Structural Alterations in Nerve Fibers Produced by Hypotonic and Hypertonic Solutions

Abstract

In sections of KMnO₄-fixed, developing mouse sciatic nerves, the central gap of mesaxons in myelinating fibers is normally closed with close apposition of the outside ∼20 A dense strata of the two ∼75 A Schwann cell membranes. The two combined outside strata make the intraperiod line bisecting each myelin lamella. The ∼150 A mesaxon is elaborated spirally around the axon in either a right hand or left hand spiral, and its inside (cytoplasmic) ∼20 A strata in apposition form the major dense lines of myelin. In hypotonic solutions the lamellae of adult frog sciatic myelinated fibers split apart along the outside membrane strata apposed at the intraperiod line throughout the spiral. Under similar conditions the inside (cytoplasmic) strata of the membranes, in apposition at the major dense lines, do not separate. The ∼150 A membranous structure resulting from this is called an "internal compound membrane."