Schwann cells

Schwann cells (SC) named after physiologist Theodor Schwann

: neurolemmocytes that are the principle glia throughout the entire peripheral nervous system (PNS).

The PNS includes all nerves going out to muscles as well as sensory nerves coming from the muscles back to the spinal cord. Schwann cells are a type of “support” cell in the PNS.

There are two types of Schwann cell, myelinating and nonmyelinating. Myelinating Schwann cells wrap around axons of motor and sensory neurons to form the myelin sheath. The sheath is not continuous. Individual myelinating Schwann cells cover about 100 micrometres of an axon—equating to approximately 10,000 Schwann cells along a 1 metre length of the axon—which can be up to a metre or more in length. Myelin is necessary for sending appropriate electrical signals throughout the nervous system.

The gaps between adjacent Schwann cells are called nodes of Ranvier. The vertebrate nervous system relies on the myelin sheath for insulation and as a method of decreasing membrane capacitance in the axon.

 
Schwann cells are not stem cells, they are adult cells and can only be Schwann cells.

Schwann cells are absolutely essential for regeneration in the injured PNS.

Netrin:

Secreted proteins that regulate axon guidance and neuronal migration.

Netrins are genetically conserved across nematode worms, fruit flies, frogs, mice, and humans. 

Netrins are chemotropic; a growing axon will either move towards or away from a higher concentration of netrin

The detailed mechanism is not understood:  netrin attraction is mediated through UNC-40/DCC cell surface receptors and repulsion is mediated through UNC-5 receptors.

 Netrins also act as growth factors, encouraging cell growth activities in target cells. Mice deficient in netrin fail to form the hippocampal commissure or the corpus callosum.

Semaphorin 3

The Semaphorins are the largest family of axon guidance. Semaphorins are divided into 8 classes (classes 3-7 found in vertebrates). Class 3 Semaphorins are secreted.it is a protein that in humans is encoded by the SEMA3A gene.

They are characterized structurally by a conserved 400 amino acid sema domain. 

This secreted protein can function as either a chemorepulsive agent, inhibiting axonal outgrowth, or as a chemoattractive agent, stimulating the growth of apical dendrites. In both cases, the protein is vital for normal neuronal pattern development.

It is secreted by surrounding tissues to guide migrating cells and axons in the developing nervous system of an organism which is critical for the precise formation of neurons and vasculature.

Sema3a repels axons from the dorsal root ganglia, facial nerves, vagal nerves, olfactory-sensory, cortical nerves, hippocampal nerves and cerebellar nerves.

Increased expression of this protein is associated with schizophrenia and is seen in a variety of human tumor cell lines and aberrant release of this protein is associated with the progression of Alzheimer's disease.

http://www.nature.com/nrm/journal/v8/n4/fig_tab/nrm2142_F7.html

http://www.sciencemag.org/site/feature/data/pharmacia/2001/HiramotoF2.xhtml