Perineurial glia.

Although the ultrastructure of peripheral nerves has been known for nearly 200 years, the developmental origins and functional roles of all five main components of these specialized nervous system conduits are still poorly understood. One of these understudied nerve elements, the perineurium, is a component of the blood-nerve barrier and is essential for protecting axons and their associated Schwann cells from ionic flux, toxins, and infection. However, until recently, it was thought that this vital nerve tissue was derived from the mesoderm and simply served a structural/barrier function with no other influence on the development, maintenance, or regeneration of peripheral nerves. Recent work in zebrafish using in vivo time-lapse imaging, genetic manipulation, and laser axotomy is shedding light on the origin and roles of this previously ignored glial nerve component and is changing how we view development of the nervous system.