Structure, development, and maintenance of the nerve net of the body column in Hydra

The anatomy and developmental dynamics of the nerve net in the body column of Hydra viridissima were examined immunocytochemically with a monoclonal antibody (CC04) that recognizes an antigen in nerve cells and with an antiserum against vasopressin. CC04⁺ neuron cell bodies, their neurites, and vasopressin-like-immunoreactive (VLI⁺) neurites could be clearly visualized on whole-mount preparations. All neurites of the CC04⁺ neurons in the body column were VLI⁺. However, only half of the VLI⁺ neurites in the body column were CC04⁺. Immunocytochemical analysis of macerated preparations showed that half of the neurons in the gastric region of the body column were CC04⁺. These results suggest that most of the neurons in the gastric region are VLI⁺. The density of the VLI⁺ neurites was uniform along the entire length of the body column. The CC04⁺ neuron density in the gastric region remained constant at all stages of asexual development and during foot regeneration. After pulse-labeling with 5-bromo-2'-deoxyuridine (BrdU), CC04⁺ neurons with labeled nuclei appeared in the body column. We conclude that neuron density in the gastric region is maintained at a constant value by insertion of new neurons in parallel with continuous epithelial cell division.