The perikaryal surface of spinal ganglion neurons: differences between domains in contact with satellite cells and in contact with the extracellular matrix

The perikaryal surface of spinal ganglion neurons undergoes dynamic changes throughout life. In particular, numerous slender projections develop and r etract continuously from this surface. We showed previously that the outgro wth of these projections, while an intrinsic property of spinal ganglion ne urons, is also influenced by the surrounding microenvironment. Since the la tter consists of satellite cells and the extracellular matrix, we sought to determine the relative contributions of each of these components to the ou tgrowth of perikaryal projections. To this end, we took advantage of a litt le known characteristic of the satellite cell sheaths: in the rabbit, these sheaths can exhibit gaps that leave the nerve cell body surface directly e xposed to the extracellular matrix. We compared the surface domains covered by satellite cells with those in direct contact with the extracellular mat rix. We found that the perikaryal projections are abundant in the former do mains but are absent in the latter. We also found that the perineuronal ext racellular matrix of rabbit spinal ganglia contains laminin and fibronectin , two glycoproteins that have been reported to promote the growth of axonal processes from sensory ganglion neurons. Laminin and fibronectin were also present at the level of the gaps in the satellite cell sheath. These resul ts: (1) provide additional evidence that environmental factors influence th e outgrowth of perikaryal projections from spinal ganglion neurons; (2) sug gest that satellite cells permit the outgrowth of these projections; (3) su ggest that in the spinal ganglia of adult rabbits the perineuronal extracel lular matrix is not in itself able to promote the outgrowth of these projec tions. This study provides a further example of the influence that supporti ng neuroglial cells have on sensory ganglion neurons.