Use of a monoclonal antibody to classify neurons isolated from the head region of Hydra

A mouse monoclonal antibody (JD1) to Hydra attenuata using the peroxidase‐antiperoxidase (PAP) method revealed unipolar, bipolar, and multipolar sensory and ganglion cells in the head region of H. littoralis. Neurons isolated from macerated hypostomes and tentacles were classified according to the number of their cytoplasmic processes and the position of the cilium, when present, relative to the perikaryon. PAP‐stained sensory cells had an apical ciliary cone, whereas ganglion cells did not. Neurons with cytoplasmic processes longer than 50 μm stained faintly, whereas those with processes shorter than 50μm in length stained mainly dense brown. Unipolar neurons had an oval, crescent, round, or elliptic perikaryon with a single short axon. The perikaryal shape of bipolar neurons varied from round to tall triangular, short triangular, crescent, oval, or elliptic with two oppositely directed symmetric or asymmetric processes. Asymmetric processes were present in a bipolar sensory cell with a long apical cilium typical of gastrodermal sensory cells. One type of bipolar ganglion cell had a short perikaryal cilium. Another type had neurites longer than 50 μm. We found seven morphological variations of multipolar neurons, including one with an apical knob, two with a short perikaryal cilium, two with cytoplasmic loops near the perikaryon, one with perpendicular processes projecting from the major neurites, and one with a branched process longer than 50 μm opposite a tangled mass of neurites.