Nerve net differentiation in medusa development of <i>Podocoryne carnea</i>

Abstract

The phylum Cnidaria is the most primitive phylum with a well-developed nervous system. Planula larvae and polyps display a diffuse nerve net (plexus), which is densest in the polyp hypostome. In contrast, the nervous system of the medusa is more complexly structured and reflects the anatomical needs of a well differentiated non-sessile animal. We analyzed the nervous system of two life stages of the hydrozoan Podocoryne carnea. Nerve nets of both polyps and developing medusae were examined in whole mounts and gelatin sections by using antibodies and vital staining with reduced Methylene Blue. In the polyp, both RFamide-positive nerve cells and tyrosine-tubulin containing nerve cells form an ectodermal plexus. However, apical neuronal concentration is stressed by a particular nerve ring formed by tyrosine-tubulin positive nerve cells in the hypostome above the tentacle zone. This apical nerve ring is not detected with antisera against RFamide. In developing medusa buds, the earliest detected RFamide positive nerve cells occur at stage 4 at the location of the prospective ring canal. The nerve net of the developing medusa is fully differentiated at bud stage 8. Similar results were obtained with the anti tyrosine-tubulin antibody. Strikingly, two different nerve nets were discovered which connect the medusa bud with the plexus of the gonozoid, suggesting neuronal control by the polyp during medusa bud development. Vital staining with reduced Methylene Blue (Unna's) identified not only nerve cells at the ring canal but also bipolar cells within the radial canal. These cells may fulfill sensory functions.