A stem cell will go through five phases of development during its plight to become an interneuron in the external cuneate nucleus (ECN). The interneuron in the ECN is part of the perception circuitry for the upper extremity stretch reflex that relays sensory information to the cerebral cortex. Its axon projects to the contralateral neurons in the ventroposteriolateral (VPL) thalamus via the internal arcuate fibers and medial lemniscus.
In order for a stem cell to become an interneuron in the ECN, it first undergoes determination. By the 18th day of gestation, neural plate formation is occurring, and a stem cell from the mesoderm receives a chemical signal. The positional sense of the signal impacts the fate of the stem cell. For the ECN, the signal occurs rostrally. Once the signal is received by the stem cell, cell determination is irreversible.
Next, massive cell proliferation occurs in the ventricular zone. Many genetic and environmental factors influence how rapidly the neuroblast progresses through the cell cycle.
Progression through the proliferation phase is regulated by the cell number. As the neuroblast proliferates, it bounces up and down within the ventricular zone. At the last division, the neuroblast enters the subventricular zone.
Once the neuroblast reaches the subventricular zone, it is in the migration phase and is now called a neuron. Further differentiation also occurs during the last division as the parent cell passes on more genetic information to the neuroblast. Then the neuron must migrate to the ECN in the mantle layer. Cell migration occurs through three mechanisms. (1) A cell can move along a scaffold formed by radial glial cells. The neuron latches on and rolls along the glia to the ECN. Communication between the glia and neuron helps to guide the neuron down the right path. (2) Neurons move down a...