The Days of our Neurons Part I: Prenatal Neurogenesis

By Janani Rajan

Edited by Keshav Ratra

Graphic Designs by Josephine Wang and Olivia Tang

Each person on Earth has unique experiences that cause preality (a personal reality). No two people have the exact same experiences throughout their life. Populations with the same religion, geographical location, as well as other factors, may have some strong, shared social beliefs, but they all have different life experiences that shape them, making them human beings.

Neurons are no different. No two neurons experience the same biological experience. Like humans, each one has its own neuronal preality. In humans, the time in history in which people are born has a profound impact on their social environment. Similarly, for neurons, the time they are born determines their biological environment. For humans, the people we encounter and the people who influence us have a profound impact on our social environment. Likewise, neurons form billions and billions of synaptic connections, making them special and unique from one another.

The life of a neuron begins in the developing embryo, starting with the formation of the neural tube. The neural tube is the embryonic structure that will ultimately make up the brain and spinal cord. During early embryonic development, there are three layers of tissues that develop: the endoderm (the innermost layer of cells), mesoderm (the middle layer), and the ectoderm (the outermost layer). The nervous system develops from a region of the ectoderm called the neural plate. The edges of the neural plate then elevate to form neural folds, which curve upward and fuse to form a pipe-like structure called the neural tube. The holes at both ends of the neural tube (called neuropores) will close, and regions of the neural tube will thicken, creating the basis for the brain and spinal cord.

After this formation is complete, the neuroepithelial cells (the cells that make up the inner walls of the neural tube) will form the neurons, astrocytes, and oligodendrocytes of the spinal cord. This occurs when neuroepithelial cells differentiate into neuroblasts, precursors that commit themselves to becoming a neuron. These neuroblasts migrate to the lower part of the neural tube to form the mantle layer, which will later become the gray matter of the spinal cord. Other neuroepithelial cells differentiate into glioblasts or spongioblasts, cells that commit to becoming glial cells. These cells migrate to the intermediate and marginal zones, two distinct layers of the neural tube. Here, they become astroblasts and oligodendroblasts, which then become astrocytes and oligodendrocytes respectively.

The development of the brain starts at the rostral end of the neural tube, which contains three vesicles: the prosencephalon (which forms the forebrain), mesencephalon (which forms the midbrain), and rhombencephalon (which forms the hindbrain). The prosencephalon further develops into the telencephalon (which becomes the cerebrum) and diencephalon (which becomes the retina, hypothalamus, and thalamus), while the rhombencephalon splits into the metencephalon (which becomes the pons and cerebellum) and myelencephalon (which becomes the medulla oblongata), forming the five secondary brain vesicles. Neuroblasts are constantly generated in the subventricular zone, which is the region situated outside the walls of each of the five ventricles. These neuroblasts must migrate long distances to their final destinations in the brain, before they can specialize and form synaptic connections, which will make up the cerebral cortex. They migrate through the rostral migratory stream, a specialized migratory route found in the brain. This migratory route passes through the olfactory bulb, a rounded mass of tissue that contains the nerve cells involved in smell.

A set of specialized cells in the developing nervous system of all vertebrates, called Radial Glia, help facilitate this migratory route. Their long processes guide neuroblast migration from the ventricular zone to the mantle layer. Astrocytes are also integral to the process of prenatal neurogenesis in the brain in a number of different ways. Some astrocytes differentiate into neurons, and these neurons cover the chains of neuroblast that migrate across the rostral migratory stream in their journey to the olfactory bulb. This means that astrocytes are also precursors for neurons. Additionally, astrocytes also form the parallel blood vessels that carry the migrating neuroblasts to the olfactory bulb. After developing and differentiating in the olfactory bulb, the neuroblasts are integrated into the circuitry of the brain, where they form synaptic connections, creating the cerebral cortex and all the different regions of the brain.

In order for neurons to develop their own individual “personalities,” and, they must undergo this intricate process of neurogenesis. Most neurons are developed during prenatal neurogenesis, and are present throughout a person’s life, forming more and more synaptic connections as a person makes memories and learns new things. However, neurogenesis doesn’t stop after birth. In fact, new neurons are consistently produced in the olfactory epithelium throughout adulthood. More information about this can be found in the second part of this blog post: Neurogenesis Part II: Olfactory Neurogenesis. Neurogenesis is what enables the brain to function the way it functions, and it is important to understand the origins of neurons in order to further understand the roles they play in the nervous system.

Approved by Dr. Charles Pidgeon.

This blog is protected by US Copyright law and is owned by Dr. Charles Pidgeon.

References:

1. Mira, Helena, and Javier Morante. “Neurogenesis From Embryo to Adult – Lessons From Flies and Mice.” Frontiers, Frontiers, 8 June 2020, www.frontiersin.org/articles/10.3389/fcell.2020.00533/full.

2. Solomon, Nadia. “Development of the Central Nervous System.” Kenhub, Kenhub, 29 Oct. 2020, www.kenhub.com/en/library/anatomy/development-of-the-central-nervous-system.

3. Urbán, Noelia, and François Guillemot. “Neurogenesis in the Embryonic and Adult Brain: Same Regulators, Different Roles.” Frontiers in Cellular Neuroscience, Frontiers Media S.A., 27 Nov. 2014, www.ncbi.nlm.nih.gov/pmc/articles/PMC4245909/.