Myelination: all about Rac 'n' roll

Abstract

During the development of the peripheral nervous system, Schwann cells select individual axons from a nerve bundle and establish a one-to-one relationship through a process termed "radial sorting". Recent findings identify the Rho family GTPase Rac1 as the downstream effector molecule responsible for process extension and lamellipodia formation in Schwann cells, allowing for proper radial sorting and myelination. These findings begin to shed light on our understanding of the distinct and yet essential molecular mechanisms involved in developmental processes preceding myelination.

Figure 1.

Illustration of Schwann cell (SC) development in the mouse sciatic nerve using electron microscopy. During peripheral nerve development, Schwann cells proliferate, migrate, and ensheath axon (Ax) bundles (A). Schwann cells organize a basal lamina (BL, outlined in blue), which completely surrounds each individual cell and the associated axon(s). Upon receiving the appropriate signals from the local environment, Schwann cells will sort through the axon bundle, isolate an individual axon, and then initiate myelination (B). Bottom panels are schematic representations of the electron micrographs in the top panels, and represent radial sorting, initiation, and myelination of a single axon.