292 Nature Vol. 290 26 March 1981 however, since the affinity of myosin for affinity for scallop light chains. Although Saya de Malha Bank is apparently part of light chain drops dramatically after the this may be a fortuitous property of scallop the same physiographical feature as the first one is bound. The affinity of light myosin, it is intriguing to ask what might be Seychelles, the Mozambique Plateau is chain-depleted scallop myosin for foreign learned using other light chain-depleted close to Africa and thought to be a piece light chains is as great or greater than the myosins in similar experiments. D broken from it, and Lord Howe Rise is likewise thought to be a fragment from nearby Australia, although in these three cases there is apparently no direct evidence of the presence of continental material. Two distinct types of But direct evidence or not, the seismic velocity-depth profiles suggest that these six platforms are of similar construction oceanic plateau and thus members of a class. In three cases (Seychelles, Rockall, Lord Howe) upper from Peter J. Smith mantle velocities (>8.0kms I) are observed at about 30 km below sea level, THE many refraction studies carried out bution. But others (for example, Broken and immediately above the Moho there is a over the oceans during the past twenty Ridge and Shatsky Rise) are thought to be 14-18km thick lower crustal layer with years have shown the basic structure of the truly oceanic, although the processes by seismic P wave velocities in the range oceanic crust to be remarkably uniform. which they are formed are unknown. To 6.0-7.0kms I. Unfortunately, the profiles The three-layer model proposed by Raitt test this two-fold classification and put it from the three other plateaus (Mozam­ (in The Sea, Wiley, 1963) in pre-plate onto a more systematic basis, Carlson et al. bique, Saya de Malha, Falkland) do not tectonic, and almost pre-ocean floor (Earth planet. Sci. Letts 51, 171; 1980) have extend down to the Moho, although the spreading, days still generally holds good, now collated the seismic refraction data same basal crustal layer is present in notwithstanding the discovery of from II of the many oceanic plateaus Mozambique and Saya de Malha. Still systematic variations in some lithospheric known, showing that the two types are more unfortunately, the Falkland profile properties with age. At the same time, structually distinct as might well be does not even reach that far, but is con­ however, the ocean basins contain some inferred from their supposedly different sistent with the others in having a wide obvious anomalous features, of which origins. range of velocities of up to 6.4km S-I in its active ridges are the most conspicuous and Of the II features studied, three were upper parts. hot spots the most controversial. And then already known to be underlain by rock of This upper-level velocity variability is there are the 'ocean plateaus' - broad continental type. Thus Rockall contains also evident in eight seismic profiles from aseismic platforms standing high above the Lower Eocene granites; granitic rocks are the five platforms suspected not to be of ocean floor and usually topped with thick exposed on the Seychelles Plateau; and continental origin. But there the resemb­ deposits of calcareous sediments. gneissic rocks have been recovered from lance ends. Generally (five profiles - Some of these plateaus (Rockall Bank the basement of the Falkland Plateau by Shatsky, Broken Ridge, Fiji) the mantle is and Seychelles Plateau, for example) have the Deep Sea Drilling Project. In addition, reached at depths of only 18-25 km, and long been regarded as continental frag­ immediately above the Moho there is a ments, presumably broken off from larger lower crustal layer 6-15km thick having landmasses during continental redistri- Peter J. Smith is the Editor Of 'Open Earth'. seismic velocities in the range

Ice-mountain of which we hear so much, and the remains of which are not unfrequentIy to be seen even by summer visitors. The average height of this is about half the total height of the Fall, but this winter it has attained to the unprecedented height of within twenty feet of the top of the Fall! THE FALLS OF NIAGARA IN WINTER Within the last few years a considerable portion of "Table Rock" has fallen away. In IN the first week of last February it fell to my its present condition a stream of water about lot to make very hurriedly the transcontinental one foot in thickness falls over it in summer, journey of 3500 miles from San Francisco to and, owing to the amount of its overhanging, New York. Before starting I resolved that the it is easy to get between this Fall and the rock, one stoppage which I could allow myself en and thus to be "behind Niagara." At the time route should be made at Niagara. I was of my visit (February 8th), however, the whole confirmed in my resolve by seeing statements of this portion of the Fall was completely in various American papers to the effect that, frost-bound. Enormous icicles, of the most owing to the long-continued and exceptionally surpassing beauty, depended from the rock severe cold of the present winter, the Ice­ above, while at my feet were masses of the mountains at the Falls were higher than had frozen spray from the Horse-Shoe Fall. The ever been previously known. intense emerald green of the water of that Fall, In the Niagara gorge numerous springs seen through and between these magnificent discharge themselves into the chasm at various ice-pendants, could be reproduced by no points in the precipitous rocky sides, and at artist, but will never be effaced from my these points numerous collections of huge and memory. The accompanying woodcut, photo­ massive icicles appeared as though adherent to graphed on to the wood block from a the rock, measuring perhaps seventy or eighty photographic picture taken a few days prior to feet in length, and eight to ten feet in irregular my visit, will, to those who know the place, diameter. give some faint idea of the beauty of the scene, Gigantic icicle under Table Rock. The upper right+hand The boulders in front of the American Fall and of the gigantic scale of the icicles. corner is rock; a portion of the Canadian (or Horse-Shoe Fall) is on the left. The whole of the apparent ground is a mass of are, as it were, gigantic nuclei, round which the frozen spray which has accllmulaled many feel in thickness on frozen spray accumulates, and produces the From Nature 23, 31 March, 511,1881. the shingle at the foot of the rock.

0028-0836/81! 130292-02$01.00 1981 i\1acmillan Journals Ltd Nature Vol. 290 26 March 1981 293 7.3-7.6kms-,. Then above that there is a velocities evident in the second pattern lymphocytes from many long-standing unit with typical oceanic layer 3 velocities. have also been observed at the base of MG patients make anti-AChR in culture, it The only exception to all this (apart from normal crusts in parts of the Pacific and the seems that the thymus is not the primary the Manihiki Plateau whose profile is too Atlantic, as well as in some ophiolites, site of anti-AChR synthesis (J. Newsom­ short to indicate much) is the Ontong-Java which in fact strengthens the view that Davis, Royal Free Hospital, London). Plateau, the two profiles of which demon­ oceanic plateaus of the second kind are Thymic lymphocyte preparations strate a curious mixture of both genuinely of marine origin. (irradiated to destroy B cells) can also 'continental' and oceanic characteristics. Carlson and his colleagues admit that the enhance the in vitro anti-AChR synthesis On the face of it, this spoils the story number of plateaus for which velocity by MG peripheral blood lymphocytes, rather, although it could quite genuinely profiles arc available is small and therefore possibly due either to antigenic stimulation represent a composite structure with both regard their analysis as tentative, although by AChR on thymic cells, or to the presence continental and oceanic elements. the agreement between the seismic data and of a thymic helper-T cell population Leaving that anomaly aside, however, prior suppositions about two different specific for anti-AChR (or both) (Newsom­ there are clearly just two basic and distinct origins inspires confidence. The mode of Davis). Other cell types may also be structural patterns, suggesting two formation of the non-continental plateaus involved, however, as N. Shinomiya separate modes of origin. The two types of remains open, however. The new analysis (Tokyo Medical and Dental School) structurc also correlate well with their seems to confirm the conclusion by showed that normal, but not MG, respective counterparts elsewhere. Thus Hussong and his colleagues (J. ~eophys. peripheral blood T cells inhibited anti­ the first (continental) pattern is very similar Res. 84,6003; 1979) that the truly marine AChR synthesis by MG lymphocytes. This to the seismic velocity-depth structure of plateaus are expanded sections of normal suggests the absence of a specific continental shields, whereas the second oceanic crust, but the cause of the suppressor-T cell in MG subjects. (oceanic) is not. Then, the high basal expansion is a mystery. IJ This latter finding indicates that even normal subjects may have AChR-sensitive lymphocytes and display low levels of anti­ AChR. Anti-AChR has been detected in the sera of 18 per cent of MG relatives (R. Pirskanen, Karolinska Institute) although Idiotype restriction in it did not correlate with HLA-B8, the HLA antigen most closely associated with MG, nor with the electromyographic myasthenia gravis antibodies abnormality also found in a proportion of MG relatives. Others have demonstrated from Angela Vincent very low levels of anti-AChR in normal subjects (T. Mittag, Mount Sinai School of IT is now reasonably certain that anti­ showed that a factor in MG serum can Medicine), in cultures of normal acetylcholine receptor antibodies (anti­ slightly increase the open lifetime of the lymphocytes with MG T cells (R. Kelley, AChR) play a pathogenic role in the individual AChR-associated ion channels. UCLA School of Medicine) and in purified reduction in end plate acetylcholine This effect could only be seen, however, Torpedo AChR (Shinomiya, Tokyo receptors (AChR) that underlies the when the serum had been applied in the Medical and Dental School). The existence defective neuromuscular transmission presence of histrionicotoxin, a toxin that of sensitized B lymphocytes would not be characteristic of myasthenia gravis (MG). reversibly blocks the channel in its open surprising if the normal immune system Experimental autoimmune MG (EAMG) state. was exposed to AChR, and it may be in animals immunized against purified Although the pathogenesis of the disease relevant that AChR-like molecules have AChR,human anti-AChR in over 800/0 of appears to be established, the fundamental been detected on normal peripheral MG patients, and passive transfer ofMG to aetiology and cellular immune mechanisms lymphocytes (G. Morrel, Wayne State mice using MG immunoglobulins had all are not. Is there a precipitating event such University; D. Richman, University of been reported by 1975 and progress made as virus infection, and how are genetic Chicago) and mouse thymic lymphocytes since then was the subject of a recent factors involved? Is there a specific (S. Fuchs, Weizmann Institute) although conference* . lymphocyte abnormality or a failure of a-bungarotoxin binding has not been It has been shown that, although in any normal control mechanisms? Any answer demonstrated. individual the initial anti-AChR titre gives to these questions must include an Work on EAMG may help to elucidate no indication of the severity of the disease, explanation of the role of the thymus. the involvement of genetic factors. When subsequent anti-AChR levels tend to Many patients have 'thymitis' with animals are immunized against AChR, a correlate with changes in the clinical state. germinal centres, and some have a thymic proportion of the anti-AChR antibodies Various mechanisms thought to be tumour. MG symptoms, particularly in the cross-react with the animals' endplate involved in loss of AChR function have former group, often respond well to AChR, resulting in weakness and neuro­ been demonstrated, including thymectomy although improvement may muscular block. Not all species or strains complement-dependent damage to the take months or years. One possibility is respond equally and P. Berman (Salk postsynaptic membrane (A. Engel, Mayo that the thymus is the site of both the Institute) showed that susceptibility to Clinic), increased AChR degradation antigen and the initiating event. It has been clinical EAMG in inbred strains of mice caused by cross-linking of AChR by recognized for several years that cultured was not related to the anti-AChR titre, its divalent antibody (D. Drachman, Johns thymic epithelial cells can display muscle­ ability to increase AChR degradation, or a Hopkins University), and direct 'block' of like antigens including AChRs (K. difference in the safety factor for AChR function by binding of antibody to Wekerle, Max Planck Institute). Wekerle transmission at endplates from different the ACh/ a-bungarotoxin binding site (D. proposed that helper-T cells were sensitized genetic strains. The greatest susceptibility Drachman; B. Fulpius, University of to AChR in the thymus and then emigrated was found in those with a particular major Geneva). Antibodies can also modify to the periphery where anti-AChR histocompatability complex (H-2b) and rather than block AChR function, as E. synthesis took place. This may well be the immunoglobulin (Ig]b) genotype. The involvement of the IgC region suggests Albuquerque (University of Maryland) case because, although thymic H *The sixth conference on myasthenia gravis was held in New that overt myasthenia in mice depends on York frum December 2nd tu4th, 1980, organized by Dr D. Grub Angela Vincent is in the Department of (Maimonides Medical Center) and sponsored by the New York Neurological Science, The Royal Free Hospital, particular polymorphic genes coding for Academy of Sciences and the Myasthenia Gravis Foundation London. the constant region of immunoglobulin

0028·0836/81/130293-02$01.00 'c': 1981 Macmillan Journals Ltd