A death gene is only expressed when a cell has "decided" to undergo apoptosis. The expression of death genes has important functions in apoptotic cascades. 

Death Genes

There are many different types of apoptotic cascades involving death genes. Our understanding of apoptosis is outlined by the following: 

-An influx of calcium ions (Ca+2) from the exterior of the cell, as well as interior stores of calcium, enters the mitochondria.

-The mitochondria releases a pro-apoptotic protein called Diablo into the cell. This may be blocked by the Bcl-2 anti-apoptotic proteins. 

-Diablo binds to and inhibits a family of proteins known as the inhibitors of apoptosis proteins (IAPs) that normally inhibit proteins called caspases. Caspases are types of enzymes that degrade proteins and nuclear DNA.

-A cascade of caspase enzymes are activated, leading to destruction of proteins and nuclear DNA needed for cell survival. 

Apoptotic Cascade

How does a cell "decide" to enter apoptosis by expressing death genes and activating the apoptotic cascade? A cell's competition for resources with other cells is an important apoptosis determinant. These limited resources may include the following: 

-Synaptic targets: There are only a limited number of synaptic targets (e.g., muscles). 

-Neurotrophic factors: Neurotrophic factors are chemicals produced by synaptic targets that enables the neuron to survive. There are a limited number of neurotrophic factors.

-Experience: Experience may either continuously stimulate a synapse to strengthen it, or it may never use the synapse and discard of it. This is because synapses are energetically costly to maintain throughout the life span.

Apoptosis Determinants

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. 

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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). 

Development of the Nervous System

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

Behavioral Neuroscience 8th Edition

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

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.

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