The prebifurcation section of the axon of the rat spinal ganglion cell.

The long nonmyelinated portion of the unipolar process of spinal ganglion cells resembles in many instances the perikaryon and is characterized by the following structural pecularities: 1. The surface membrane displays numerous invaginations and evaginations, which interdigitate with folds of the investing satellite cells, resulting in a considerable increase of the area of intercellular contact. The intercellular gap frequently widens to intercellular cisternae. 2. The axolemma of the most distal part of the nonmyelinated portion is undercoated by dense material and thus resembles the "initial segment" of multipolar nerve cells. 3. The unipolar process of spinal ganglion cells shows a conspicuously high density of neurotubules. The neurotubules frequently collect into fascicles in the same manner as was described for the initial segment of multipolar nerve cells. 4. The nonmyelinated part as well as the first internodes of the myelinated part of the unipolar cell process contain a highly developed axoplasmic reticulum, many-partly huge-mitochondria, a striking number of dense bodies and clusters of ribosomes. The myelin sheath of the first internodes of the unipolar process is unusually thin in relation to their axon diameters. At successive internodes the thickness of the myelin sheath increases stepwise, the Schwann cell loops of the paranodal zones changing their appearence correspondingly. In the great number of ultrathin sections scanned in this study, not one synapse was found, neither in the unmyelinated initial segment nor in the soma.