Biology:Telencephalization

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Telencephalization is the process by which the brain, during very early stages of development, begins to be regulated by the more complex processes of the forebrain (mainly the telencephalon) rather than the more basic, or primitive neural centers as is observed in early development of the neural tube [1]. This intricate phenomenon is initiated by the asymmetrical division of progenitor cells, which not only gives rise to neuronal cells of the central nervous system, but also to glial cells [2].

Development of cerebral cortex

On the twenty-first day after conception for the human embryo, the neural tube develops, which ultimately becomes the central nervous system (brain and spinal cord). Throughout a period of a few short days, the neural tube becomes closed off and three chambers develop towards the anterior portion of the neural tube [2]. These chambers are known as the ventricles, and the tissue that surrounds them constitutes the three major sections of the brain: the forebrain, midbrain, and hindbrain. The telencephalon, the embryonic structure from which the mature cerebrum develops, resides in the forebrain.

In development from the neural tube to the brain, which weighs approximately three pounds and contains billions of neurons, the cerebral cortex forms from the inside out. This means that the first progenitor cells to be produced travel a little distance and makes the first, or deepest, layer. The following cells that are produced must pass through each layer that was made before it, until all six layers of the cerebral cortex are fully realized. [2]

Symmetrical and asymmetrical division of the ventricular zone

Progenitor cells inhabit an area outside of the early neural tube called the ventricular zone. Progenitor cells divide during a phase called symmetrical division. These new cells can then move to an area termed the subventricular zone, where they continue to divide. After a period of approximately seven weeks or so for human development, the progenitor cells trigger a new form of division called asymmetrical cell division [2]. Like the name suggests, this division results in not only a duplicate progenitor cell, but also a neuronal precursor cell [2]. Radial glia, which play an integral role in the migration of neuronal precursor cells during cerebral cortex development, are the first brain cells to be produced through asymmetrical division [2]. The cell bodies of radial glia stay in close proximity to the wall of the neural tube within the ventricular and subventricular zones, but they extend processes outward and connect to the pia mater, the innermost layer of the meninges (a group of protective tissues that surround the brain). As the cerebral cortex becomes thicker, the fibers of radial glia extend and preserve their connections with the pia mater [2]. The job of these radial glia are to provide guidance for neurons migrating throughout the developing brain.

Asymmetrical division goes on for approximately three months during human development [2]. An estimate of how many neurons the human brain possesses comes out to be about 100 billion neurons, so that means on any given day during this three-month period of asymmetrical division, there are around one billion neurons traveling along these radial glia fibers throughout the cerebral cortex [2]. For the first layer of the cerebral cortex, the migration of the neurons along the radial glia is extremely short and takes around a single day. For the neurons that reside in the sixth, or outermost layer, their migration takes almost two weeks. Cortical development ends when programmed cell death (apoptosis) is triggered in progenitor cells via activation of “killer genes”.

Why And how the brain gets its size

Genetic Duplication could be an implicit factor in how the human cerebral cortexes are more advanced than early vertebrates [2]. Geneticists have discovered that genes can sometimes duplicate themselves, and if this duplication occurs in gametes (ova or sperm), then this new gene can be passed on to the offspring of an organism [2]. This offspring will have the ‘old’ gene to perform the functions necessary for the survival of that species, but with this extra gene, if it becomes mutated, can be utilized to perform higher-level processes (if this gene aids in the survival of this species).

Rakic (2011) posed the theory on how an extremely large brain of a human could arise. He proposed that the division of progenitor cells during symmetrical division increased the ventricular zone of the brain [3]. A delay in the termination of symmetrical and asymmetrical divisions could result in the larger size of the human brain when compared to other species [2]. This delay could be caused by mutations of the gene controlling the timing of brain development [2]. Also, the convoluted appearance of the brain (sulci and gyri) increase the surface area of the cerebral cortex, which allows for more complex circuitry of interconnected neurons within the brain [2].

The ventricular zone produces more neurons than are needed. Around 50 percent of neurons do not find appropriate synaptic connections with other postsynaptic cells and don’t receiving a life-sustaining signal that is received from the postsynaptic cell, so these neurons die by apoptosis [2]. This process of producing more neurons than is needed might be seen as wasteful, but an evolutionary process deemed it necessary to produce this surplus of neurons, rather than producing just the right amount of each type of neuron [2].

References




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