Neuronal Development

The nervous system. Central and peripheral, this system is integral to every bodily function happening in you right now and at every waking moment. From the rhythm of your heartbeat to the tiniest sensation of a gentle itch, the nervous system drives all functions that contribute to your survival.

Without it, well, you would be no more than a bag of bones with the intelligence of a tardigrade (no offense to tardigrades).

So, where does the nervous system begin, and what makes it so important?

Neurons and glial cells are perhaps the most important functioning components of the nervous system. They are vital to the reception and distribution of information from stimuli both inside and outside of your body. These cells emerge before you even become a conscious being: at embryonic development.

Embryonic Development: The Beginning

By now, you’re probably familiar with the “Big 3” (I don’t know if anyone actually calls them that, maybe it’s just me): the endoderm, mesoderm, and ectoderm. The three layers which give rise to every physiological system in your body, beginning at embryonic development.

Your nervous system stems (pun intended) from the outermost layer, the ectoderm. At the very beginning of this process, the cells begin to form what is called the “neural plate” in the central portion of the ectoderm. As the ectoderm continues to grow on either side of the neural plate, the varied mitotic rates result in a sort of “U” shape from this neural plate, which is then known as the “neural groove.”

The tops of this groove eventually join to become the “neural tube.” The neural tube later becomes cerebral ventricles of the brain and the central canal of the spinal cord.

Now, the good stuff.

Neuronal Development

There are four stages recognized in the process of neuron development and the formation of the nervous system.

Throughout these formative stages, all cells which form the basis of every organ and system that come together to ultimately develop into the sentient creature that you are, are formed and assigned to their lifelong destinations and physiological roles.

Stage 1: Neurogenesis

This is the very first stage of the foundational process of neuronal development. During this stage, cells that have yet to be differentiated will undergo mitosis to produce either stem cells, or neuroblasts, which will ultimately be differentiated into many different types of neurons.

For now, neuroblasts and stem cells can be thought of as essentially the same things. They are, functionally, precursors to what eventually becomes a neuron.

The main difference is that neuroblasts are the stars of the show when it comes to embryonic development, whereas stem cells have a stronger presence in adult neurogenesis (development of neurons takes place throughout the entire lifetime).

These cells continue dividing until they eventually form what is called the “ventricular zone,” which is a densely-packed layer of… well, cells! This ultimately leads to the formation of three separate zones (another Big 3, eh?): the ventricular zone, intermediate zone, and the marginal zone.

The neurons and glial cells are formed in the intermediate zone.

Stage 2: Cell Migration

Here is where things start to get even more exciting and take on a bit more complexity. In this stage, cells that were previously responsible for creating the ventricular zone now must move great distances to establish distinct cell populations for further embryonic development.

These migrations are genetically pre-determined and so are not random in any way.

Keep in mind that the formation of the ventricular, intermediate, marginal zone is due to the movement of cells. So, even though the formal process of migration has not officially begun, this is the mechanism behind the formation of these zones.

Further, cells in the intermediate zone have already begun developing into neural and glial cells. (But these are not as special as the ones coming up.)

What constitutes cell migration is the movement of more cells (which are being formed during the ongoing process of neurogenesis) along the radial glia toward the marginal zone from the ventricular zone. Once they reach the marginal zone, they begin differentiation.

Stage 3: Differentiation

Now, the process of differentiation is different from normal cell mitosis in that the embryo’s DNA dictates the nerve cells’ specific physiology for their future core functions. Here is where it is determined what type of nerve cell they will become.

This process continues on, hand-in-hand, with Stage 4.

Stage 4: Outgrowth

Okay, I know I said the last part was exciting, but this is even more exciting! In this stage, the foundational cells of the nervous system truly begin to take the shape we all know and love.

Here is where the axons and dendrites begin to form – differentiation continues through this stage to direct these developments, as each neuron requires a distinct physicality based on their ultimate function.

The proliferation of synapses – the junction between nerve cells which allows for communication throughout the entirety of the nervous system – begins here as well. The synapses, axons, and dendrites all work together to create this system of communication: Dendrites receive information from a given stimulus, pass it through the axon and to the synapse, which then transfers that information to another neuron until all information ends up in your brain.

Going back to differentiation: As previously noted, there are many – many – different types of neurons. What the developing neurons will become is not only determined by DNA expression, but also by the location of the cell during development and relative position to neighboring nerve cells.

Neuronal Development in Adults

As previously mentioned, neuron development continues throughout your entire lifetime. This is because the development of neurons is essential to functions such as memory and learning.

In fact, without the ongoing process of neurogenesis into adult life, scientists have found that intellectual and mental disorders and/or challenges are likely to emerge (Olde et al., 2011).

Neuronal development in adults takes place in the hippocampus. Researchers learned that both a complete lack of, and low rates of, neurogenesis in adults is directly associated with difficulty in differentiating stimuli that may be similar to one another: such as two food items that smell the same but are completely different in taste or ingredients.

This can also result in problems with long-term memory and has been found to be related to conditions such as depression, anxiety, and post-traumatic stress disorder. Other factors that can decrease rates of neurogenesis in adults are stress, sleep deprivation, and aging (bummer, right?).

Don’t fret, though. There are things you can do to combat the slowing of neurogenesis throughout your adult life. This same lab confirmed that exercise, learning (whether by schooling, reading a book, or other ways of intaking new information) and even simple conversation all effectively increase rates of neurogenesis in adults.

To illustrate just how active the hippocampus is in producing neurons, the lab performed an experiment with mice to find that over 30,000 neurons were produced every single day. This demonstrates just how much energy goes into constantly producing the proper synapses and cells that allow you to take in new information and create memories every second of every day.

Neuronal Development for Memory and Learning

According to Olde et al. (2011), new proteins must be synthesized at the synapses between neurons in order to create the ability to form memories. This also constitutes the need to synthesize proteins at dendrites as well.

Imagine this process happening every time you learn someone’s name. When you meet someone at a coffee shop or follow a new person on social media, you make a subconscious (or conscious) effort to remember their name, right? Well, as minute as this act may seem, there are proteins being synthesized and new neurons being created to aid you in creating that one single memory.

Even now, as you are reading this information, synapses are firing, dendrites and neurons are working, and proteins are being synthesized in order for you to retain this new knowledge. Crazy, right?

Some of the major components to the development and functioning of neurons have been identified as miRNA, a non-coding RNA involved in gene expression, circRNA, another non-coding RNA, sodium (Na), potassium (K) and more.

miRNA and circRNA are integral to the formation of neurons in that they are involved in numerous stages of neuronal development: dendritic branching, neuronal differentiation, etc. (Olde et al., 2011) and have also been linked to some intellectual and psychological disorders.

Sodium and potassium, along with the previously mentioned RNA molecules, of course, are vital to the neuron’s ability to receive and transmit information via synapses (Gjedde, 2002).

Neuronal development is central to life’s most essential system: the nervous system. Beginning at embryonic development, this process gives rise to the very cells that allow us to function as sentient, feeling, seeing, all-around perceptive organisms.

From the smallest sensation of the gentle touch of a feather to core instinctual reflex mechanisms like “Fight or Flight,” growth and creation of neurons is a part of human growth and development that we could never do without.

References

1. Gjedde, A. (2002). Coupling of blood flow to neuronal excitability. In W. Walz (Ed.), The neuronal environment: Brain homeostasis in health and disease (pp. 233-257). Totowa, NJ: Humana Press.
2. Olde Loohuis, N. F., Kos, A., Martens, G. J., Van Bokhoven, H., Nadif Kasri, N., & Aschrafi, A. (2011). MicroRNA networks direct neuronal development and plasticity. Cellular and Molecular Life Sciences, 69(1), 89-102. doi:10.1007/s00018-011-0788-1