By Melina Fatemi
Our brain is a complex organ that is responsible for everything that you are able to do: from balance to more complex tasks such as planning or solving problems. But have you ever wondered how your brain becomes the way it does?
Embryonic Development
At the very beginning, during embryonic development, there are three layers that have emerged, which ultimately determine the DNA instructions for development. These three layers are the ectoderm, mesoderm, and endoderm in their respective orders from the outer layer to the inner.
Image from Charles Hanson — Research Gate
It is important to note that all of these cells include the exact same DNA. So, you may be wondering, what sets these apart then? The answer is the signals sent by the surrounding tissues reach certain layers but not all, which impacts which genes are turned on and which ones are not.
Neural Induction
The signals sent by the mesoderm trigger some ectoderm cells leading them to become nerve tissue. This process is known as neural induction. Later signals determine whether these nerve tissues will become neurons or glia. For example, through the sonic hedgehog signaling molecule released by the mesodermal tissue, the cells nearby specialize into glia. The cells that are farther away become motor neurons and those the farthest away become interneurons. In fact, interestingly enough, flies use the same mechanism!
Proliferation
You may already be familiar with the concept of cell division from your biology textbook. The same concept applies to developing neurons: in order to increase in number, they must divide! These daughter cells come from stem and progenitor cells, which divide symmetrically and asymmetrically to form the developed brain. Complications in this stage of the brain are proposed to be the cause of microcephaly and megalocephaly.
Migration
The next step in the development of the brain would be getting the neurons to where they need to be — a process known as migration. There are two types of migrations taking place in the neural tube: radial migration and tangential migration. As demonstrated in the diagram below, radial migration involves moving outward with the help of radial glia while tangential migration involves moving upward.
Image from Frontiers
Synapse Development
To better understand the process of synapse development, it is important to first consider the purpose of dendrites and axons.
Image from National Institute of Child Health and Human Development
A message travels down a myelinated axon, reaching the axon terminal. The axon terminal of a neuron is connected to the dendrites of other neurons. A gap exists between the axon terminal of the neuron and the dendrite of the receiving neuron. This gap is known as the synapse. Due to the existence of this gap, the electrical messages must be converted into chemical messengers to reach the receiving neuron. The dendrites of the receiving neuron have receptors, which act as a lock to these neurotransmitters (the key).
The growth of the axon is possible thanks to its growth cone, which explores the environment and receives molecular cues (e.g., netrin, semaphorin, ephrin) enabling it to move to its final location and grow. Moreover, as the synapse develops, the presynaptic axon packages itself with neurotransmitters while receptors open on the post-synaptic dendrite. In addition, molecules discharged by astrocytes (a form of glia) are important for the regulation of synaptic development.
The myelin sheath covering the axon is crucial to synaptic communication and is another aspect that occurs throughout a person’s lifespan. These fatty layers insulate the neuron and ensure that the electrical messages are sent quickly.
Image from The Brain From Top to Bottom
Synaptic Pruning
Initially, the brain’s neurons develop too many connections. However, to ensure efficiency is maintained by the brain, the weaker connections are eliminated through a process called synaptic pruning. On the other hand, stronger connections survive. Glia are once again important players in this process, especially astrocytes.
The development of the brain is a complex and fascinating process that starts during embryonic development. While much remains elusive about the brain, ongoing research continues to uncover new insights into this incredible organ and how it functions.
References
Brain Facts: A Primer on the Brain and Nervous System. (2002). Society for Neuroscience.
National Center for Biotechnology Information. (n.d.). The Axonal Growth Cone — Neuroscience. National Library of Medicine. Retrieved August 12, 2023, from https://www.ncbi.nlm.nih.gov/books/NBK11114/
Professor Galvan. (2018, January 30). 9.2 neuronal proliferation, migration, & aggregation. YouTube. https://www.youtube.com/watch?v=9_RWypigbho
Santos, E., Noggle, C.A. (2011). Synaptic Pruning. In: Goldstein, S., Naglieri, J.A. (eds) Encyclopedia of Child Behavior and Development. Springer, Boston, MA. https://doi.org/10.1007/978-0-387-79061-9_2856