Mechanics of Myelin Sheath Expansion: Changes in Mesaxons and Schwann Cell Cytoplasm Upon Sheath Expansion

Abstract

This chapter focuses on certain aspects of the geometry of the myelin sheath, particularly in regard to the rearrangement of the myelin lamellae upon changes in \the caliber of the axis cylinder. The process deserves attention because it may provide clues to the mechanism that controls the amount of myelin formed per fiber. The chapter emphasizes on the pertinent data from earlier studies and also on recent studies on changes in mesaxon and Schwann cell cytoplasm upon sheath expansion. Fragmentation of Schwann cell cytoplasm into islands is observed in stumps of nerves and in degenerating fibers by several investigators and was interpreted as regenerating axons as pseudopods of proliferating Schwann cells or as Bungner's bands. In conclusion, the data present in the chapter indicate that stretching and expansion of the myelin sheath is transmitted onto the outer mesaxon and the plasma membranes of the Schwann cells. This process provides a mechanism by which the need for additional myelin formation may be signaled to the Schwann cell.