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“Lens Mitochondria” in the Retinal Cones of the Tree-shrew Tupaia belangeri
VisionRes., Vol.
Pergamon
Copyright 01996
PII: SO042-6989(96)00199-X
37, No. 3, pp. 267–271, 1997 Elsevier Science Ltd. All rights reserved Printed in GreatBritain
0042-6989/97 $17.00+ 0.00
Research Note “Lens Mitochondria” in the Retinal Cones of the Tree-shrew Tupaia belangeri WOLFGANG KNABE,* SERGUEI SKATCHKOV,~HANS-J~G KUHN*$ Received 26 February 1996; in revisedform 10 July 1996 In all mammals, the mitochondria of the cones of the retina are concentrated in the inner segment. Uniquely in tree-shrews (Tupaiu, Scandentia, Mammalia), a “megamitochondrion” exhibiting highly specialized systems of densely packed cristae and a very electron dense matrix, is located apically in the inner segment. The ellipsoid is a solid body containing several megamitochondria and, towards its base, a large number of smaller mitochondria. The refractive index of isolated, but not oriented, inner segments of Tupaia belangeri is higher (xA= 1.405) than in any other mammal studied so far. The consistent geometrical pattern of the multilamellar crista-matrix systems, oriented longitudinally towards the outer segment, suggests an additional optical function of the megamitochondria. Copyright 01996 Elsevier Science Ltd
Retina Coneinnersegment =mitochondria
Refraction Tupaia belangeri
been tied off from the finger-likeprocesses.A high Na+– K+-ATPase-activity has been found at the plasma membrane of the inner segments of the cones in several vertebrates (Stirling & Sarthy, 1985; Yazulla & Studholme, 1987; Madreperla et al., 1989; Adler, 1993). Providedthat a similardistributionpattern of this enzyme existsin the cones of Tupaia, itseems reasonableto relate the constant finding of many small mitochondria in the vicinity of the plasma membrane to the energy requirement of the cells. Small mitochondriahave a favorable surface-to-volumeratio comparedto the centrally located megamitochondrialmass. The characteristic distribution of small mitochondria and megamitochondria in the retinal cones of adult Tupaia is establishedwhen young Tupaia open their eyes at the age of 17 days (Knabe, 1994) and during improvementof visual acuity, which occurs rapidly after eye opening (Hertenstein et al., 1988). The megamitochondria of Tupaia are characterized, when compared to the mitochondriaof the neighboring Miillercells and to the mitochondriaof the cones of other mammals, by their electron dense matrix containing many mitochondrial granules, as seen in transmission electron microscopy (Figs 1, 2). In some teleost fishes, Nag and Bhattacharjee(1995) noted the unusually dense matrix of the single apical ellipsosomes, which are derived from inner segment mitochondria. A “curious *AnatomischesInstitut der Georg-August-Universitat,Kreuzbergring photostablepigment” was found in the inner segment of 36, D-37075 Gottingen,Germany. Tupaia, close to the base of the outer segment (Bow~Laboratory of Neurobiology, University of Puerto Rico, San Juan, maker et al., 1991 p. 11). This pigment seems to be Puerto Rico. identicalwith or similar to pigments in a wide variety of $To whom all correspondence should be addressed [Fax: +49-551species, e.g. within the ellipsosomesof fish (MacNichol 397043].
Besides the cones of two functional types (Miiller & Peichl, 1989;Petry & H&-osi,1990;Petry et al., 1993)the retina of the tree-shrew Tupaia contains only a small number of rods (Kuhne, 1983; Muller & Peichl, 1989). Generally, in the adult Tupaia, the large body of a single megamitochondrion, about 5 pm in diameter, fills the apicalportion of each cone inner segment(Figs 1, 2). The apical megamitochondrionsits on top of between three and six closely packed megamitochondria.Together the megamitochondriaform a solid mass that fillsthe conical upper region of the inner segment and, widening towards the base of the inner segment, comes close to neighboring cones [Fig.2(a)]. In a study on the developmentof the megamitochondria in Tupaia belangeri (Knabe, 1994; Knabe & Kuhn, 1996;who also describeMethodsfor the present study), it was demonstrated that these ultrastructurally specialized organelles arise by growth of single mitochondria, and not, as was suggested earlier (Foelix et al., 1987), by fusion of smaller mitochondria. From their bodies the megamitochondriaemit finger-like processes stretching towards the nucleus of the cones, primarily in the vicinity of the plasma membrane of the inner segments (Fig. 3). In sections, these processes closely resemble unspecialized mitochondria. Additionally, towards the base of the inner segments, there are many isolatedsmallermitochondriawhich appearto have
267
268
RESEARCHNOTE
FIGURE 1. Transverse section of the apical megamitochondrionin the inner segment of a cone. Central retina of an adult Tupaiabelangeri.Plates of four or five cristae are longitudinallyoriented towardsthe outer segment and, in transverse sections, traverse the megamitochondrioneither in a wavy or in a plane fashion. pe, processes of pigmentepithelial cells; cc, connecting cilium. Transmissionelectron micrograph, scale 1 pm.
et al., 1978) or the mitochondria of catarrhine primates (Bowm.akeret al., 1991).Whether and to what extent this pigment contributes to the dense matrix in Tupaia is unknown. Possibly, this matrix and the tightly packed cristae are responsible for the stability of the megamitochondria of Tupaia. On two occasions, a smaller residualbody was found instead of a megamitochondrion which must have existed there and left a cytosole-tilled exact ghostof the originalmitochondrionborderedby the megamitochondriain the vicinity (Knabe, 1994; Knabe & Kuhn, 1996). The megamitochondriaof Tupaia are filledwith tightly packed cristae in geometrical patterns, forming tubes, funnels, waves, and plane stacks. Where their outer membranes are immediately adjacent to each other, the orientation of the cristae is often carried over from one mitochondrion to the next. The tubes are oriented longitudinally from the base towards the outer segment of each cone. In transverse sections of the single apical megamitochondrion,the parallel bundles of wavy cristae crossing the maximum diameter of the mitochondrion revealed several remarkable features (Fig. 1). In three adult Tupaia we found (a) an almost constant maximum amplitude (0.31 pm) for each wave; (b) an almost constant distance between two maxima (0.6 pm); and (c) an almost constant intercristal distance (0.035 pm). The light penetrating the inner segment toward the outer segment is propagated along stacks of distinct strata:
membranes of the cristae, electron dense matrix of the mitochondrion and, in the transmission electron microscope, clear intracristal space. The optics of such a system needs further clarification. The bodies of between four and seven megamitochondria fill the upper part and the central core of the inner segment(Figs 2, 3). The interveningcytosol is reduced to a minimum. Therefore, the metabolically active surface of this assemblage is small in relation to the volume as well as to the high density of cristae of the megamitochondria.Should the megamitochondriain the inner segments of Tupaia merely indicate a high metabolic activity (Samorajski et al., 1966; Foelix et al., 1987), their unique size and unique geometrical pattern of cristae would remain unexplained. Based on microwave models of photoreceptorcells Govardovskiiet al. (1981) provided evidence, by analogy, for accessory optical functions of the mitochondriaof the cone ellipsoid. The maximum diameter of the inner segments of the cones of Tupaia is much wider than the diameter of the outer segments(Figs 2, 4). Therefore, the outer segments are not denselypacked, as in the retina of most mammals rich in rods, but widely spaced. In a transverse section parallel to the outer limiting membrane, only about onequarter to one-third of the cut surface is taken up by the outer segments (Fig. 2). Processes of the pigment epitheliumsproject deep into the spaces between them (Figs 2, 4). This means that a considerablepercentage of
RESEARCHNOTE
269
(a)
(b)
(c)
FIGURE2. Transmissionelectron micrographsof cones from the central retina of an adult Tupaiabelangeri. (a) Longitudinal section of cone outer (COS)and inner segments. Mitochondriaare concentrated in the ellipsoid (e) of the inner segment. Each ellipsoidcontains a single megamitochondrion(mm) at the top, close to the outer segment,Outer segmentsand the apical half of the ellipsoids are surroundedby processes of the pigment epithelial cells (pe). Scale 1 pm. (b) Transverse section of inner segments at a level where only a few microvilli of the pigment epitheliumsare found between them. Each cone contains one apical megamitochondrion.(c) Transverse section of outer segments widely separated by processes of the pigment epithelial cells.
the light penetrating the inner segment would miss the effects, which have been postulated for the cones receptors in the outer segment, if the light were not (Horowitz, 1981) do, indeed, occur within the bodies of refracted stronglytowards the central longitudinalaxis of the megamitochondriaof Tupaia: both the inner and the outer segment. 1. Many of the unique structural properties of the In 11 cone inner segments of Tupaia belangeri, the megamitochondria are not sufficiently well exrefractive index was determined by phase contrast plained by the known metabolic functions of microscopy (for method, see Sidman, 1957). The index mitochondria. varied from 1.387 to 1.419,the average value was 1.405. 2. There is evidence that membrane bound “oil In comparison to other mammals (Sidman, 1957; droplets” as well as double membrane bound Borwein, 1981), this is the highest average refractive index of cone inner segments measured so far. However, “ellipsosomes” of many vertebrates originate from the method did not take into accountthe orientationof the mitochondria (Pedler & Tansley, 1963; Berger, lamellar system of the cristae. So far, it has not been 1966). Oil droplets definitivelyact as cut-off filters possible to measure refraction exactly along the long(Wolbarsht, 1976; Jane & Bowmaker, 1988) and, itudinal axis of the inner segments of Tupaia. additionally, inner segments with oil droplets are There are several indications, albeit teleological in known to function as lenses collecting the light for part, that higher refraction and pronounced wave-guide the receptors in the outer segments (Baylor &
270
RESEARCHNOTE
FIGURE3. Groupof five megamitochondriain the apical portion of the inner segment of a 17-day-oldTupaiabelangeri.The bodies of the megamitochondriaoccupy the central longitudinalcore of the ellipsoid. From the voluminousbody of the single apical megamitochondrion(mm), several finger-likeprocesses (arrows) project, close to the plasma membrane (pm), towards the base (b) of the inner segment. The cone outer segment (COS),partially embedded in the depression at the top of the inner segment and the apical megamitochondrion, is connected to the inner segment by the cilium (cc). Three-dimensional reconstructionfrom 100 serial ultrathin sections (50 rim). x22,000.
FIGURE4. Scanningelectron micrographof a groupof cone inner segmentsand adjacent pigment epitheliums.Central retina of an adult Tupaiabelangeri.Clusteredprocesses of the pigmentepithelial cells (pe) completelyhide the cone outer segments and occupy the spaces between the apical portions of the cone inner segments (cis) carrying the apical megamitochondrion. Microvilli of the pigment epitheliumsare broken off from the cone on the right, therefore, the outer segment is visible (arrow). Scale 2 flm.
RESEARCHNOTE
Fettiplace, 1975; Young & Martin, 1984). If mitochondriawere alwaysopticallyinert, selectivity for evolutionary optimization into megamitochondria, ellipsosomes or oil droplets could not have occurred. 3. Although it is, in principle, diurnal and despite its cone-dominated retina Tupaia belangeri can be quite active at night (Aue, 1988),when it probably depends on residual light for orientation. Without strong refractionof the light towardsthe receptorsin the outer segments,most of the lightwould be lost in the spaces between the outer segments and be absorbed in the melanin granules within the processes of the pigment epitheliums.This would be an unsatisfactorysituation for the organism. For these reasons, we would like to suggest that the unique size, dense matrix and specialized multilamellar system of cristae of the highly refractive megamitochondria of Tupaia have accessory optical functions, chanelling the light towards the receptorsin the outer segments. REFERENCES
271
Jane, S. D. & Bowmaker,J. K. (1988).Tetrachromaticcolour vision in the duck (Anapkujv-hynchosL.): microspectrophotometryof visual pigments and oil droplets. Journal of Comparative Physiology A 162, 225-235. Knabe, W. (1994).Die Morphogeneseder Megamitochondrienin den Zapfen von Tupaia belangeri. Doctoral thesis (medicine), Gottingen. Knabe, W. & Kuhn, H.-J. (1996). Morphogenesisof megamitochondria in the retinal cone inner segments of Tupaia belangeri (Scandentia).Cell and TissueResearch, 285, 1-9. Kiihne, J.-H. (1983). Rod receptors in the retina of Tupaia belangeri. Anatomy and Embryology, 167, 95–102. MacNichol,E. F., Kunz,Y. W., Levine,J. S., Htirosi,F. I. & Collins,B. A. (1978). Ellipsosomes: Organelles containing a cytochrome-like pigmentin the retinal cones of certain fishes.Science, 200, 549–552. Madreperla, S. A., Edidin, M. & Adler, R. (1989). Na + –K+ adenosine triphosphatase polarity in retinal photoreceptors: a role for cytoskeletal attachments. Journal of Cell Biology, 109, 1483– 1493. Mtiller, B. & Peichl, L. (1989). Topographyof cones and rods in the tree shrew retina.Journal of ComparativeNeurology,282, 581–594. Nag, T. C. & Bhattacharjee, J. (1995). Retinal ellipsosomes: morphology,development, identification, and comparison with oil droplets. Cell and Tissue Research, 279, 633437. Pedler, C. & Tansley, K. (1963). The fine structure of the cone of a diurnal gecko (Phelsurnairrunguis).Experimental Eye Research, 2, 39A7. Petry, H. M., Erichsen, J. T. & Szel, A. (1993). Immunocytochemical identificationof photoreceptorpopulations in the tree shrew retina. Brain Research, 616, 344-350. Petry, H. M. & H6rosi,F. L (1990). Visual pigments of the tree shrew (Tupaia belangeri) and greater galago (Galago crassicaudatus):a microspectrophotometricinvestigation. VisionResearch, 306, 839– 851. Samorajski, T., Ordy, J. M. & Keefe, J. R. (1966). Structural organization of the retina in the tree shrew (Tupaia glis). Journal of Cell Biology, 28, 489–504. Sidman, R. L. (1957). The structure and concentration of solids in photoreceptor cells studied by refractometry and interference microscopy. Journal of Biophysical and Biochemical Cytology, 31, 15–30. Stirling, C. E. & Sarthy, P. V. (1985).Localization of the Na–K pump in turtle retina. Journal of Neurocytology, 14, 33-47. Wolbarsht,M. L. (1976).The functionof intraocularfilters.Federation Proceedings, 351, 44-50. Yazulla, S. & Studholme,K. M. (1987).Ultracytochemicaldistribution of ouabain-sensitive, K+-dependent, p-nitrophenylphosphatasein the synaptic layers of goldfish retina. Journal of Comparative Neurology, 261, 74-84. Young, S. R. & Martin, G. R. (1984). Optics of retinal oil droplets: a model of light collection and polarization detection in the avian retina. VisionResearch, 24, 129–137.
Adler, R. (1993). Plasticity and differentiation of retinal precursor cells. InternationalReview of Cytology, 146, 145–190. Aue, D. (1988). Konfrontationenzwischen mannlichenSpitzhornchen (Tupaiabelangeri):Konsequenzender Sozialkontaktefur Verhalten und Physiologic sowie der Einfluss individueller und ausserer Faktoren auf die Dominanzentscheidungen.Doctoral thesis (biology), GOttingen. Baylor, D.A. & Fettiplace, R. (1975).Light path and photoncapture in turtle photoreceptors.Journal of Hrysiology, 248, 433-464. Berger, E. R. (1966). On the mitochondrialorigin of oil drops in the retinal double cone inner segments. Journal of Ultrastructure Research, 14, 143-157. Borwein, B. (1981). The retinal receptor: A description. In Enoch, J. M. & Tobey, F. L. (Eds), Vertebratephotoreceptor optics (pp. 11– 81). Berlin–Heidelberg-NewYork: Springer. Bowmaker, J. K., Astell, S., Hunt, D. M. & Mellon, J. D. (1991). Photosensitiveand photostablepigments in the retinae of old world monkeys.Journal of ExperimentalBiology, 156, 1–19. Foelix, R. F., Kretz, R. & Rager, G. (1987). Structure and postnatal developmentof photoreceptorsand their synapsesin the retina of the tree shrew (Tupaiabelangeri). Cell and TissueResearch, 247, 287– 297. Govardovskii,V. I., Golovanevskii,E. L, Zueva, L. V. & Vasileva, 1. L. (1981).The role of cellular organoidsin the photoreceptoroptics (studies on microwave models) (in Russian). Journal of Evolutionary Biochemistry and Physiology, 17, 492497. Hertenstein, B., Zimmerman, E. & Rahmann, H. (1988). The development of visual acuity in treeshrews (Tupaia belangeri). Acknowledgements-We thank Dr Peter Schwartz, Gottingen, for Zeitschriflfiir Siiugetierkundej53, 29&300. scanning electron microscopy, Natalia Mojaiskaia (St Petersburg,/ Horowitz, B. R. (1981). Theoretical considerations of the retinal Russia), Renate Rasch and Irmgard Weiss for technical assistance, receptor as a waveguide. In Enoch, J. M. & Tobey, F. L. (Eds), Cyrilla Maelicke for revising the English manuscript, Dr Andreas Vertebratephotoreceptor optics (pp. 219–300).Berlin–Heidelberg– Reichenbachand Jens Grosche(Leipzig), as well as the two referees of New York: Springer. VisionResearch for helpful criticism and suggestions.
Pergamon
Copyright 01996
PII: SO042-6989(96)00199-X
37, No. 3, pp. 267–271, 1997 Elsevier Science Ltd. All rights reserved Printed in GreatBritain
0042-6989/97 $17.00+ 0.00
Research Note “Lens Mitochondria” in the Retinal Cones of the Tree-shrew Tupaia belangeri WOLFGANG KNABE,* SERGUEI SKATCHKOV,~HANS-J~G KUHN*$ Received 26 February 1996; in revisedform 10 July 1996 In all mammals, the mitochondria of the cones of the retina are concentrated in the inner segment. Uniquely in tree-shrews (Tupaiu, Scandentia, Mammalia), a “megamitochondrion” exhibiting highly specialized systems of densely packed cristae and a very electron dense matrix, is located apically in the inner segment. The ellipsoid is a solid body containing several megamitochondria and, towards its base, a large number of smaller mitochondria. The refractive index of isolated, but not oriented, inner segments of Tupaia belangeri is higher (xA= 1.405) than in any other mammal studied so far. The consistent geometrical pattern of the multilamellar crista-matrix systems, oriented longitudinally towards the outer segment, suggests an additional optical function of the megamitochondria. Copyright 01996 Elsevier Science Ltd
Retina Coneinnersegment =mitochondria
Refraction Tupaia belangeri
been tied off from the finger-likeprocesses.A high Na+– K+-ATPase-activity has been found at the plasma membrane of the inner segments of the cones in several vertebrates (Stirling & Sarthy, 1985; Yazulla & Studholme, 1987; Madreperla et al., 1989; Adler, 1993). Providedthat a similardistributionpattern of this enzyme existsin the cones of Tupaia, itseems reasonableto relate the constant finding of many small mitochondria in the vicinity of the plasma membrane to the energy requirement of the cells. Small mitochondriahave a favorable surface-to-volumeratio comparedto the centrally located megamitochondrialmass. The characteristic distribution of small mitochondria and megamitochondria in the retinal cones of adult Tupaia is establishedwhen young Tupaia open their eyes at the age of 17 days (Knabe, 1994) and during improvementof visual acuity, which occurs rapidly after eye opening (Hertenstein et al., 1988). The megamitochondria of Tupaia are characterized, when compared to the mitochondriaof the neighboring Miillercells and to the mitochondriaof the cones of other mammals, by their electron dense matrix containing many mitochondrial granules, as seen in transmission electron microscopy (Figs 1, 2). In some teleost fishes, Nag and Bhattacharjee(1995) noted the unusually dense matrix of the single apical ellipsosomes, which are derived from inner segment mitochondria. A “curious *AnatomischesInstitut der Georg-August-Universitat,Kreuzbergring photostablepigment” was found in the inner segment of 36, D-37075 Gottingen,Germany. Tupaia, close to the base of the outer segment (Bow~Laboratory of Neurobiology, University of Puerto Rico, San Juan, maker et al., 1991 p. 11). This pigment seems to be Puerto Rico. identicalwith or similar to pigments in a wide variety of $To whom all correspondence should be addressed [Fax: +49-551species, e.g. within the ellipsosomesof fish (MacNichol 397043].
Besides the cones of two functional types (Miiller & Peichl, 1989;Petry & H&-osi,1990;Petry et al., 1993)the retina of the tree-shrew Tupaia contains only a small number of rods (Kuhne, 1983; Muller & Peichl, 1989). Generally, in the adult Tupaia, the large body of a single megamitochondrion, about 5 pm in diameter, fills the apicalportion of each cone inner segment(Figs 1, 2). The apical megamitochondrionsits on top of between three and six closely packed megamitochondria.Together the megamitochondriaform a solid mass that fillsthe conical upper region of the inner segment and, widening towards the base of the inner segment, comes close to neighboring cones [Fig.2(a)]. In a study on the developmentof the megamitochondria in Tupaia belangeri (Knabe, 1994; Knabe & Kuhn, 1996;who also describeMethodsfor the present study), it was demonstrated that these ultrastructurally specialized organelles arise by growth of single mitochondria, and not, as was suggested earlier (Foelix et al., 1987), by fusion of smaller mitochondria. From their bodies the megamitochondriaemit finger-like processes stretching towards the nucleus of the cones, primarily in the vicinity of the plasma membrane of the inner segments (Fig. 3). In sections, these processes closely resemble unspecialized mitochondria. Additionally, towards the base of the inner segments, there are many isolatedsmallermitochondriawhich appearto have
267
268
RESEARCHNOTE
FIGURE 1. Transverse section of the apical megamitochondrionin the inner segment of a cone. Central retina of an adult Tupaiabelangeri.Plates of four or five cristae are longitudinallyoriented towardsthe outer segment and, in transverse sections, traverse the megamitochondrioneither in a wavy or in a plane fashion. pe, processes of pigmentepithelial cells; cc, connecting cilium. Transmissionelectron micrograph, scale 1 pm.
et al., 1978) or the mitochondria of catarrhine primates (Bowm.akeret al., 1991).Whether and to what extent this pigment contributes to the dense matrix in Tupaia is unknown. Possibly, this matrix and the tightly packed cristae are responsible for the stability of the megamitochondria of Tupaia. On two occasions, a smaller residualbody was found instead of a megamitochondrion which must have existed there and left a cytosole-tilled exact ghostof the originalmitochondrionborderedby the megamitochondriain the vicinity (Knabe, 1994; Knabe & Kuhn, 1996). The megamitochondriaof Tupaia are filledwith tightly packed cristae in geometrical patterns, forming tubes, funnels, waves, and plane stacks. Where their outer membranes are immediately adjacent to each other, the orientation of the cristae is often carried over from one mitochondrion to the next. The tubes are oriented longitudinally from the base towards the outer segment of each cone. In transverse sections of the single apical megamitochondrion,the parallel bundles of wavy cristae crossing the maximum diameter of the mitochondrion revealed several remarkable features (Fig. 1). In three adult Tupaia we found (a) an almost constant maximum amplitude (0.31 pm) for each wave; (b) an almost constant distance between two maxima (0.6 pm); and (c) an almost constant intercristal distance (0.035 pm). The light penetrating the inner segment toward the outer segment is propagated along stacks of distinct strata:
membranes of the cristae, electron dense matrix of the mitochondrion and, in the transmission electron microscope, clear intracristal space. The optics of such a system needs further clarification. The bodies of between four and seven megamitochondria fill the upper part and the central core of the inner segment(Figs 2, 3). The interveningcytosol is reduced to a minimum. Therefore, the metabolically active surface of this assemblage is small in relation to the volume as well as to the high density of cristae of the megamitochondria.Should the megamitochondriain the inner segments of Tupaia merely indicate a high metabolic activity (Samorajski et al., 1966; Foelix et al., 1987), their unique size and unique geometrical pattern of cristae would remain unexplained. Based on microwave models of photoreceptorcells Govardovskiiet al. (1981) provided evidence, by analogy, for accessory optical functions of the mitochondriaof the cone ellipsoid. The maximum diameter of the inner segments of the cones of Tupaia is much wider than the diameter of the outer segments(Figs 2, 4). Therefore, the outer segments are not denselypacked, as in the retina of most mammals rich in rods, but widely spaced. In a transverse section parallel to the outer limiting membrane, only about onequarter to one-third of the cut surface is taken up by the outer segments (Fig. 2). Processes of the pigment epitheliumsproject deep into the spaces between them (Figs 2, 4). This means that a considerablepercentage of
RESEARCHNOTE
269
(a)
(b)
(c)
FIGURE2. Transmissionelectron micrographsof cones from the central retina of an adult Tupaiabelangeri. (a) Longitudinal section of cone outer (COS)and inner segments. Mitochondriaare concentrated in the ellipsoid (e) of the inner segment. Each ellipsoidcontains a single megamitochondrion(mm) at the top, close to the outer segment,Outer segmentsand the apical half of the ellipsoids are surroundedby processes of the pigment epithelial cells (pe). Scale 1 pm. (b) Transverse section of inner segments at a level where only a few microvilli of the pigment epitheliumsare found between them. Each cone contains one apical megamitochondrion.(c) Transverse section of outer segments widely separated by processes of the pigment epithelial cells.
the light penetrating the inner segment would miss the effects, which have been postulated for the cones receptors in the outer segment, if the light were not (Horowitz, 1981) do, indeed, occur within the bodies of refracted stronglytowards the central longitudinalaxis of the megamitochondriaof Tupaia: both the inner and the outer segment. 1. Many of the unique structural properties of the In 11 cone inner segments of Tupaia belangeri, the megamitochondria are not sufficiently well exrefractive index was determined by phase contrast plained by the known metabolic functions of microscopy (for method, see Sidman, 1957). The index mitochondria. varied from 1.387 to 1.419,the average value was 1.405. 2. There is evidence that membrane bound “oil In comparison to other mammals (Sidman, 1957; droplets” as well as double membrane bound Borwein, 1981), this is the highest average refractive index of cone inner segments measured so far. However, “ellipsosomes” of many vertebrates originate from the method did not take into accountthe orientationof the mitochondria (Pedler & Tansley, 1963; Berger, lamellar system of the cristae. So far, it has not been 1966). Oil droplets definitivelyact as cut-off filters possible to measure refraction exactly along the long(Wolbarsht, 1976; Jane & Bowmaker, 1988) and, itudinal axis of the inner segments of Tupaia. additionally, inner segments with oil droplets are There are several indications, albeit teleological in known to function as lenses collecting the light for part, that higher refraction and pronounced wave-guide the receptors in the outer segments (Baylor &
270
RESEARCHNOTE
FIGURE3. Groupof five megamitochondriain the apical portion of the inner segment of a 17-day-oldTupaiabelangeri.The bodies of the megamitochondriaoccupy the central longitudinalcore of the ellipsoid. From the voluminousbody of the single apical megamitochondrion(mm), several finger-likeprocesses (arrows) project, close to the plasma membrane (pm), towards the base (b) of the inner segment. The cone outer segment (COS),partially embedded in the depression at the top of the inner segment and the apical megamitochondrion, is connected to the inner segment by the cilium (cc). Three-dimensional reconstructionfrom 100 serial ultrathin sections (50 rim). x22,000.
FIGURE4. Scanningelectron micrographof a groupof cone inner segmentsand adjacent pigment epitheliums.Central retina of an adult Tupaiabelangeri.Clusteredprocesses of the pigmentepithelial cells (pe) completelyhide the cone outer segments and occupy the spaces between the apical portions of the cone inner segments (cis) carrying the apical megamitochondrion. Microvilli of the pigment epitheliumsare broken off from the cone on the right, therefore, the outer segment is visible (arrow). Scale 2 flm.
RESEARCHNOTE
Fettiplace, 1975; Young & Martin, 1984). If mitochondriawere alwaysopticallyinert, selectivity for evolutionary optimization into megamitochondria, ellipsosomes or oil droplets could not have occurred. 3. Although it is, in principle, diurnal and despite its cone-dominated retina Tupaia belangeri can be quite active at night (Aue, 1988),when it probably depends on residual light for orientation. Without strong refractionof the light towardsthe receptorsin the outer segments,most of the lightwould be lost in the spaces between the outer segments and be absorbed in the melanin granules within the processes of the pigment epitheliums.This would be an unsatisfactorysituation for the organism. For these reasons, we would like to suggest that the unique size, dense matrix and specialized multilamellar system of cristae of the highly refractive megamitochondria of Tupaia have accessory optical functions, chanelling the light towards the receptorsin the outer segments. REFERENCES
271
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