Role of Galactocerebroside in the Formation of Peripheral Myelin

Abstract

The formation of mature myelin lamellae in developing peripheral nerve is preceeded by a series of events during which axons are segregated by invading Schwann cells into those that will become myelinated and into others that will be ensheathed by non-myelinating Schwann cells. Initially columns of Schwann cells align along the outside of large nerve bundles (1). These Schwann cells migrate toward the interior of the bundle and rapidly proliferate in response to the axons (2, 3). Clusters of Schwann cells (called families) become organised around smaller groups of 5–20 neurites and acquire a common basal lamina. The larger axons become segregated, first at the periphery of the small bundle, and eventually into a separate furrow in the Schwann cell surface. Thus some Schwann cells adopt a 1:1 relationship with a single axon segment and become isolated from other Schwann cells and axons by a separate basal lamina and extracellular collagen fibrils (4). The Schwann cells that claim a 1:1 relationship with the larger axons are destined to form myelin sheaths around the axons they ensheath, while Schwann cells that retain a relationship to several smaller axons do not form myelin but ensheath each axon in a separate trough. The myelinating Schwann cell ceases division once it has established a 1:1 relationship with an axon segment and forms a mesaxon that elongates to spiral around the axon producing multiple membrane layers. As a final step, these myelin lamellae compact to form the mature myelin sheath (5).