User:Mkelliott/sandbox

Editing the first paragraph/entire intro
My contributions to the intro are italicized for easier editing; the current paragraph structure is also my suggestion

Neurogenesis is the production of new nervous system cells, including neurons and glia, from neural stem cells. In embryonic development, the nervous system develops from the ectoderm, whose cells differentiate into epithelium (skin) and precursors of the nervous system. Several types of stem cells are involved in neurogenesis, including neuroepithelial (NEP) stem cells, radial glial cells (RGCs), basal progenitors (BPs), subventricular zone astrocytes, and subgranular zone radial astrocytes, among others. Neurogenesis is most abundant during embryonic development, but recent evidence suggests that to some extent it continues throughout adult life.

Neurogenesis is characterized by several key processes, including the proliferation of the neural progenitor cells, the differentiation into the vast diversity of neurons in the adult brain, and cell migration to the neurons' final location. The process of neurogenesis then involves a final cell division of the parent neural stem cell, which produces daughter neurons that will never divide again. Most neurons of the human central nervous system live the lifetime of the individual. Many molecular and genetic factors affect neurogenesis, from promoting proliferation to affecting differentiation, and even regulating cell death, or apoptosis. Several of these key factors appear in a broad range of species, some crossing the divide between vertebrates and invertebrates, although they may play different roles in different groups.

Stages of neurogenesis
The process of neurogenesis takes several stages to be fully complete. The ectodermal cells that will eventually become neurons must multiply, enough to cause a thickening of the ectoderm into what is known as the neural plate. After this proliferation occurs, the neural plate invaginates, or forms a valley, called the neural groove. The neural groove then seals, becoming the neural tube, and some neural precursors remain above and to either side of the tube, forming the neural crest. This overall process is called primary neurulation.

A property of stem cells is that they are pluripotent, meaning they carry the potential for many different fates, so in order to become neurons, the cells must receive a signal telling them as much. This signaling is accomplished through a variety of signaling molecules, discussed below, and the process of becoming a particular cell type from a stem cell is known as differentiation. A topic of scientific debate at the moment is whether the "default" fate of ectodermal cells is skin or neural tissue. From the default, cells must receive an additional signal telling them to become the non-default option. Evidence has been presented for both sides of the debate: neurons as the default and epithelium as the default.

Once the cells have developed into neurons, they must follow another set of chemical signals to migrate to their final location in the organism. The cells of the central nervous system, which includes the brain and spinal cord, come from neuroepithelial stem cells in the ventricular zone of the neural tube. These cells divide rapidly and symmetrically, such that they create two of the same offspring, in a process known as proliferation. Neuroepithelial cells transform into radial glia, which are stem cells in their own right, not the supportive glial cells of the adult nervous system despite what the name may suggest. Radial glia are involved in migration; their long processes help other neurons find their correct destinations in the embryo.

Genetic and molecular factors in vertebrate neurogenesis
Several genes have been widely implicated in neurogenesis across many species, suggesting that they have been conserved through evolutionary time. This is by no means a complete list, but just a few important molecules involved.
 * Notch: Notch participates in several aspects of neurogenesis, including the initial multiplication of neural progenitors and the differentiation of these cells into their final forms. Notch is also important in choosing between different options for a cell's fate, which in neurogenesis involves choosing between becoming a skin cell or a nervous system cell, since both are derived from the ectoderm.
 * Delta: Delta works with Notch in determining cell fate in neurogenesis; the delta-notch signaling pathway allows one cell in a proneural cluster to become a neuron while inhibiting the surrounding cells.
 * Pax6: Pax6 is a transcription factor involved in both embryonic and adult neurogenesis. In the embyro, Pax6 is expressed in the ventricular zone, along the inner surface of the neural tube, and there is evidence for its involvement in both proliferation and differentiation.
 * Sonic Hedgehog: Sonic Hedgehog, or Shh, is involved in development of the forebrain, retina, and spinal cord and is secreted by the floor plate of the neural tube, and is involved in neural tube formation and development.
 * Bone morphogenetic proteins (BMPs): research suggests that BMPs inhibit neurogenesis, and thus for neurogenesis to take place other factors must inhibit BMP signaling.
 * Neuro-D: Neuro-D is a basic helix-loop-helix transcription factor involved in vertebrate nervous cell differentiation, for many different cell types and areas of the developing embryo. Neuro-D has also been implicated in development of nervous system cells in the retina.