By ERIK DAHL, BENGT FALCK, CLAES VON MECKLENBURG, and HARRY MYHRBERG. Summary

Transcription

1 53 1 An adrenergic nervous system in sea anemones By ERIK DAHL, BENGT FALCK, CLAES VON MECKLENBURG, and HARRY MYHRBERG (From the Departments of Zoology and of Histology, University of Lund, Sweden) With one plate (fig. i) Summary A new method for the demonstration of certain mono-amines by means of fluorescence microscopy was applied to the tentacles and oral disk of the sea anemones Metridium senile and Tealia felina. A fluorescent substance was found in the cells and fibres of the tentacular ectodermal nervous system. This nervous system probably has a double sensory and motor function. Material and methods THE results briefly recorded below were obtained in the course of a study of adrenergic mechanisms in lower invertebrates by means of the method recently described by Falck (1962). According to this method tissue pieces are frozen-dried, treated with formaldehyde gas, sectioned in paraffin, and studied in the fluorescence microscope. Certain mono-amines condense with formaldehyde gas to form products with a yellow (5-hydroxytryptamine) or green (adrenaline, noradrenaline, dopamine) fluorescent colour. The species used in the present investigation were the actinians Metridium senile and Tealia felina. The hydrozoan Hydractinia echinata was also tested. In the actinians, preparations from the tentacles and oral disk, columnar bodywall, mesenteries, and mesenterial filaments (and in Metridium, also acontia) were studied. Various fluorescent structures were observed, but only those belonging to the nervous system will be dealt with here. Results The only nervous structures showing any fluorescence after the treatment described above are situated in the tentacular apparatus of both actinians, while in H. echinata no fluorescent neurones were found. The general morphology of the nervous system of the Actiniaria is comparatively well known. This is especially the case with the nervous structures of the column and mesenteries, which were the objects of comprehensive investigations by Pantin and his collaborators (Pantin, 1952; Batham, Pantin, and Robson, i960). The most relevant observations on the innervation of tentacles and oral disk are recorded in the earlier papers of Hertwig and Hertwig (1879) and Groselj (1909) (see also Leghissa, 1949). The fluorescent system observed in the tentacular apparatus had a bright green colour. The cell-bodies of the neurones lie in the ectodermal epithelium, [Quart. J. micr. Sci., Vol. 104, pt. 4, pp , 1963.]

2 532 Dahl, Falck, von Mecklenburg, and Myhrberg either comparatively close to the external surface or, in most cases, at about mid-level (fig. i, A to E). In either case the cell sends a fibre to the surface. On this fibre there may be one or more varicosities and generally there exists a terminal or subterminal expansion with a diameter several times that of the fibre. From the proximal end of the cell-body another fibre passes centripetally towards the base of the epithelium, often, however, splitting up into two fibres well before reaching the level of the basal ends of the epithelial cells. These fibres also generally show numerous varicosities. From the level of the bases of the epithelial cells down to the ectodermal muscular layer the fluorescent fibres form a dense network. No fibres, however, enter the mesogloea or the endoderm, which are both devoid of fluorescent material. The fluorescent basi-epithelial network contains no fluorescent cells but only varicose fibres. It seems not improbable that these fibres form an interconnected nerve-net of the type described by Batham, Pantin, and Robson (i960). Many varicose fibres are superimposed on dark, non-fluorescent structures in the meshes of the network. It has not yet been possible to establish whether these fibres are in contact with non-fluorescent nervous elements or whether they form terminals on non-nervous structures. In Metridium the fluorescent neurones are very numerous in the distal part of the tentacles, where they occur in approximately the same numbers as the nematocysts. In some limited areas it was possible to trace a regular alternation of nerve-cell-nematocyst nerve-cell, but it is not yet possible to say whether this is the normal arrangement in the distal part of the tentacle. Towards the base of the tentacle the fluorescent neurones become much less numerous, both absolutely and in comparison with the nematocysts, a marked decrease being generally obvious from about the middle of the tentacle. The base of the tentacle seems to be devoid of fluorescent neurones and the same applies to the oral disk. It should perhaps be mentioned that the comparison between the distribution of nematocysts and neurones made above does not imply that we have found anything indicating an innervation of the nematocysts or cnidoblasts (fig. 1, c, E). The density of the basi-epithelial fluorescent network also decreases towards the tentacular base, although scattered fibres can still be found there and on rare occasions also in the oral disk (fig. 1, E). FIG. 1 (plate). The arrangement of fluorescent neurones in the tentacles of T. felina and M. senile. The bright structures in the endoderm (en) seen in some of the figures are photographic artifacts. Visually, the endodermal epithelium is only slightly brighter than e.g. the mesogloea (mg). A, transverse section through tentacle wall of T. felina. B, detail of a transverse section through the ectoderm of T. felina. c, longitudinal section through the distal part of a tentacle of M. senile. D, oblique horizontal section through the basal part of the ectodermal epithelium of a tentacle of M. senile. E, longitudinal section through the base of a tentacle of M. senile. The arrow indicates the decrease in density of the basi-epithelial network towards the tentacular base, ec, ectoderm; ecm, ectodermal muscle sheet; en, endoderm; mg, mesogloea; nc, nerve-cells in ectoderm; nn, basi-epithelial fluorescent nerve-net.

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4 Adrenergic system in sea anemones 53 3 In Tealia the arrangement of the fluorescent neurones is essentially similar, although they are comparatively less numerous and the cell-bodies are generally situated close to the surface of the epithelium. A rough estimate gives at least 5, perhaps as many as 10 nematocysts to every neurone even in the distal part of the tentacle. Proximally the density of the neurones and nerve-net decreases as in Metridium. The fluorescent substance in the tentacular nerve-net has not yet been identified. Its bright green fluorescence suggests a catechol amine, and in its reactions to formaldehyde it recalls those of primary catechol amines and almost excludes secondary catechol amines like adrenaline (see Falck, 1962). Discussion Ross (i960 a, b) investigated the effect of drugs on column and sphincter preparations from Calliactis and Metridium and found no effect of acetylcholine, 5-hydroxytryptamine, noradrenaline or dopamine, while some effect was obtained with adrenaline and tryptamine. No inhibitory drug was found among the numerous substances tested. Since no adrenergic structures were found by us in the sphincter or column, these results are not directly relevant to the present discussion. The effect of adrenaline, however, is slightly puzzling. According to Ross it raises tonus, causes direct contractions, and enhances activity in the column. In the sphincter it causes slow contraction and some enhancement of slow reaction to stimuli. It produces no enhancement of quick reaction to stimuli. As far as is known to us the effect of drugs on tentacle and oral disk reactions has not been studied. In connexion with a discussion of the nervous mechanisms underlying the through-conduction and column action in actinians, Pantin (1952) drew attention to the fact that a different mechanism must be involved in the reactions of the tentacles and oral disk. He pointed out that 'stimulation of the disk or tentacles calls up only a local response in the neighbourhood of the stimulus'. Increased stimulation also increases the area affected, and repeated electric shocks cause a spreading of the response and may ultimately also affect the through-conduction system of the column and mesenteries. The arrangement of the tentacular nerve-net described above suggests that it may be a mechanism for producing, or at least contributing to, the reactions studied by Pantin and Pantin (1943) and Pantin (1952). Another aspect of the tentacular nerve-net appears to be of considerable general interest. The cells of the tentacular net were interpreted by the Hertwigs and by Groselj as sensory cells and this has also been the opinion of other authors dealing with the subject. Our own observations of the histological arrangement in the epithelium support this interpretation. Further support can be drawn from the investigation of Pantin and Pantin (1943) on the reactions to stimuli of the tentacles and oral disk, for the reactions observed are scarcely comprehensible without the assumption of a sensory mechanism, and no other cells are known which could be supposed to act as receptors.

5 534 Dahl, Falck, von Mecklenburg, and Myhrberg If this is correct, however, it implies that we have here an adrenergic sensory or possibly combined sensory and motor system. This would open new and highly interesting perspectives on the function of primitive nervous systems (compare Parker 1919, Pantin, 1956). It is of interest to note that the fluorescent nerve-cells were provided with varicose processes, the varicosities being particularly numerous in the basiepithelial network. Varicose nerve terminals have now been demonstrated by means of the present method in various vertebrates such as mammals (Carlsson and others, 1962; Falck, 1962), Amphibia (Falck and Haggendal (in MS.)), fishes (Fahlen and Falck (in MS.)), and also in various invertebrates, such as molluscs (Dahl and others, 1962), annelids and turbellarians (unpublished observations), and coelenterates. Consequently this arrangement appears to be characteristic of adrenergic systems throughout the animal kingdom. We are indebted to Professor G. Thorson, Helsingor, for the material used in the present investigation, and to Professor C. F. A. Pantin and Dr. Elaine A. Robson for valuable discussions and suggestions. References BATHAM, E. J., PANTIN, C. F. A., and ROBSON, E. A., i960. Quart. J. micr. Sci., 101, 487. CARLSSON, A., FALCK, B., and HILLARP, N.-A., Acta physiol. scand., 56, Suppl DAHL, E., FALCK, B., LINDQVIST, M., and MECKLENBURG, C. VON, K. Fysiogr. Sallsk. Lund Forh., 32, No. 8. FAHLEN, G., and FALCK, B., 1963 (to be published). FALCK, B., Acta physiol. scand., 56, Suppl and HAGGENDAL, J., 1962 (to be published). GROSELJ, P., Arb. zool. Inst. Univ. Wien, 17, 269. HERTWIG, O., and HERTWIG, R., Jen. Z. Naturwiss., 13, 457. LEGHISSA, S., Pubbl. Staz. Zool. Napoli, zi, 272. PANTIN, C. F. A., Proc. Roy. Soc. B, 140, Pubbl. Staz. Zool. Napoli, 38, 171. and PANTIN, A. M. P., J. exp. Biol., 20, 6. PARKER, G. H., The elementary nervous system. Philadelphia (Lippincott). Ross, D. M., 1960a. J. exp. Biol., 37, &. Ibid., 37, 753.