Changes

*Myelination in the PNS is achieved by the Schwann cell, a derivative of neural crest cells, which bud off from the neuroepithelium at a very early stage of neurogenesis.  

*Myelination in the PNS is achieved by the Schwann cell, a derivative of neural crest cells, which bud off from the neuroepithelium at a very early stage of neurogenesis.  

*During development, Schwann cells engage many small axons.  

*During development, Schwann cells engage many small axons.  

*As axonal diameter increases, Schwann cells eventually relate with only a single axon c.f [[Microscopic Anatomy of the CNS#Oligodendrocytes|oligodendrocytes]].  

*As axonal diameter increases, Schwann cells eventually relate with only a single axon c.f [[Neurones - Anatomy & Physiology#Oligodendrocytes|oligodendrocytes]].  

**This single axon is enveloped in a trough by the Schwann cell processes that engulf it.  

**This single axon is enveloped in a trough by the Schwann cell processes that engulf it.  

**As the processes come together, an inner mesaxon is formed.  

**As the processes come together, an inner mesaxon is formed.  

* During development of the nervous system, Schwann cells interact with many small axons.  

* During development of the nervous system, Schwann cells interact with many small axons.  

** Schwann cells eventually relate to only one axon, as axonal diameter increases with maturation of the system.

** Schwann cells eventually relate to only one axon, as axonal diameter increases with maturation of the system.

***[[Microscopic Anatomy of the CNS#Oligodendrocytes|Oligodendrocytes]], that myelinate the CNS, differ from Schwann cells in that they interact with many axons.  

***[[Neurones - Anatomy & Physiology#Oligodendrocytes|Oligodendrocytes]], that myelinate the CNS, differ from Schwann cells in that they interact with many axons.  

====The Process of Myelination====

====The Process of Myelination====