[The Schmidt-Lantermann incisures of the myelin sheath of Mauthner axons: site of longitudinal myelin growth].

The myelin sheath of the Mauthner axons in the spinal cord of young and adult fish belonging to the family Cyprinidae was examined by phase and electron microscopy. The sheath thickened considerably with age, the number of lamellae increasing from 200-230 in young fish (length 2--4 cm) to 250-300 in adult animals (length 20-25 cm). During this growth, the myelin sheath remained fairly compact and of optimal thickness for impulse propagation, the axon/fiber thickness ratio being 0.67-0.72 in young and 0.76-0.78 in adult fish. As already observed by previous investigators, the Mauthner axons lacked nodes of Ranvier. However, Schmidt-Lantermann's clefts (SLC) were present at irregular intervals. Within the SLC, parallel arranged osmiophilic bands of variable length and with a thickness of approx. 42-44 nm were observed to form a transitional zone between the cytoplasmic areas of the oligodendrocyte and the typical myelin structure. Between such adjacent 'D-bands', which thinned out to build up the major dense lines, an electron translucent area measuring approx. 25--26 nm in width contained a thin 'I-band' (8--10 nm) which was continuous with the intraperiod line. It is speculated that, in the Mauthner axon, the SLCs may be sites where freshly synthesized myelin is added to the lamellae already present, thus permitting their longitudinal growth.