. After the structure of the neural tube has been laid out, the cells that line the innermost part of the tube, "called the ventricular zone, proliferate at a logarithmic rate". As these cells grow and multiply, a second zone called the marginal zone is formed, which will contain the axons and dendrites. This stage continues for quite some time. The overproduction of neurons is balanced by the process of apoptosis, or programmed cell death. -
0-1 years
At birth, the brain has 200 billion brain cells called neurons. 60% of an infant's energy intake from food is used for brain growth. To help aid the infant's brain growth, social factors are just as crucial as nutrition is. During the first year of life specifically, brain volume doubles as connections between neurons and other developmental processes start to occur. -
1 Year Old
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Cell Migration
After the birth of the cells in the brain, they travel to their final destinations. The cerebral cortex is composed of multilayered tissue several millimeters thick. The earliest migrating cells go to the deepest cortical layers, whereas the subsequent migrations pass through previously formed layers to form the outer layers. This entire process, about 25 weeks after conception, is when all six layers of the cortex will have been formed. -
0-1 years
A brain growth that occurs during this time period includes myelination, which helps boost signaling speed between neurons, allowing for faster connections to be made. White matter pathways between regions of the cortex also start to develop. -
Sensorimotor Stage
The sensorimotor stage is the overarching stage of brain development that occurs from birth to infancy. One of the key changes during the sensorimotor stage is the biological changes in the brain. From the brain development that had already occurred from conception to birth, babies are born with 100 billion neurons which are connected by 50 trillion synapses. The brain has also reached 25% of its adult size at birth. Milestones during this stage include babies having basic reflexes for survival. -
Synaptogenesis
A synapse is defined as the point of contact between two brain cells, being a dendrite and an axon. First synapses in the brain are observed by about the 23rd week of gestation. Typically, there will be a massive overproduction of synapses which are then gradually reduced through pruning. However, different structures in the brain reach their peak of synapse production at different time points. This difference is important since it plays a factor in the plasticity in those regions of the brain. -
1-2 Years Old
Brain volume continues to grow during this stage, increasing by an estimated 15%. -
2 Years Old
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0-2 Years Old
During this period, the key biological changes in the brain include the fact that the cerebellum and prefrontal cortex continue to make new neurons. And, by the age of two, synaptic density is already halved. Myelination continues to occur, starting from the back to the front of the brain and from the center to the sides. The brain reaches 50% of its adult size at the age of 1. Some milestones during this stage are that motor development such as sitting without support and walking occur. -
Synapse pruning
The overproduction of synapses is resolved through a process called pruning, which removes the unused synapses. Until the stage of synaptogenesis is reached, brain development is largely gene driven. However, once pruning starts to occur more often, this process is more experience driven. Pruning, just like synaptogenesis, occurs at different time periods in the brain. The ability for the brain to prune itself is crucial in the neuroplasticity necessary for adaptive capabilities. -
Myelination
Myelination is the process of wrapping axons of neurons in fatty cells. This ultimately helps facilitate neuronal activity and communication since the insulation the fatty tissues provide allows the axons to transmit electrical signals faster. Myelination starts from birth but reaches its peak levels during two years old. Just like pruning, the process of myelination occurs at different time periods in different regions of the brain. -
Summary
In general, brain development starts since conception and continues even into early adulthood. However, the basic structure of the brain "is laid down primarily during the prenatal period and early childhood". The brain, demonstrating its neuroplasticity, matures at different times for different regions of the brain. Although aspects such as basic sensations and perception systems are fully developed by kindergarten, other processes don't fully mature until much later. -
Citations
https://www.psychologytoday.com/us/basics/neuroscience/how-the-brain-develops https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3722610/ https://www.verywellfamily.com/everything-you-need-to-know-about-fetal-brain-development-4707581#:~:text=How%20Your%20Baby%27s%20Brain%20Develops%201%20First%20Trimester,is%20rapid%20development%20of%20neurons%20and%20wiring.%20 https://www.frontiersin.org/articles/10.3389/fncel.2019.00576/full -