The ventricular zone is a dense region of dividing cells in the developing brain that are the precursors cells of neurons or glia (i.e., the cells that will differentiate to become either neurons or glia). Note that neurons do not form by dividing themselves, but rather by division and differentiation of cells in the ventricular zone.

Ventricular Zone

There are many ways in which a newly formed neuron precisely identifies its destination. Cell migration occurs in an inside-out manner, where the newest cells form the outermost layer of the brain. Two mechanisms of cell migration are known as the following: 

-Radial glial cell migration describes a process in which cells migrate using the scaffolding provided by radial glial cells. Cells might migrate perpendicularly or rostrally to the radial glial cells. 

-Cell adhesion molecules (CAMs) are proteins located on the surface of cells that help migrating cells or developing axons connect with the correct structures of the developing nervous system.

Mechanisms of Cell Migration

On the intercellular level, there are two main mechanisms of neurodegeneration as a result of injury: 

-In retrograde degeneration, a neuron is injured close to the cell body, which initiates cell death of the neuron. If the entire injured neuron atrophies or dies, the neurons that were previously connected to the injured neuron may experience degeneration as well in a process known as transneuronal degeneration. 

-In anterograde degeneration, a neuron is injured farther from the cell body along its axon. The part of the axon that is severed from the neuron is degraded, but the axon may regrow, and new axonal sprouts can connect with postsynaptic cells by the help of CAMs and other factors. 

Mechanisms of Neural Regeneration

There are chemical and structural phenomena that contribute to the development of synapses. Two examples of chemical and structural phenomena are the following: 

-Growth cones are swellings at the tips of axons off which thin, threadlike fibers called filopodia extend. The filopodia of these growth cones can latch onto cellular adhesion molecules (CAMs) , and then contract move the neuron in the direction of the CAM (and, presumably, the location of the postsynaptic cell). Growth cones can also facilitate movement of dendrites, and thereby plasticity, in adults.  

-Chemoattractants and chemorepellents are chemical signals that either attract or repel axonal growth cones, respectively. Gradients of these signaling molecules affect the distribution of neurons during the development of the nervous system. 

Mechanisms of Synaptogensis

Apoptosis is a term used to describe programmed cell death. Cell death is an important aspect of development in both animals and plants, and apoptosis is a selective and organized way to kill cells that was likely established long ago in the course of evolution. 

Apoptosis

Abnormalities in the development of the nervous system that are influenced by both our genes and the environment. Developmental disorders commonly manifest as behavioral abnormalities.

Developmental Disorders

Cell-cell interactions describe the process in which one cell influences the differentiation of neighboring cells during the development of the nervous system. This process is a key reason why vertebrate brains are less hardwired and more susceptible to environmental cues and experience. In contrast, invertebrate development, such as in C. elegans, is more hardwired and relies less on cell-cell interactions, making their mitotic lineages strictly predictable and easier to trace.

Cell-Cell Interactions

In this case, development of the nervous system marks the six distinct stages of cellular development of neurons. The six distinct stages are the following; 

-Neurons form via neurogenesis, or the mitotic division of precursor cells to form neurons.

-The newly formed neurons migrate to their destinations in the developing brain via a process known as cell migration.

-The newly formed neurons become further specialized to execute their assigned tasks via cell differentiation.

-The specialized neurons network with other cells by forming synaptic connections in a process known as synaptogenesis.

-When newly formed neurons or synapses are deemed unnecessary, they are selectively removed (such cell death and synaptic rearrangement are important aspects of plasticity). 

Macalester College

Brandeis University

The brain is a highly complex organ that develops in an orderly fashion, as directed by genes and the environment. There are four main stages of brain development that describe how the brain changes in morphology throughout prenatal life. Within these four main stages of brain morphological development, there are six stages describing the development of the nervous system that enable the brain to change in morphology. (Page 194 has images of the morphological changes of the developing brain). 

Stages of Brain Development

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

The brain starts off as a single cell: The fertilized egg, or zygote. 

-12 hours after fertilization: The zygote begins dividing such that it forms a small ball of identical cells about 200 micrometers in diameter by three days after fertilization.

-A week after fertilization: The embryo develops three distinct tissue layers that will go on to become the tissues of the body: The ectoderm, the endoderm, and the mesoderm. The nervous system will develop from the ectoderm.

-18 days after fertilization: The ectoderm thickens to develop a structure called the neural plate.

-20 days after fertilization: Cell division at the neural plate proceeds unevenly such that a neural groove forms, essentially an indentation of the neural plate.

-22 days after fertilization: The neural tube forms when the ridges of the neural groove, known as the neural crest. fuse. 

-24 days after fertilization: Development of the nervous system quickens, as the "head" end of the neural tube begins to display the beginning features of the forebrain, midbrain, and hindbrain. The neural tube becomes the interior canal within the central nervous system (CNS) carrying cerebrospinal fluid. Development of the nervous system leads to further differentiation of brain structures. 

Brain Morphological Development

Development of the Nervous System

Contrary to the historical belief that all brain cells are present at birth, recent research indicates that neurogenesis, the formation of new neurons, can persist into adulthood. This finding highlights the brain's ongoing capacity for change and development throughout the lifespan.

Neurogenesis in Adulthood

Analyze the following scenario and identify the primary cellular process of brain development that has been disrupted. Explain the cascading effects this disruption would have on at least one subsequent developmental stage.

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