Sea anemones feed by discharging nematocysts into their prey, but the pathway for control of nematocyst discharge is unknown. The purpose of this study was to investigate the ultrastructural evidence of neuro‐nematocyte synapses and to determine the types of synaptic vesicles present at different kinds of nematocyst‐containing cells. The tip and middle of tentacles from small specimens of Aiptasia pallida were prepared for electron microscopy and serial micrographs were examined. We found clear vesicles in synapses on mastigophore‐containing nematocytes and dense‐cored vesicles in synapses on basitrich‐containing nematocytes and on one cnidoblast with a developing nematocyst. In addition, we found reciprocal neuro‐neuronal and sequential neuro‐neuro‐nematocyte synapses in which dense‐cored vesicles were present. It was concluded that : (1) neuro‐nematocyte synapses are present in sea anemones, (2) different kinds of synaptic vesicles are present at cells containing different types of nematocysts, (3) synapses are present on cnidoblasts before the developing nematocyst can be identified and these synapses may have a trophic influence on nematocyst differentiation, and (4) both reciprocal and sequential synapses are present at the nematocyte, suggesting a complex pathway for neural control of nematocyst discharge. J. Morphol. 238:53–62, 1998. © 1998 Wiley‐Liss, Inc.
Using transmission electron microscopy of serially sectioned tentacles from the sea anemone Aiptasia pallida, we located and characterized two types of neuro-spirocyte synapses. Clear vesicles were observed at 10 synapses and dense-cored vesicles at five synapses. The diameters of vesicles at each neuro-spirocyte synapse were averaged; clear vesicles ranged from 49-89 nm in diameter, whereas the dense-cored vesicles ranged from 97-120 nm in diameter. One sequential pair of synapses included a neuro-spirocyte synapse with clear vesicles (81 nm) and a neuro-neuronal synapse with dense-cored vesicles (168 nm). A second synapse on the same cell had dense-cored vesicles (103 nm). An Antho-RFamide-labeled ganglion cell and three different neurites were observed adjacent to spirocytes, but no neuro-spirocyte synapses were present. Many of the spirocytes also were immunoreactive to Antho-RFamide. The presence of sequential neuro-neuro-spirocyte synapses suggests that synaptic modulation may be involved in the neural control of spirocyst discharge. The occurrence of either dense-cored or clear vesicles at neuro-spirocyte synapses suggests that at least two types of neurotransmitter substances control the discharge of spirocysts in sea anemones.
Using transmission electron microscopy of serially sectioned tentacles from the sea anemone Aiptasia pallida, we located and characterized two types of neuro‐spirocyte synapses. Clear vesicles were observed at 10 synapses and dense‐cored vesicles at five synapses. The diameters of vesicles at each neuro‐spirocyte synapse were averaged; clear vesicles ranged from 49–89 nm in diameter, whereas the dense‐cored vesicles ranged from 97–120 nm in diameter. One sequential pair of synapses included a neuro‐spirocyte synapse with clear vesicles (81 nm) and a neuro‐neuronal synapse with dense‐cored vesicles (168 nm). A second synapse on the same cell had dense‐cored vesicles (103 nm). An Antho‐RFamide‐labeled ganglion cell and three different neurites were observed adjacent to spirocytes, but no neuro‐spirocyte synapses were present. Many of the spirocytes also were immunoreactive to Antho‐RFamide. The presence of sequential neuro‐neuro‐spirocyte synapses suggests that synaptic modulation may be involved in the neural control of spirocyst discharge. The occurrence of either dense‐cored or clear vesicles at neuro‐spirocyte synapses suggests that at least two types of neurotransmitter substances control the discharge of spirocysts in sea anemones. J. Morphol. 241:165–173, 1999. © 1999 Wiley‐Liss, Inc.
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