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Degeneration of visual pathways in the bottlenose dolphin
346
Brain Research, 88 (1975) 346~-352 ~.~/ ElsevierScientificPublishing Company,Amsterdam - Printed in The Netherlands
Degeneration of visual pathways in the bottlenose dolphin
MYRON S. JACOBS, PETER J. MORGANE ANDWILLARDL. McFARLAND Department of Pathology, New York University, College of Dentistry, New York, N.Y. 10010, Worcester Foundation for Experimental Biology, Shrewsbury, Mass. 01545 and National Institutes of Health, Bethesda, Md. 20014 (U.S.A.)
(Accepted January23rd, 1975)
As part of a continuing study of the central nervous system of Cetacea, i.e., dolphins and great whales, we have carried out the first degeneration study of the visual system in the bottlenose dolphin, Tursiops truncatus, employing silver stains to trace anterograde axonal degeneration. Visual capabilities in Cetacea, based on quantitative examination of the optic projections, are apparently quite variable. In their observations of the rudimentary eyes of the blind river dolphin, Platanista gangetica, Herald et aL 17 noted that the optic nerves were extremely small and contained only a few hundred nerve fibers. The paucity of optic nerve fibers in P. gangetica is intriguing since this species is found only in muddy waters (Ganges, Indus Rivers), and it may be at the lower limits of visual potential amongst all dolphin and whale species. In other cetacean species, in which optic nerve fiber counts were reported by Morgane and Jacobs 26, including another of the so-called blind river dolphins, Inia geoffrensis, the numbers range from 15,500 in Inia to 420,800 in the sei whale, Balaenoptera borealis. In the bottlenose dolphin the optic nerve was found to contain 147,000 fibers. In addition to great variability in the numbers of retinofugal fibers among cetacean species, the fact that the cetacean head is generally large, with'eyes laterally placed, makes it likely that binocular vision is either greatly reduced or, possibly, absent in most cetaceans. In Tursiops, at least, some binocular vision may be possible when the eyes are brought forward and the beak is elevated. An earlier opinion by Hatschek14 that such vision was not possible in the dolphin was based on his observation of total decussation of the optic nerve across the optic chiasm in a Weigertstained series of brain stem sections from a captured animal that had lost one eye accidentally. Until the present study, his conclusion, often cited in the literature on vision in Cetacea, was never experimentally tested. It was only with the development by Nagel et aL 27 of a safe and adequate method of anesthesia for the dolphin that it became feasible to undertake major surgery in cetaceans with good assurance that the animal would survive surgery and the critical early postoperative period. In order to investigate the cetacean visual system employing silver degeneration techniques, enucleation of the left eye was carried out in a 107 kg male Tursiops truncatus measuring 2.39 m (beak-fluke length), using a nitrous oxide-oxygen mix-
347 ture previously described by Nagel et al. 27. After retraction of the globe, a clean surgical cut of the optic nerve was made and the entire globe was removed. The orbit was then packed with gel foam, the orbital area sutured closed, and the animal returned to its salt water tank following recovery from the anesthesia. The dolphin recovered uneventfully during a 14-day postoperative survival period. It was then sacrificed, under anesthesia, by perfusion through the aorta with neutral buffered formalin, and the brain was removed from the skull using a vertical milling machine. The perfused brain was post-fixed by immersion in frequent changes of fresh neutral formalin for a period of 3 months. The fixed brain weighed 1400 g. It was blocked appropriately to include the diencephalon, midbrain and pons and was then cut transversely into 1 cm thick slabs (7 pieces), each measuring approximately 9 cm latero-laterad by 6.5 cm dorso-ventrad. Each piece was immersed in a 30 % sucrose5~o formalin solution to minimize ice crystal formation during frozen sectioning and stored under refrigeration for 3 weeks. Serial frozen sections, individually stored in small, alternating groups of four 25-#m and three 40-#m thick slices, were cut using a sliding microtome in conjunction with a large thermoelectric freezing plate (Bailey Instrument Co.). Adjacent thin sections from each group were stained by the uranyl nitrate modification of the Nauta 2s method for silver impregnation of degenerating axons and by the second Fink-Heimer 9 method for terminal degeneration. Adjacent thick sections from each group were stained for cells with cresyl violet and for myelin sheaths with Weigert's hematoxylin. Although the postoperative degeneration time was quite adequate for demonstrating anterograde axonal changes by the Nauta method (Fig. 1), it was inappropriate for impregnation of degenerating axon terminals in the dolphin. Thus, sections stained by the Fink-Heimer method revealed, as with the Nauta method, axonal beading and fragmentation, but few dust-like silver profiles that could be interpreted definitely as degenerating terminals. If the time requirements for such degeneration to appear in the dolphin are similar to those reported for land mammals 1,10,33, then it is apparent that a shorter interval between surgery and sacrifice is needed for silver impregnation of degenerating boutons. Degenerating axons in the left optic nerve decussated in the optic chiasm into the contralateral optic tract and passed directly to the dorsal part of the lateral geniculate nucleus and, via the brachium of the superior colliculus, to the stratum opticum of the superior colliculus (Fig. 2). No evidence of degeneration was found in the corresponding regions on the ipsilateral side (Fig. 1). Degenerating fibers entered the lateral geniculate nucleus in a rostrocaudal sequence, appeared first in the more dorsal region and then gradually filled the ventral regions of the nucleus. As has been pointed out by Morgane and Jacobs 20, and by Kruger 21, the lateral geniculate of Tursiops occupies an extreme dorsolateral position in the thalamus that conforms to the wide bilateral flare of this animal's cerebral hemispheres. Owing, perhaps, to the complete decussation of optic nerve fibers in the dolphin, the laminar pattern of geniculate cells characteristic of bilateral retinogeniculate projections that has been described in land mammalsT, 20, especially carnivoresZ2,15, 31 and primates4,24, 25,35,3s, is absent from the cetacean brain.
349
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s.V 247
Fig. 2. Rostrocaudal series of traced transverse sections through brain of Tursiops truncatus showing course of degeneration of completely crossed retinal projections, s. III 138, at level of optic chiasm (OC); s. IV 68, at level of rostral amygdala (Am); s. IV 217, at mid-amygdalar level; s. V 97, at level of posterior commissure (PC) and caudal Am; s. V 247, at rostral level of superior colliculus (SC); s. VI 127, at level of splenium (SpCC). Other abbreviations: BSC, brachium of superior colliculus; CC, corpus callosum; CM, nucleus centrum medianum; CN, caudate nucleus; CoB, corticobulbar tract; CoP, corticopontine tract; Fo, fornix; GP, globus pallidus; Hab, habenula; IC, inferior colliculus; ICp, internal capsule; IPF, interpeduncular fossa; LG, dorsal lateral geniculate nucleus; LV, lateral ventricle; MI, massa intermedia; MG, medial geniculate nucleus; NE, nucleus ellipticus; NLL, nucleus of lateral lemniscus; ON, optic nerve; OT, optic tract; Pul, pulvinar; Put, putamen; R, reticular nucleus; StM, stria medullaris; VB, ventrobasal nucleus; VL, ventrolateral nucleus; VM, ventromedial nucleus; 3V, third ventricle.
According to Duke-Elder e, 'the stratified configuration is associated in a general sense with the partial decussation of the optic nerve fibers in the chiasma and the development of binocular vision.' It is of interest that, although the dolphin has been separated from terrestrial lines of mammalian descent for many millions of years, its geniculate displays a dorsoventral gradient of increasing neuronal cell size. One wonders whether this cytomorphological feature may represent the cetacean counterpart of the great concentration of large geniculate neurons seen in the more
Fig. 1. A - I: degeneration of visual pathway in Tursiops truncatus. A: optic chiasm showing alternation of crossing bundles of degenerating and normal optic nerve fibers. B-I are in pairs to show, in opposite sides of the same section, contralateral degeneration and ipsilateral sparing of visual structures. B and C are of optic tracts, D and E are of dorsal lateral geniculate nuclei, F and G are of brachia of superior colliculi, H and I are of superior colliculi. Figure scales both indicate 100 pm. Magnification of photomicrographs A - C shown by scale on A; magnification of D - I shown by scale on D.
350 ventral cell laminae of this nucleus in the more highly evolved land mammals, such as has been described in primates and man by LeGros Clark24, 2~ and Solnitzky and Harman aS. The functional significance of this size gradient in the dolphin remains obscure. Axonal degeneration in the optic nerve, optic chiasm (Fig. 1A), optic tract (Fig. 1B) and brachium of the superior colliculus (Fig. 1F) consisted of linear arrays of rather coarse argyrophilic fibers with a swollen, beaded and partially fragmented appearance. In the optic chiasm, bundles of fibers exhibiting this pattern of degeneration alternated with unstained retinofugal axon bundles from the intact right eye (Fig. IA). In the contralateral lateral geniculate (Fig. 1D) and superior colliculus (Fig. 1H), the degenerating axons had a diffusely plexiform organization. In the case of the geniculate, this fiber pattern may reflect the absence of cell lamination, a feature most readily apparent in cell-stained sections. No degenerating retinofugal fibers were found in the posterior commissure or passing to the pretectal region on either side as has been demonstrated in a variety of terrestrial mammals~Z,32,34. Within the pretectal region, however, we26 previously identified a well-developed olivary nucleus, which has been related through connections with the accessory optic system in other mammals3,11. The pretectal olivary nucleus also has been described in other cetacean species by Fuse 1°. Although no system of accessory optic fibers was found in the present study, it is probable that future studies may demonstrate its existence in cetaceans. The presence of substantial degeneration in the superior colliculus and its absence in the pretectal region suggest a distribution pattern of retinofugat fibers in the dolphin that more closely resembles that of primates 2,zz than of carnivores z4. Since Dawson et al. 5 recently have shown that a vigorous pupillary light response is present in Tursiops, the absence of demonstrable degenerating fibers passing to the pretectal region is puzzling. It would appear either that the retinopretectal projection in the dolphin is a multisynaptic one or, more likely, that by 14 days after eye enucleation such fibers have degenerated and been cleared from the field. In regard to the second possibility, it may be said that in view of large body size and the many technical and practical problems associated with anesthesia, surgery and handling of dolphins, only one animal could be used for the present study. It is possible that an incomplete picture of the retinofugal projections was obtained in this animal, for as Ebbesson s has pointed out, the staining selectivity of degenerating axons for heavy metals is influenced by many variables including species, time of degeneration, fiber systems affected, fiber size and temperature. While the 2-week survival used in this study was particularly effective for demonstrating relatively coarse fibers, it was probably ineffective at bringing out the majority of retinofugal fibers which are rather thinZg,30,36. Our early work is using light microscope fiber counts of the optic nerve and optic tract showed that in the bottlenose dolphin, the tract contains an average of 19 ~ fewer axons than the nerve. Since the present study suggests that only crossed fibers are present in the optic tract, then many axons must leave the pathway approximately at the chiasmatic level. Degenerating axons passing into the hypothalamic
351 region, however, were n o t found, a n d the answer to this n u m e r i c a l d i s c r e p a n c y in fibers between nerve a n d t r a c t is still unanswered. I n the absence o f evidence o f n o n - d e c u s s a t i n g r e t i n o f u g a l fibers in the p r e s e n t study, one w o n d e r s w h e t h e r Tursiops, a n d p e r h a p s o t h e r cetaceans, m a y r e p r e s e n t unique m a m m a l s in n o t c o n f o r m i n g to the hypothesis o f N e w t o n - M f i l l e r - G u d d e n 7 which indicates t h a t in m a m m a l s , u n c r o s s e d as well as crossed p r o j e c t i o n s arise f r o m each eye. I n this r e g a r d , the d o l p h i n exhibits a n a p p a r e n t l y t o t a l d e c u s s a t i o n o f o p t i c nerve fibers that, with few exceptions 13,19, is t y p i c a l o f the p a t t e r n p r e s e n t in inf r a m a m m a l i a n vertebrates~, 37. S u p p o r t e d in p a r t b y G r a n t N S 06582, N S F G r a n t G B 8066 a n d b y the N e w Y o r k Z o o l o g i c a l Society. W e t h a n k Dr. W . J. H. N a u t a a n d Dr. F. Scalia for their suggestions as well as F. L y n n Michel, G i n o M e r l i n o a n d W i l l i a m G r e a v e s for technical a n d p h o t o g r a p h i c assistance.
1 BLACKSTAD,T. W., Commissural connections of the hippocampal region in the rat, with special references to their mode of termination, J. comp. NeuroL, 105 (1956) 417-537. 2 CAmeoS-ORTEGA,J. A., AND Gt~ES, P., The subcortical distribution of optic fibers in Saimiri sciureus (squirrel monkey), 2. comp. Neurol., 131 (1967) 131-142. 3 CARPr.NTER,M. B., AND PmRSOr~,R. J., Pretectal region and the pupillary light reflex. An anatomical analysis in the monkey, J. comp. NeuroL, 149 (1973) 271-300. 4 Co~¢Ko, L. W., The laminar pattern of the lateral geniculate body in the primates, 2. Neurol. Neurosurg. Psychiat., 11 (1948) 211-219. 5 DAWSON,W. M., BmNDOgr, L. A., AND PEREZ, J. M., Gross anatomy and optics of the dolphin eye (Tursiops truncatus), Cytology, 10 (1972) 1-12. 6 DUKE-ELDER, S., The eyes of mammals. In S. DUKE-ELDER (Ed.), System of Ophthalmology, VoL 1, The Eye in Evolution, Mosby, St. Louis, Mo., 1958, pp. 429-507. 7 DUKE-ELDER, S., AND WYBAR, K. C., The visual system. In S. DUKE-ELDER (Ed.), System of Ophthalmology, Vol. 2, The Anatomy of the Visual System, Mosby, St. Louis, Mo., 1961, pp. 585-693. 8 EBBFSSON,S. O. E., The selective silver impregnation of degenerating axons and their synaptic endings in nonmammalian species. In W. J. H. NAtrrA AND S. O. E. EBB~SSON (Eds.), Contemporary Research Methods in Neuroanatomy, Springer, New York, 1970, pp. 132-161. 9 FINK, R. P., AND HEIMER, L., TWO methods for selective silver impregnation of degenerating axons and their synaptic endings in the central nervous system, Brain Research, 4 (1967) 369-374. 10 FUSE, G., 12ber einen bisher unbekannten, dem Nucleus olivaris corporis quadrigemini anterioris gleichstellbaren Kern bei einigen Delphinen, Arb. Anat. Inst. Sendai, 21 (1938) 349-357. 11 GIOLLI,R. A., AND GurrmIE, M. D., The primary optic projections in the rabbit. An experimental degeneration study, .L comp. NeuroL, 136 (1969) 99-126. 12 GUmLERY,R. W., The laminar distribution of retinal fibers in the dorsal lateral geniculate nucleus of the cat: a new interpretation, 2. comp. NeuroL, 138 (1970) 339-368. 13 HALPERN,M., Retinal projections in blind snakes, Science, 182 (1973) 390-391. 14 HATSCrmK,R., Sehnervenatrophie bei einem Delphin, Arb. Neurol. Inst. Univ. Wien, I0 (1903) 223-229. 15 HAYHOW,W. R., The cytoarchitecture of the lateral geniculate body in the cat in relation to the distribution of crossed and uncrossed optic fibers, J. comp. NeuroL, 110 (1958) 1-63. 16 HEIMER, L., Selective silver-impregnation of degenerating axoplasm. In W. J. H. NAUTA AND S. O. E. EBB~SON (Eds.), Contemporary Research Methods in Neuroanatomy, Springer, New York, 1970, pp. 106-131. 17 HERALD,E. S., BROWNELL,JR., R. L., FRYE, F. L., MORRIS,E. J., EVANS,W. E., ANDSCOTT,A. B., Blind river dolphin: first side-swimming cetacean, Science, 166 (1969) 1408-1410.
352 18 JACOBS,M. S., AND MORGANE, P. J., Retino-hypothalamic connexions in Cetacea, Nature (Lond.), 203 (1964) 778-780. 19 JAKWAY, J. S., AND RISS, W., Retinal projections in the tiger salamander, Ambystoma tigrinum, Brain Behav. Evolut., 5 (1972) 401-442. 20 KAAS, J. H., GUILLERY, R. W., AND ALLMAN, J. M., Some principles of organization in the dorsal lateral geniculate nucleus, Brain Behav. Evolut., 6 (1972) 253-299. 21 KRUGER, L., The thalamus of the dolphin (Tursiops truncatus) and comparison with other mare mals, J. eomp. Neurol., 111 (1959) 133-194. 22 LAEMLE, L. K., Retinal projections of Tupaia glis, Brain Behav. Evolut., 1 (1968) 473-499. 23 LAEMLE,L. K., AND NOBACK, C. R., The visual pathways of the lorisid lemurs (Nycticebus coucang and Galago crassicaudatus), J. comp. Neurol., 138 (1969) 49-62. 24 LEGROS CLARK, W. E., A morphological study of the lateral geniculate body, Brit. J, Ophthal., 16 (1932) 264-284. 25 LEGROS CLARK, W. E., The laminar organization and cell content of the lateral geniculate body in the monkey, J. Anat. (Lond.), 75 (1941) 419-433. 26 MORGANE, P. J., AND JACOBS, M. S., Comparative anatomy of the cetacean nervous system. In R. J. HARRISON(Ed.), Functional Anatomy of Marine Mammals, Vol. 1, Academic Press, London, 1972, pp. 117-244, 27 NAGEL, E. L., MORGANE, P. J., AND MCFARLAND,W. L., Anesthesia for the bottlenose dolphin, Tursiops truncatus, Science, 146 (1964) 1591-1593. 28 NAUTA,W. J. H., Silver impregnation of degenerating axons. In W. F. WINDLE(Ed.), New Research Techniques of Neuroanatomy, Thomas, Springfield, II1., 1957, pp. 17-26. 29 OGDEN, T. E., AND MILLER, R. F., Studies of the optic nerve of the rhesus monkey: nerve fiber spectrum and physiological properties, Vision Res., 6 (1966) 485-506. 30 POTTS, A. M., HODGES, D., SHEIMAN, C. B., FRITZ, K. J., LEVY, N. S., AND MANGNALL, Y., Morphology of the primate optic nerve. 11. Total fiber size distribution and fiber density distribution, Invest. Ophthal., I1 (1972)989-1016. 31 SANDERSON, K. J., Lamination of the dorsal lateral geniculate nucleus in carnivores of the weasel (Mustelidae), racoon (Procyonidae) and fox (Canidae) families, J. comp. Neurol., 153 (1974) 239-266. 32 SCALIA, F., The termination of retinal axons in the pretectal region of mammals, J. eomp. Neurol., 145 (1972) 223-258. 33 SCHNEIDER, G. E., Retinal projections characterized by differential rate of degeneration revealed by silver impregnation, Anat. Rec., 160 (1968) 423. 34 SINGLETON, M. C., AND PEELE, T. L., Distribution of optic fibers in the cat, J. comp. Neurol., 125 (1965) 303-328. 35 SOLNITZKY,O., AND HARMAN, P. J., A comparative study of the central and peripheral sectors of the visual cortex in primates, with observations on the lateral geniculate body, J. comp. Neurol., 85 (1946) 313-420. 36 STONE, J., AND HOLLANDER, U., Optic nerve axon diameters measured in the cat retina: some functional considerations, Exp. Brain Res., 13 (1971) 498-503. 37 WALLS, G. L., The Vertebrate Eye and its Adaptive Radiation, Hafner, New York, 1963, 785 pp. 38 WONG-RILEV,M. T. T., Neuronal and synaptic organization of the normal dorsal lateral geniculate nucleus of the squirrel monkey, Sairniri sciureus, J. comp. Neurol., 144 (1972) 25-60.
Brain Research, 88 (1975) 346~-352 ~.~/ ElsevierScientificPublishing Company,Amsterdam - Printed in The Netherlands
Degeneration of visual pathways in the bottlenose dolphin
MYRON S. JACOBS, PETER J. MORGANE ANDWILLARDL. McFARLAND Department of Pathology, New York University, College of Dentistry, New York, N.Y. 10010, Worcester Foundation for Experimental Biology, Shrewsbury, Mass. 01545 and National Institutes of Health, Bethesda, Md. 20014 (U.S.A.)
(Accepted January23rd, 1975)
As part of a continuing study of the central nervous system of Cetacea, i.e., dolphins and great whales, we have carried out the first degeneration study of the visual system in the bottlenose dolphin, Tursiops truncatus, employing silver stains to trace anterograde axonal degeneration. Visual capabilities in Cetacea, based on quantitative examination of the optic projections, are apparently quite variable. In their observations of the rudimentary eyes of the blind river dolphin, Platanista gangetica, Herald et aL 17 noted that the optic nerves were extremely small and contained only a few hundred nerve fibers. The paucity of optic nerve fibers in P. gangetica is intriguing since this species is found only in muddy waters (Ganges, Indus Rivers), and it may be at the lower limits of visual potential amongst all dolphin and whale species. In other cetacean species, in which optic nerve fiber counts were reported by Morgane and Jacobs 26, including another of the so-called blind river dolphins, Inia geoffrensis, the numbers range from 15,500 in Inia to 420,800 in the sei whale, Balaenoptera borealis. In the bottlenose dolphin the optic nerve was found to contain 147,000 fibers. In addition to great variability in the numbers of retinofugal fibers among cetacean species, the fact that the cetacean head is generally large, with'eyes laterally placed, makes it likely that binocular vision is either greatly reduced or, possibly, absent in most cetaceans. In Tursiops, at least, some binocular vision may be possible when the eyes are brought forward and the beak is elevated. An earlier opinion by Hatschek14 that such vision was not possible in the dolphin was based on his observation of total decussation of the optic nerve across the optic chiasm in a Weigertstained series of brain stem sections from a captured animal that had lost one eye accidentally. Until the present study, his conclusion, often cited in the literature on vision in Cetacea, was never experimentally tested. It was only with the development by Nagel et aL 27 of a safe and adequate method of anesthesia for the dolphin that it became feasible to undertake major surgery in cetaceans with good assurance that the animal would survive surgery and the critical early postoperative period. In order to investigate the cetacean visual system employing silver degeneration techniques, enucleation of the left eye was carried out in a 107 kg male Tursiops truncatus measuring 2.39 m (beak-fluke length), using a nitrous oxide-oxygen mix-
347 ture previously described by Nagel et al. 27. After retraction of the globe, a clean surgical cut of the optic nerve was made and the entire globe was removed. The orbit was then packed with gel foam, the orbital area sutured closed, and the animal returned to its salt water tank following recovery from the anesthesia. The dolphin recovered uneventfully during a 14-day postoperative survival period. It was then sacrificed, under anesthesia, by perfusion through the aorta with neutral buffered formalin, and the brain was removed from the skull using a vertical milling machine. The perfused brain was post-fixed by immersion in frequent changes of fresh neutral formalin for a period of 3 months. The fixed brain weighed 1400 g. It was blocked appropriately to include the diencephalon, midbrain and pons and was then cut transversely into 1 cm thick slabs (7 pieces), each measuring approximately 9 cm latero-laterad by 6.5 cm dorso-ventrad. Each piece was immersed in a 30 % sucrose5~o formalin solution to minimize ice crystal formation during frozen sectioning and stored under refrigeration for 3 weeks. Serial frozen sections, individually stored in small, alternating groups of four 25-#m and three 40-#m thick slices, were cut using a sliding microtome in conjunction with a large thermoelectric freezing plate (Bailey Instrument Co.). Adjacent thin sections from each group were stained by the uranyl nitrate modification of the Nauta 2s method for silver impregnation of degenerating axons and by the second Fink-Heimer 9 method for terminal degeneration. Adjacent thick sections from each group were stained for cells with cresyl violet and for myelin sheaths with Weigert's hematoxylin. Although the postoperative degeneration time was quite adequate for demonstrating anterograde axonal changes by the Nauta method (Fig. 1), it was inappropriate for impregnation of degenerating axon terminals in the dolphin. Thus, sections stained by the Fink-Heimer method revealed, as with the Nauta method, axonal beading and fragmentation, but few dust-like silver profiles that could be interpreted definitely as degenerating terminals. If the time requirements for such degeneration to appear in the dolphin are similar to those reported for land mammals 1,10,33, then it is apparent that a shorter interval between surgery and sacrifice is needed for silver impregnation of degenerating boutons. Degenerating axons in the left optic nerve decussated in the optic chiasm into the contralateral optic tract and passed directly to the dorsal part of the lateral geniculate nucleus and, via the brachium of the superior colliculus, to the stratum opticum of the superior colliculus (Fig. 2). No evidence of degeneration was found in the corresponding regions on the ipsilateral side (Fig. 1). Degenerating fibers entered the lateral geniculate nucleus in a rostrocaudal sequence, appeared first in the more dorsal region and then gradually filled the ventral regions of the nucleus. As has been pointed out by Morgane and Jacobs 20, and by Kruger 21, the lateral geniculate of Tursiops occupies an extreme dorsolateral position in the thalamus that conforms to the wide bilateral flare of this animal's cerebral hemispheres. Owing, perhaps, to the complete decussation of optic nerve fibers in the dolphin, the laminar pattern of geniculate cells characteristic of bilateral retinogeniculate projections that has been described in land mammalsT, 20, especially carnivoresZ2,15, 31 and primates4,24, 25,35,3s, is absent from the cetacean brain.
349
I ,\....'%.) ~III 18B OO
7O.
s , ~
s.V 247
Fig. 2. Rostrocaudal series of traced transverse sections through brain of Tursiops truncatus showing course of degeneration of completely crossed retinal projections, s. III 138, at level of optic chiasm (OC); s. IV 68, at level of rostral amygdala (Am); s. IV 217, at mid-amygdalar level; s. V 97, at level of posterior commissure (PC) and caudal Am; s. V 247, at rostral level of superior colliculus (SC); s. VI 127, at level of splenium (SpCC). Other abbreviations: BSC, brachium of superior colliculus; CC, corpus callosum; CM, nucleus centrum medianum; CN, caudate nucleus; CoB, corticobulbar tract; CoP, corticopontine tract; Fo, fornix; GP, globus pallidus; Hab, habenula; IC, inferior colliculus; ICp, internal capsule; IPF, interpeduncular fossa; LG, dorsal lateral geniculate nucleus; LV, lateral ventricle; MI, massa intermedia; MG, medial geniculate nucleus; NE, nucleus ellipticus; NLL, nucleus of lateral lemniscus; ON, optic nerve; OT, optic tract; Pul, pulvinar; Put, putamen; R, reticular nucleus; StM, stria medullaris; VB, ventrobasal nucleus; VL, ventrolateral nucleus; VM, ventromedial nucleus; 3V, third ventricle.
According to Duke-Elder e, 'the stratified configuration is associated in a general sense with the partial decussation of the optic nerve fibers in the chiasma and the development of binocular vision.' It is of interest that, although the dolphin has been separated from terrestrial lines of mammalian descent for many millions of years, its geniculate displays a dorsoventral gradient of increasing neuronal cell size. One wonders whether this cytomorphological feature may represent the cetacean counterpart of the great concentration of large geniculate neurons seen in the more
Fig. 1. A - I: degeneration of visual pathway in Tursiops truncatus. A: optic chiasm showing alternation of crossing bundles of degenerating and normal optic nerve fibers. B-I are in pairs to show, in opposite sides of the same section, contralateral degeneration and ipsilateral sparing of visual structures. B and C are of optic tracts, D and E are of dorsal lateral geniculate nuclei, F and G are of brachia of superior colliculi, H and I are of superior colliculi. Figure scales both indicate 100 pm. Magnification of photomicrographs A - C shown by scale on A; magnification of D - I shown by scale on D.
350 ventral cell laminae of this nucleus in the more highly evolved land mammals, such as has been described in primates and man by LeGros Clark24, 2~ and Solnitzky and Harman aS. The functional significance of this size gradient in the dolphin remains obscure. Axonal degeneration in the optic nerve, optic chiasm (Fig. 1A), optic tract (Fig. 1B) and brachium of the superior colliculus (Fig. 1F) consisted of linear arrays of rather coarse argyrophilic fibers with a swollen, beaded and partially fragmented appearance. In the optic chiasm, bundles of fibers exhibiting this pattern of degeneration alternated with unstained retinofugal axon bundles from the intact right eye (Fig. IA). In the contralateral lateral geniculate (Fig. 1D) and superior colliculus (Fig. 1H), the degenerating axons had a diffusely plexiform organization. In the case of the geniculate, this fiber pattern may reflect the absence of cell lamination, a feature most readily apparent in cell-stained sections. No degenerating retinofugal fibers were found in the posterior commissure or passing to the pretectal region on either side as has been demonstrated in a variety of terrestrial mammals~Z,32,34. Within the pretectal region, however, we26 previously identified a well-developed olivary nucleus, which has been related through connections with the accessory optic system in other mammals3,11. The pretectal olivary nucleus also has been described in other cetacean species by Fuse 1°. Although no system of accessory optic fibers was found in the present study, it is probable that future studies may demonstrate its existence in cetaceans. The presence of substantial degeneration in the superior colliculus and its absence in the pretectal region suggest a distribution pattern of retinofugat fibers in the dolphin that more closely resembles that of primates 2,zz than of carnivores z4. Since Dawson et al. 5 recently have shown that a vigorous pupillary light response is present in Tursiops, the absence of demonstrable degenerating fibers passing to the pretectal region is puzzling. It would appear either that the retinopretectal projection in the dolphin is a multisynaptic one or, more likely, that by 14 days after eye enucleation such fibers have degenerated and been cleared from the field. In regard to the second possibility, it may be said that in view of large body size and the many technical and practical problems associated with anesthesia, surgery and handling of dolphins, only one animal could be used for the present study. It is possible that an incomplete picture of the retinofugal projections was obtained in this animal, for as Ebbesson s has pointed out, the staining selectivity of degenerating axons for heavy metals is influenced by many variables including species, time of degeneration, fiber systems affected, fiber size and temperature. While the 2-week survival used in this study was particularly effective for demonstrating relatively coarse fibers, it was probably ineffective at bringing out the majority of retinofugal fibers which are rather thinZg,30,36. Our early work is using light microscope fiber counts of the optic nerve and optic tract showed that in the bottlenose dolphin, the tract contains an average of 19 ~ fewer axons than the nerve. Since the present study suggests that only crossed fibers are present in the optic tract, then many axons must leave the pathway approximately at the chiasmatic level. Degenerating axons passing into the hypothalamic
351 region, however, were n o t found, a n d the answer to this n u m e r i c a l d i s c r e p a n c y in fibers between nerve a n d t r a c t is still unanswered. I n the absence o f evidence o f n o n - d e c u s s a t i n g r e t i n o f u g a l fibers in the p r e s e n t study, one w o n d e r s w h e t h e r Tursiops, a n d p e r h a p s o t h e r cetaceans, m a y r e p r e s e n t unique m a m m a l s in n o t c o n f o r m i n g to the hypothesis o f N e w t o n - M f i l l e r - G u d d e n 7 which indicates t h a t in m a m m a l s , u n c r o s s e d as well as crossed p r o j e c t i o n s arise f r o m each eye. I n this r e g a r d , the d o l p h i n exhibits a n a p p a r e n t l y t o t a l d e c u s s a t i o n o f o p t i c nerve fibers that, with few exceptions 13,19, is t y p i c a l o f the p a t t e r n p r e s e n t in inf r a m a m m a l i a n vertebrates~, 37. S u p p o r t e d in p a r t b y G r a n t N S 06582, N S F G r a n t G B 8066 a n d b y the N e w Y o r k Z o o l o g i c a l Society. W e t h a n k Dr. W . J. H. N a u t a a n d Dr. F. Scalia for their suggestions as well as F. L y n n Michel, G i n o M e r l i n o a n d W i l l i a m G r e a v e s for technical a n d p h o t o g r a p h i c assistance.
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