Mechanism of ocular dominance segregation in the lateral geniculate nucleus: competitive elimination hypothesis

TINS- January 1986

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Mechanism of ocular dominance segregation in the lateral genlculate nucleus: competitive elimination hypothesis Recent studies on the development of retinogeniculate connections in a variety of mammalian species support the hypothesis that competitive interactions between fibers from two eyes for their brain targets early in development lead to the elimination of a large fraction of the initial population of optic axons and eventually to segregation of ocular dominance domains. The balance between the overproduction and elimination of optic axons may ultimately determine the size of territories devoted to fibers from each eye, as well as the ratio of crossed and uncrossed retinal axons. Similar phenomena observed in other neural systems indicate that competitive elimination plays a significant role in the developing mammalian brain. In vertebrates with laterally placed eyes and separate, largely non-overlapping visual fields, connections between the retina and the brain are organized so that ganglion cells project mostly to the central neurons in the opposite hemisphere. However, in species with more frontally directed eyes and overlapping fields of view, all optic axons do not cross to the other hemisphere. An often large fraction of fibers (which usually originate from the temporal part of the retina) terminate on the same side of the brain that they originated from, and most importantly these uncrossed fbers terminate exclusively within discrete regions, in complete segregation from the contingent of crossed fibers 1'2. Most higher primates, including humans, have a six-layered lateral geniculate nucleus (LGd) in which layers 1, 4 and 6 receive input from the eye on the other side of the body and layers 2, 3 and 5 receive input from the eye on the same side (Fig. 1B). The discovery made ten years ago that projections from the two eyes overlap transiently in the LGd of the rhesus monkey early in development3 raised the questions about cellular mechanisms underlying the segregation of retinal terminals in this structure. The concept that development of binocular connections proceeds from a diffuse to a segregated phase was based on the fact that radioactive tracers injected into one eye of early monkey embryos label both LGds equally3. This indicated that initially there is a total overlap of the projections from the two eyes (Fig. 1A). The segregation phase, during which fibers originating from the left and right eyes become restricted to separate layers of the LGd, occurs during the third

quarter of gestation3'4. Similar progression in the formation of retinogeniculate connections has subsequently been shown in other mammalian orders 5-15. There is, however, considerable variation in the extent of initial overlap and in the sequence and timing of arrival and withdrawal of contralateral and ipsilateral projections. Although the extent of axonal intermixing during development is not always related to the degree of binocularity in adult animals, in general it appears that species with a larger binocular visual field have a more extensive initial overlap of the projections from the two eyes. Two basic mechanisms could account for the segregation of axons from both eyes in the LGd. One possibility is that retinal fibers project-

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ing to the inappropriate layers of the LGd are eliminated (Fig. 2C). The other possibility is that only the terminal branches are diverted from inappropriate to appropriate layers (Fig. 2B). These two possibilities are discussed below in the light of some recent experimental findings. Competitive elimination hypothesis The competitive elimination hypothesis implies that the input from each eye to the inappropriate territory in the LGd loses in competition with the input from the appropriate one. To be tenable, this hypothesis requires that (i) an excess of axons be sent to the LGd prior to segregation; (ii) the elimination of excess axons coincides with segregation; and (iii) the loss of axons involves mostly axons terminating in inappropriate territories. Good evidence supports each of these requirements. First, it has been shown that in the monkey around midgestation, when the projections from the two eyes overlap fully, the optic nerve contains 2.85 million axons, which is almost 2.5 times the number found in the adult ~6. Second, about 1 million fibers are eliminated during the third quarter of gestation, the very

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Fig. 1. Sketch of the distribution of fbers from the lef (L) and right (R) eyes over six layers (1-6) of the dorsal lateral geniculate nucleus (LGd) in the embryonic (,4) and mature rhesus monkey (B). A. As revealed by autoradiographic labelling ~, during the second quarter of gestation projections from the left and right eyes (represented by hatched lines and dots, respectively) overlap in the LGd on both sides. Although the borders between laminae (dashed lines) are as yet not clearly outlined, all neurons of the LGd have been generated and have attained their final positions at that time~. Segregation of fibers occurs during the third quarter of gestation. B, Distribution of projections from the two eyes to the appropriate LGd layers of adult monkeys. Note that projections from the two eyes do not alternate regularly since layers 2 and 3 on both sides receive input from the ipsilateral eye. 1986, Elsevier Science .Publishers B.V., Amsterdam 0378 - 5912t86/$02.00

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same period during which fibers from the left and right eyes segregate (Fig. 3). Third, the loss of axons seems to be selective, since a random competition for limited postsynaptic sites would result in an indiscriminate rather than layer-specific elimination. The fulfilment of these three requirements is essential, but does not provide conclusive evidence for the competitive elimination hypothesis. Theoretically, the elimination of axons could occur with or without competition for specific postsynaptic space in the LGd. However, several lines of evidence support the role of competition. The most obvious one is that the segregation of axons in the monkey does not result in regularly alternating inputs to ipsi- and contralateral layers of the LGd. Rather, the adjacent layers 2 and 3 on both sides receive ipsilateral input (Fig. 1B). Although layers 1, 2 and 3 through 6 receive physiologically different sets of retinal axons, the reverse order of input originating from left and right eyes that occurs between layers 2 and 3 must be induced by information generated by the LGd neurons. Convincing evidence that this process involves loss of axons rather than re-routing of their terminal branches is that when monocular enucleation is performed before the stage of segregation, the projections of the remaining eye fail to withdraw from the inappropriate territories 17. Such animals retain 30-40% of their supernumerary embryonic axons (Fig. 3), indicating that axonal survival depends on the availability of specific synaptic space in the LGd Is. Even more compelling is the evidence that early monocular enucleation rescues mostly ganglion cells which project to inappropriate territories 19-2~. Thus, it appears that erroneous projections normally lose in competition with the appropriate set of fibers. This selectivity affects the ratio of crossed and uncrossed fibers: following removal of one eye in newborn rats, the number of ipsilateral fibers in the optic tract (after decussation) is larger than normal22. On the basis of such information, it seems reasonable to conclude that elimination regulated by competition between axons from the two eyes for two sets of LGd neurons plays a significant role in segregation of ocular territories in the brain centers and, indirectly, in determining the final ratio of crossed and uncrossed fibers 17:s. The molecular mechanism involved in this competition is un-

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Fig. 2. Illustrations of two possible modes of development o f retinogeniculate connections in the rhesus monkey from the diffuse to the segregated state. As shown in column A, the projections originating from the left (L, solid black line) and right (R, white line) eyes are intermixed among undifferentiated and uncommitted LGd neurons (represented with circles outlined by broken lines). In the second half of gestation, inputs from the left and right eyes become connected exclusively to neurons subserving the left eye (black circles) or right eye (open circles). Theoretically, this segregation can be achieved either by the rearrangement of connections (column B) or by competitive elimination of half the inappropriate retinal axons (column C). In the latter case, the optic nerve at some stage of overlap must have an excess of optic axons that should be eliminated during the segregation phase. As discussed in the text, it is likely that both mechanisms operate.

subserving each eye in the cortex. The question relevant to the present article is to what extent similar rearrangement of fibers occurs during segregation of retinal projections to the LGd. Recently, it has been reported that individual optic axons send their terminal branches into both contralateral Rearraag~nent hyl~thesis The second possibility is that segre- and ipsilateral layers of the LGd in the gation of inputs subserving the two fetal cat26. The injection of tracers into eyes is achieved by rearrangement of the optic tract could not determine the terminal axonal arborization from the origin of fibers which become rearraninappropriate to appropriate layers ged, but both electron microscopic27 (Fig. 2B). According to this hypothe- and physiological studies28 indicate sis, projections to the inappropriate that at least some inappropriately layers that are observed following located axons from transient synaptic early monocular enucleation are not contacts in the wrong territories. Thus, the result of retaining inappropriate considerable rearrangement of termiprojections (Fig. 4C), but are due to nals undoubtedly occurs during the the wider spread of appropriate pro- segregation phase also in the LGd. jections to inappropriate layers (Fig. However, it is unlikely that the 4D). Such a possibility was first breakdown of already established synsuggested for the segregation of geni- apses is the sole or main mechanism for culo-cortical projections during the segregation of binocular connections, formation of ocular dominance since the majority of synapses in the columns24. Although, at the present LGd are formed long after segregation time, the supporting evidence for this is completed in both monkey29 and hypothesis has not been adequately cat 3°. Furthermore, even during the confirmed, it is in harmony with the height of the overlapping phase in the recent finding that the number of cat's LGd, axons from the left and neurons in the monkey LGd remains right eyes terminate in restricted zones relatively stable during the formation that presumably correspond to the of ocular dominance columns in the prospective layers, and few if any cortex25. Thus, rearrangement of axons have a widespread arborization terminal branches, rather than their along their trajectories through all elimination, may be the mechanism three laminae26. The most plausible underlying segregation of territories interpretation of these findings is that

known, but blocking the inward sodium current of ganglion cell somata with tetrodotoxin indicates that electrical activity might contribute to the regulation of ganglion cell death in the retina23.

TINS-January 1986

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in the binocular system that occurred during the process of evolution. The major difficulty in discussing this complex subject is that many variables probably influence the ratio of crossed and uncrossed fibers in each species for example, the initial ratios of crossed and uncrossed fibers; the magnitude of axonal overproduction; the degree of elimination of ipsi- and contralateral axons and the number of axons projecting to extrageniculate centers that are unrelated to binocular vision. Available data on these variables are still fragmentary, and in some cases tentative, but a rough picture begins to emerge. For example, in rats and mice, which have a visual field overlap of less than 30° and only 2% uncrossed fibers32, neonatal unilateral eye enucleation spares only a small Phylogenetic considerations number of supernumerary optic The hypothesis of competitive elimi- axons w'21'22'31. Likewise, in hamsters, nation and its role in the formation of which also have a relatively small the connections subserving binocular overlap, the remaining eyes of early vision are consistent with differences in enucleates had an average of 8% more experimental results obtained from ganglion cells than normal 33. In conspecies with variable degrees of bino- trast, in cats, which have a larger cular overlap. Review of the literature overlap (90O-100O), early enucleation indicates that removal of one eye at preserves 20-25% of the supernumethe critical developmental stage results rary axons and ganglion cells34'35. in preservation of supernumerary optic Finally, in rhesus monkeys, which axons which is roughly proportional to have 130°--140° overlap, monocular the amount of visual field over- enucleation at fetal ages preserves lapZg-22.3~,32,34.This correlation raises the over 30% of the supernumerary question of whether competitive elimi- axons TM. Since humans also have a nation has contributed also to changes large overlap of visual fields and

each eye gives rise to separate subsets of axons that terminate exclusively within either appropriate or inappropriate territories, but not in both. Therefore, simple withdrawal and expansion of branches of the individual fibers within alternating layers cannot fully explain segregation of binocular inputs in the LGd. Finally, the rearrangement hypothesis cannot account for the changes in the ratio of crossed and uncrossed fibers that occur during development and after monocular enucleation 22. Therefore, although alteration and differential growth of terminal branches contribute to the developmental changes in density of retinal projections, such alterations seem to be insufficient to explain all events associated with the phenomenon of binocular segregation.

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nearly an equal distribution of axons ~om each eye to both sides36, the loss of an eye during development may spare an even larger proportion of axons from the remaining eye. Human fetuses at mid-gestation have 3.5 million optic axons 37, compared with about 1 million in the adult 3s, which suggests that similar principles and mechanisms may apply. Another important question about the development of binocular vision throughout evolution is to what extent axonal elimination influences the ratio between crossed and uncrossed retinogeniculate fibers in any given species. To answer this question, we need to know both the initial and final proportions of crossed to uncrossed fibers, and these are not well established in any species. So far, it has been found that in fetal cat, crossed and uncrossed optic axons may be unequally distributed before they have reached the brain 39'4°. On the other hand, in primates, the ratio of ipsi- and contralateral projections is probably, from the start, close to 1:1 (Ref. 3). Obviously, such species-specific differences in the initial proportion of crossed to uncrossed fibers must have a bearing on the amount of axonal loss that is later directly associated with establishment of their binocularity. This proportion, however, appears not to be dependent solely on the degree of visual field overlap in a given species; rather, the number of those

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Fig. 3. Number of axons in the optic nerve in rhesus monkeys o f various embryonic (E) and postnatal (P) ages. The thick striped line indicates the number of optic axons in ndMt monkeys. Arrow A poir~ to the age when, ~cordin8 to our ~ H - ~ y ~ e autorndiographic ar~lys~, the genesis of reti~ff g~glion cells begins; arrow B indicates when it stops. Line C denotes the period during which retinal input from the two eyes becomes segregated in the LGd. This period coi~ides with the rapid e l ' ~ , ~ o n of more than I rmUion optic axons as indwoted by line D. The block squares indicate the n ~ e r of optic axons in postnatal animals that had one eye removed around the 65th day of gestation. For further details, see Rakic and Riley 16'is.

14 ganglion cells that are situated within the part of the temporal retina subserving binocular vision may be the most crucial variable. It should be underscored that, in many species, elimination of retinal fibers is not necessarily related to development of binocularity. For example, the chicken, which has negligible visual field overlap, nevertheless loses about 40% of its initial complement of optic axons 41. Even in the monkey, elimination of more than 500000 fibers occurs well after the phase of binocular segregation in the LGd (Fig. 3) and therefore is almost certainly not related to binocularity. In contrast, the largest proportion of optic axon loss in the cat occurs before the onset of the segregation phase 7. The elimination of some optic axons in mammals is probably involved with the establishment of extrageniculate connections42"43. Other axons may be lost because the ganglion cells from which they originate fail in competition for the input from bipolar and amacrine cells within the retina 44. The fraction of axon loss attributable to each of the several possible causes raised above is not known, but deletion of the projections to topographically incorrect points or with inappropriate structures is a likely possibility. Although overall loss of optic axons may be due to a variety of reasons, the portion of loss that occurs during the segregation phase in the LGd correlates rather well with the degree of binocularity in various species. This correlation suggests that competitive elimination plays a significant role in determining the ratio of axons projecting to the ipsilateral and contralateral LGd. However, the question still remains as to what determines the magnitude of overproduction, initial ratios of crossed to uncrossed fibers, and the extent and time course of subsequent elimination of axons in each species. We also need to know how interactions between retinal axons and geniculate neurons select which axons should be eliminated. At present, two points have become clear: first, both pre- and postsynaptic elements must be engaged in at least some aspects of this selectivity; and second, segregation of retinal connections which subserve the two eyes occurs prenatally in primates and several other mammalian species and therefore cannot be regulated by visual experience.

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Fig, 4. Schematic representation of the possible mechanisms that may be involved in attainment of distribution of retinal projections to the LGd in normal monkeys (B) and in those from which one eye was removed in the first third of pregnancy (C and D). In enucleated monkeys, the entire territory of LGd neurons receives input from the remaining eye t7. The spread of the projection from the left eye (black lines) to the LGd cells, which would under normal conditions be connected only with the right eye (open circles), could be achieved either by retaining supernumerary axons from the left eye in the absence of the axons from the right (enucleated) eye (column C) or by collateral sprouting of axons which normally originate exclusively from the left eye (column D).

Competitive elimination in other systems The biphasic mode of development, which proceeds from more diffuse to sharply defined terminal fields, has been shown also in the superior colliculus and pretectum 3,20.45--49as well as in several non-visual structures in a variety of mammalian species 5°-54. In each case, initially more widely spread terminal fields become restricted as input from another source becomes engaged in competition. For example, corticostriatal projections are diffuse before they retract from territories that become occupied by later-forming afferent systemssl. Furthermore, corticostriatal projections also have a slight contralateral component which can be enhanced by removing competitive ipsilateral corticostriatal input at the appropriate age 54. These examples reveal that competitive elimination is a part of the dynamic cellular interactions by which synaptic connections are established in complex nervous systems. However, some classes of neuronal connections may be numerically and anatomically more rigidly predetermined. For example, in arthropods each neuron appears to have a precisely prescribed, exclusive target, and a phase of axonal overproduction and overlap has not been observed 55 . However, development of complex neuronal systems in most vertebrates,

particularly primates, is a more dynamic process which involves both rearrangement and elimination of transient connections through competitive interaction among heterogeneous inputs and heterogeneous targets.

Acknowledgements This work was supported by grant EY02593 from the US Public Health Service. I am thankful to R. W. Williams for helpful discussions and reading of the manuscript.

Selected references 1 Polyak, S. (1957) The Vertebrate Visual System, University of Chicago Press, Chicago 2 Kaas, J., Guinery, R. W. and Allman, J. M. (1972) Brain Behav. Evol. 6, 253-299 3 Rakic, P. (1976) Nature (London) 261,467471 4 Rakic, P. (1977) Philos. Trans. R. Soc. London Ser. B 278, 245-260 5 Bunt, S. M., Lund, R. D. and Land, P. W. (1983) Dev. Brain. Res. 6, 147-168 6 Cavalcante, L. A. and Rocha-Miranda, C. E. (1978) Brain Res. 146, 231-348 7 Chalupa, L. M. and Williams, R. W. (1984) in Development of Visual Pathways in Mammals (Stone, J., Dreher, B. and Rapaport, D. H., eds), pp. 89-102, Alan R. Liss, New York 8 Cucchiaro, J. and Guillery, R . W . (1982) Abstr. Soc. Neurosci. 8, 814 9 Cusik, C. G. and Kaas, J. H. (1982) Dew Brain Res. 4, 275-284 10 Linden, D. C., Guillery, R. W. and Cucchiaro, J. (1981)J. Comp. Neurol. 203, 189211

T I N S - J a n u a r y 1986 11 Rager, G., Nowakowski, R. S., Lansmann, S., Tanaka, M. and Schwaier, A. (1980) Abstr. Soc. Neurosci. 6, 662 12 Sanderson, K. J., Dixon, P. G. and Pearson, L. J. (1982) Dev. Brain Res. 5, 161-180 13 Shatz, C. J. (1983) 2'. Neurosci. 3,482--499 14 So, K-F., Schneider, G.E. and Frost, D. O. (1978) Brain Res. 142, 575-583 15 Wye-Dvorak, J. (1984) J. Comp. Neurol. 228, 491-508 16 Rakic, P. and Riley, K. P. (1983) Science 219, 1441-1444 17 Rakic, P. (1981) Science 214, 928-931 18 Rakic, P. and Riley, K.P. (1983) Nature (London) 305, 135-137 19 Godement, P. and Salatin, J. (1984) J. EmbryoL Exp. Morphol. 82 (Suppl. 1),224 20 Insausti, R., Blakemore, C. and Cowan, W. M. (1984) Nature (London) 308, 362-365 21 Jacobs, D. S., Perry, V. H. and Hawken, M. J. (1984) J. Neurosci. 4, 2425-2433 22 Shirokawa, T., Fukuda, Y. and Sugimoto, T. (1983) Exp. Brain Res. 51,172-178 23 Fawcett, J. W., O'Leary, D. D. M. and Cowan, W. M. (1984) Proc. NatlAcad. Sci. USA 81, 5589-5593 24 LeVay, S. and Stryker, M. P. (1979) Soc. Neurosci. Symp. 4, 73-98 25 Williams, R. W. and Rakic, P. (1985) Abstr. Soc. Neurosci. 11,804 26 Sretavan, D. and Shatz, C. J. (1984) Nature (London) 308, 845-848 27 Campbell, G., So, K-F. and Lieberman, A. R. (1984) Neuroscience 13, 743-759 28 Shatz, C. J. and Kirkwood, P. A. (1984) J. Neurosci. 4, 1378-1397 29 Hendrickson, A. and Rakic, P. (1977) Anat. Rec. 187, 602 30 Cragg, B. (1975) J. Comp. Neurol. 160, 147166 31 Crespo, D., O'Leary, D. M. and Cowan, W. M. (1984) Abstr. Soc. Neurosci. 10, 464 32 Dr~iger, U. C. (1980)J. Comp. Neurol. 191, 383--412 33 Sengelaub, D. R., Windrem, M.S. and Finlay, B.L. (1983) Exp. Brain Res. 52, 269-276 34 Williams, R. W., Bastiani, M.J. and Chalupa, L.M. (1983) J. Neurosci. 3, 133-144 35 Chalupa, L. M., Williams, R.W. and Henderson, Z. (1984) Neuroscience 12, 1139-1146 36 Kupfer, C., Chumbely, L. and Downer, J. De C. (1967) J. Anat. 101,393--401 37 Provis, J. M., van Driel, D. A., Billson, F.A. and Russell, P. (1985) J. Comp. Neurol. 238, 92-101 38 Balazsi, A. G., Rootman, J., Drance, S. M., Schulzer, M. and Douglas, G.R. (1984) Am. J. Ophthalmol. 97,760-766 39 Lia, B., Williams, R.W. and Chalupa, L. M. (1984) Abstr. Soc. Neurosci. 9, 702 40 Shatz, C. J. and Kliot, M. (1982) Nature (London) 300, 525-529 41 Rager, G. (1980) Adv Anat. Embryol. Cell Biol. 63, 1-92 42 Cunningham, T. J. (1982) Int. Rev. CytoL 74, 163-186 43 McLoon, S. C. (1982) Science 215, 14181420 44 Perry, V. H. and Linden, R. (1982) Nature (London) 297, 683--685 45 Hollander, H., Fietze, J. and Distel, H. (1979) J. Comp. Neurol. 184,783-794

15 46 Land, R. W. and Lund, R D. (1979) Science 205, 698-700 47 Williams, R. W. and Chalupa, L. M. (1982) J. Neurosci. 2, 604-622 48 Williams, R. W. and Chalupa, L. M. (1984) Neuroscience 12, 1139-1146 49 Frost, D. O., So, K-F. and Schneider, G. E. (1979) Neuroscience 4, 1649 50 Cowan, W. M., Stanfield, B.B. and Amaral, D. G. (1981) in Studies in Developmental Neurobiology (Cowan, W. M., ed.), pp. 395--435, Oxford University Press, London 51 Goldman-Rakic, P. S. (1981) J. Neurosci. 1,

721-735 52 Mihailoff, G. A., Adams, C.E. and Woodward, D. J. (1984) 2". Comp. Neurol. 222, 116 53 Schwob, J. E. and Price, J.L. (1984) J. Comp. Neurol. 223, 203 54 Goldman, P. S. (1978) Science 202,768-776 55 Goodman, C. S. and Bastiani, M. J. (1984) Sci. Am. 251, 58-66

Pasko Rakic is at the Section of Neuroanatomy, Yale UniversitySchool o[ Medicine, New Haven, CT 06510, USA.

Mitochondrial diseases J. A. M o r g a n - H u g h e s

The last two decades have witnessed the discovery o f a novel group o f inborn metabolic errors which directly impinge upon the pathways o f aerobic energy production in the mitochondrial inner membrane and matrix space. The spectrum o f reported biochemical abnormalities includes defects which impair the entry o f high energy substrates into the mitochondria or the capacity to generate reducing potential f r o m these substrates, as well as those that block the oxidative phosphorylation pathway itself. Clinical expression varies according to the nature, severity and tissue distribution o f the metabolic block. Aerobically active organs with a high demand f o r A TP, such as the brain, retina, cardiac and skeletal muscle, are particularly vulnerable to defects o f this type and may be affected either singly or in different combinations. The molecular and genetic mechanisms underlying these disorders have not yet been clearly identified but there are strong theoretical reasons f o r believing that some o f them may be due to mutations affecting the mitochondrial genetic system. T h e c o n c e p t of a p r i m a r y m i t o c h o n drial disease b e g a n to e m e r g e in the early 1960s w h e n Luft et al.l d e m o n strated a defect involving the coupling o f oxidative p h o s p h o r y l a t i o n in isolated muscle m i t o c h o n d r i a f r o m a Swedish w o m a n with l o n g s t a n d i n g fatiguable w e a k n e s s a n d a grossly elevated basal m e t a b o l i c rate which was not due to thyroid overactivity. E M e x a m i n a t i o n of the p a t i e n t ' s muscle fibres had s h o w n large peripheral aggregations o f structurally a b n o r m a l m i t o c h o n d r i a with a b u n d a n t distorted cristae a n d occasional paracrystalline inclusions. W h e n studied in a W a r b u r g m a n o m e t e r , t h e s e organelles r e t a i n e d a capacity to synthesize A T P from a d d e d A D P and inorganic p h o s p h a t e but c o n t i n u e d to take up oxygen at an almost m a x i m a l rate even in the a b s e n c e o f p h o s p h a t e acceptor. T h e lack o f r e s p i r a t o r y control as d e m o n s t r a t e d in vitro, c o r r e l a t e d well with the p a t i e n t ' s clinical condition as it suggested that e n e r g y g e n e r a t e d by the a b n o r m a l l y high basal o r state 4 (-ADP) respiratory activity, which a p p r o a c h e d that r e c o r d e d u n d e r state

3 (+ADP) conditions, was b e i n g w a s t e d as heat, t h e r e b y inducing a h y p e r m e t a b o l i c state. W i t h the increasing application o f c y t o c h e m i s t r y a n d E M to t h e study o f h u m a n muscle biopsies in the s e c o n d half o f the d e c a d e , it was soon realized that structurally a b n o r m a l muscle m i t o c h o n d r i a o f the type seen in Lufi's original patient, also c o n s t i t u t e d t h e m a j o r o r exclusive m o r p h o l o g i c a l change in a variety o f o t h e r myop a t h i e s that w e r e not associated with an e l e v a t e d basal m e t a b o l i c rate 2. T h e s e disorders mostly p r e s e n t e d with extraocular o r limb w e a k n e s s in infancy o r childhood. U n d u e fatiguability, e p i s o d e s o f increased w e a k n e s s o r paralysis, and a variable lactic acidaemia o c c u r r e d in s o m e cases but in o t h e r s , additional features such as g r o w t h failure, m e n t a l r e t a r d a t i o n , seizures o r c a r d i o m y o p a t h y already indicated m o r e w i d e s p r e a d organ inv o l v e m e n t . A l t h o u g h limited in-vitro m i t o c h o n d r i a l studies in s o m e o f t h e s e early cases s h o w e d defective coupling o f oxidative p h o s p h o r y l a t i o n , the significance o f this finding in the a b s e n c e

© 1986,ElsevierSciencePublishersB.V.,Amsterdam 0378- 5912/86/$02.00