In the past couple of weeks, we have learned about neurons and the neurotransmitters they use to function. But how exactly do these cells develop? To understand this, we must start at the very beginning.
The embryo begins with two layers: the epiblast and hypoblast. The epiblast cells differentiate into three primary stem cell lines, one of which is the neural stem cells. These cells, also known as neural progenitor cells, make up the brain and central nervous system. Neural stem cells, along with other epiblast cells, pass through the primitive streak, a slit like opening in the upper layers of the embryo, and move to the rostral end of the embryo, which will eventually develop into the head. These migrating cells will form endodermal, mesodermal and ectodermal. The ectodermal cells are further divided into epidermal and neuroectodermal cells.
The next major step is the formation of the neural tube, three weeks into pregnancy. By this time, the neural progenitor cells are differentiated and located on the neural plate. Two ridges form on the plate, rise and fuse to form the neural tube. The hollow is lined by neural progenitor cells and is known as the ventricular zone (as this is where the ventricles will form).
Over the course of the next month, brain vesicles from: prosencephalon, precursor of forebrain; mesencephalon, precursor of midbrain; and rhombencephalon, precursor of hindbrain. Later, the prosencephalon divides into telencephalon and diencephalon. The rhombencephalon divides into metencephalon and myelencephalon.
At the ninth week of pregnancy, the brain begins to develop sulci and gyri. These grooves are not randomly placed but are positioned at specific brain regions. The first fissure separates the two hemispheres of the brain and is known as longitudinal fissures. The primary sulci, such as cingulate and parieto-occipital, form later and lay the roots for secondary sulci.
The number of neural progenitor cells that have migrated to the rostral end of the embryo are too small to produce the number of neurons the brain needs so it goes through a period of rapid division in order to augment the size of the neural stem pool. For a while, it goes through symmetrical cell division, in which identical neural progenitor cells are produced. Later, asymmetric cell division begins. In this one cell is a neural progenitor and the other is a neuron. In this way, neuron production is started. The number of cells going through asymmetric cell division is originally low but gradually increases by the end of cortical neurogenesis. Structures such as the cerebellum, midbrain and spinal cord form at this time. Once the neurons are produced, they move away from the ventricular zone and form a six layered neocortical mantel. The earliest neurons use somal translocation to migrate. In this process the neuron extends beyond the VZ and attaches to the pial surface (the outer surface of the developing brain). With the help of a radial glial cell, the neuron attaches itself to the pial surface.
The later neurons use radial glial cells as scaffolding to moe themselves to their final location. Neurons that will eventually become interneurons travel using tangential migration. They use signaling molecules to guide them into the correct position. This migration creates a 6 layered structure, with the earliest neurons forming the deepest layers.
Once positioned in this manner, they differentiate by producing proteins and extending dendrites and axons. In this manner, the two cells that started the process slowly develop into the brain we see today.
Blog by : Lasya Kambhampati
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