A summary model of myelination in the absence of internodal and paranodal adhesion. Schematic representation (longitudinal view) of the forming myelin unit in the indicated genotypes. In WT mice, the inner myelin membrane along the internode (blue), as well as the flanking paranodal loops (yellow), are contacting the axon (dark line represents adhesion of the inner membrane to the axon). In the absence of internodal adhesion (Cadm4^−/−), unregulated lateral growth of the inner membrane is prevented by the paranodes. In the absence of paranodal adhesion (Caspr^−/−), internodal adhesion generally limits lateral membrane overgrowth (limited extension of the myelin beneath the paranodal loops is marked by small arrows). The absence of both internodal and paranodal adhesion results in longitudinal growth of the myelin either over or under an adjacent forming myelin segment (arrows), resulting in the formation of multimyelin profiles.