Characterization of a RFamide-positive subset of ganglionic cells in the hydrozoan planular nerve net

The complexity of the hydrozoan planular nervous system was examined. Using a whole-mount technique with indirect immunofluorescence, the spatial pattern of ganglionic cells showing RFamide-like immunoreactivity was visualized. RFamide antiserum bound a subset of ganglionic cells in the anterior and upper middle regions of the planula and a few ganglionic cells in the upper tail region. Labeled cells consisted of bipolar and multipolar neurons. Stained processes from these cells formed a three-dimensional nerve net that followed the contour of the mesoglea; such fibers were striking in terms of their large numbers, long lengths, and organization into distinct bundles. Labeled fibers were seen to contact other ganglionic cells, sensory cells, epithelio-muscle cells, the mesoglea, and the outside free surface. All stained cell bodies and fibers were found in the ectoderm. Using the same technique the reappearance of RFamide-positive ganglionic cells in epithelial tissue of chimeric grafts of planulae was observed. Interstitial cells capable of forming RFamide-positive ganglionic cells underwent extensive anterior-posterior migrations in the grafts, moved into the epithelial tissue, and differentiated into RFamide-positive ganglionic cells. Stained repopulated ganglionic cells always formed in the same position in the epithelial tissue as was observed in control planulae suggesting that the expression of RFamide-like substances may be position dependent in the planula.