The development of the nervous system of Aurelia aurita (Scyphozoa, Coelenterata).

摘要

Aurelia aurita passes through several life cycle stages during its development. Sexual reproduction occurs in the adult jellyfish and results in the free-swimming planula, which develops into the sessile scyphistoma (polyp). The polyp, which reproduces asexually by budding, develops into a strobila which, also asexually, produces free-swimming ephyrae (the initial medusoid stage) through transverse fission.;In the planula, the nervous system consists of ectodermal sensory cells and neurons and their fibres. Anti-FMRFamide antibodies label both sensory and neuronal cells in the anterior region of the planula; the neuronal processes are mostly arranged longitudinally along the anterior/posterior axis, and a few fibres run transversely. Labelled neurons do not appear to make contact with one another in the early, i.e. just released, planula and do not have the appearance of a nerve net until the late planula, i.e. just prior to metamorphosis.;Metamorphosis of the planula can be induced by exogenously applied thyroxine (10−7M) and iodine (10−7M) and, to a lesser degree, by retinoic acid (10−7M). MgCl2 (0.13M) and FMRFamide (10−7M), on the other hand, inhibit or reduce, respectively, the induction of metamorphosis. Less than 50% of planulae cut into anterior and posterior halves are undergoing metamorphosis after 10 days, and thyroxine fails to enhance the rate of metamorphosis in those larvae.;In the scyphistoma, the nervous system consists of sensory cells and neurons in the ectoderm and the endoderm. The somata of cells labelled with anti-FMRFamide are located mostly in the oral disc and the tentacles, where they, together with their processes, have the appearance of a nerve net. Nerve fibres are also found on the four muscle bands that extend the length of the scyphistoma from the pedal to the oral disc.;In the developing and adult jellyfish, conventional techniques such as methylene blue staining distinguish between a diffuse nerve net and a giant fibre system. Neuronal subsets are identified by immunohistochemical techniques such as labelling with anti-FMRFamide and a monoclonal antibody generated with ephyral tissue as the immunogen. Anti-FMRFamide labels a subset of neurons of the diffuse nerve net, whereas the monoclonal antibody labels a separate subset of neurons, some of which belong to the giant fibre system, while others do not, and none of which co-label with the anti-FMRFamide antibodies. In contrast to the subset of FMRFamide-positive neurons, which has the appearance of a nerve net in all jellyfish stages, the number of neurons labelled with the monoclonal antibody increases during the development of the jellyfish from a few scattered neurons in the ephyra to an interconnected population of neurons forming a nerve net in the adult. Whereas elements of the diffuse nerve net and the FMRFamide label are present in all life cycle stages, the giant fibre system, which innervates the swimming muscles, and the monoclonal antibody label only occur in the jellyfish stages.;Rhodamine B, which has been used as an indicator of neuronal activity in other phyla, was found to also stain neurons in the jellyfish stages of Aurelia. The number of stained neurons was significantly higher in ephyrae treated with FMRFamide or Artemia when compared to ephyrae treated with MgCl2 and FM[D-R]Famide.;The data indicate that the nervous system of Aurelia aurita is more complex than previously assumed, in that a separate nerve net or subset of neurons is present in the jellyfish whose characteristics are neither solely that of the giant fibre system nor that of the diffuse nerve net. No indications were found for neuronal cell death during the development of the nervous system.