Developmental regulation in the Schwann cell lineage.

Abstract

The main developmental stages in the Schwann cell lineage have now been identified (Fig. 1). Most Schwann cells originate in the neural crest and are found in mature nerves as two distinct cells, the non-myelinating and myelinating Schwann cells (Jessen and Mirsky, 1992; Le Douarin et al., 1991). The two Schwann cell variants are probably present in comparable numbers throughout the peripheral nervous system (PNS). The generation of these cells from neural crest cells, involves the formation of two main intermediates, the Schwann cell precursor, typically found in rat nerves at embryo day (E) 14 and 15 (mouse E12 and 13) and the immature Schwann cell, present from E17 (mouse E15) to around birth. At birth, the cells start to differentiate, first along the myelin pathway, with mature non-myelinating cells appearing later. The lineage therefore involves three main transition points, i.e. the transition of crest cells to precursors, of precursors to immature Schwann cells and lastly the final, and largely reversible, formation of the two mature Schwann cell types (Jessen and Mirsky, 1997; 1998; Mirsky and Jessen, 1996).