Morphological Characterization of Glial Cells Surrounding Cajal's Initial Glomerulus of the Dorsal Root Ganglion Neurons Revealed Myelinating Schwann Cell Production

Satellite glial cells (SGCs) cover the following two areas of a large-diameter dorsal root ganglion (DRG) neuron: neuronal soma and initial region of the neuronal projection, namely Cajal's initial glomerulus (IG). The morphological and functional features of SGCs covering the neuronal soma have been studied extensively. However, those of SGCs surrounding the IG [periaxonal SGCs (aSGCs)] are poorly understood. In the present study, we aimed to investigate the histological characteristics of aSGCs in adult rats. The IG's length was approximately 120 μm, where approximately 10 aSGCs surrounded the IG. The SGC markers, including Kca2.3, Kir4.1, and FABP7, were obviously expressed in aSGCs located in the proximal and middle parts of the IG. Contrarily, the signal intensity of these cell markers decreased in aSGCs surrounding the distal part of the IG, and these cells expressed the promyelinating Schwann cell marker Oct-6. Electron microscopy revealed aSGCs winding their thin processes around the IG. Additionally, the 5-bromo-2'-deoxyuridine incorporation study demonstrated that these glial cells matured into myelinating Schwann cells. Oct-6-positive aSGCs were also found in the IG in the human DRG. Our results collectively imply that the IG is involved in the differentiation and maturation of Schwann cells, where aSGCs gradually change their ultrastructural characteristics and immunoreactivity to differentiate and mature into myelinating Schwann cells through the promyelinating stage, and that this differentiation and maturation system may be conserved among mammals.