Development of Nerve Cells in Hydrozoan Planulae: II. Examination of Sensory Cell Differentiation Using Electron Microscopy and Immunocytochemistry

Abstract

The development of sensory cells in hydrozoan planulae of Halocordyle disticha was examined using transmission electron microscopy and light immunocytochemistry. Sensory cells arise in the anterior end of the planula in the ectoderm at 24 hours postfertilization. These cells extend from the free surface of the planula to a ganglionic plexus located just above the mesoglea. The cytomorphosis of sensory cells is characterized by the appearance of a single apical cilium, microtubules, mitochondria, one to several Golgi complexes, electron-dense droplets, dense-cored vesicles, and neurites. The basal end of the sensory cell forms one to several processes (neurites) which contribute to the ganglionic plexus. Apical specialization of the sensory cell precedes basal differentiation. Sensory cells increase in number as planulae develop and many become organized into clusters of 3-6 cells distributed along the entire length of the planula. Within some of these clusters, two morphological types of sensory cells are discernible: light sensory cells and dark sensory cells. Light sensory cells outnumber the dark sensory cells and are the first sensory cells to appear at 24 hours postfertilization. Use of immunocytochemical techniques on wholemounts and paraffin-embedded sections of planulae demonstrates the presence of FMRFamide-like immunoactivity associated with some of the sensory cells. Such FMRFamide-like expression is first detected at 24 hours postfertilization in the anterior ectoderm of the planula. By 96 hours postfertilization, the spatial distribution of FMRFamide-like positive sensory cells is such that many are found in clusters along the entire anterior-posterior axis of the planula. There is, however, an abundance of FMRFamide-like positive cells in the anterior region of the planula just prior to metamorphosis. The apices and cell bodies of the sensory cells exhibit intense immunostaining, whereas the basal processes stain faintly. This study identifies neuropeptide-like substances in nerve cells of cnidarian larvae and demonstrates a developmental correlation between the time of appearance of the synthetic machinery of sensory cells with the pattern of expression of the FMRFamide-like peptide.