Occurrence of long non-myelinated axonal segments intercalated in myelinated, presumably sensory axons: electron microscopic observations in the dog atrial endocardium.
Electron microscopy of serial sections revealed the occurrence of long non-myelinated segments in myelinated, presumably sensory axons running in the left atrial endocardium of normal adult dogs. Four such non-myelinated segments were analysed in three myelinated axons. They varied from 20 to 150 microns in length, and differed from nodes of Ranvier in being invested by Schwann cells in the manner of unmyelinated nerve fibers. Short non-myelinated portions (20-25 microns long) were associated with a single Schwann cell, whereas the longest such segment (150 microns) had five. The non-myelinated axonal segments were non-varicose and similar in diameter (1.2-3.0 microns) to adjacent myelinated segments, which had myelin sheaths 6-25 lamellae thick. The cytoplasm of the non-myelinated axonal segments contained numerous neurofilaments and microtubules, some mitochondria and smooth endoplasmic reticulum. The short non-myelinated segments were enclosed by perineurium, whereas the long non-myelinated segment was devoid of perineurium at its mid-portion; instead fibroblast-like cells made a loose boundary around the axon at this level. The significance of these non-myelinated segments was discussed with special emphasis on the question of whether they result from focal degeneration of the myelin sheath (demyelination) or are generally present in the preterminal regions of some axons.
D000818:Animals; D001369:Axons / Q000648:ultrastructure*; D004285:Dogs / Q000033:anatomy & histology*; D004699:Endocardium / Q000294:innervation*; D005260:Female; D006325:Heart Atria; D008297:Male; D008854:Microscopy, Electron; D009186:Myelin Sheath / Q000648:ultrastructure*; D009475:Neurons, Afferent / Q000648:ultrastructure*