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Fine structure of retinal synaptic organelles in lamprey and hagfish photoreceptors
Visor
Rrs. Vol. 16. pp. 137 IO 29,
Pcrgamon
Press
19i6. Pnnred
tn Great
Bnrain
FINE STRUCTURE OF RETINAL SYNAPTIC ORGANELLES IN LAMPREY AND HAGFISH PHOTORECEPTORS KAJ HOLMBERGand PETER &MAN Department of Zoology, University of Stockholm. Sweden (Received 20 April 1975) Abstract- --Synaptic ribbons comparable with those in retinal photoreceutors of gnathostome occur in ~hc‘lamprey (Larrll,cr,.tr.Pltciurilis)-The hagfishes Eptatrrt~rshuryrri. E. scouri and inr arami have instead spherical synaptic bodies. These organelles are absent in Myxine The results show that synaptic ribbons do not only occur in the gnathostome chordates in one of the agnath subdivisions, the lampreys while the other agnath subdivision. the instead have synaptic bodies.
INTRODL’CTION
Electron dense synaptic ribbons, originally described by Sjiistrand (1953. 1958) and De Robertis and Franchi (1956). are characteristic organelles in cones and rods in the retina of gnathostome chordates (i.e. fishes, amphibians, reptiles, birds and mammals). Since the appearance of these synaptic ribbons and their location in the receptor terminals is strikingly the same in all gnathostome species so far examined (for references see Dowling. 1970; Steli, 1972). it is common to see in the literature that the occurrence of these organelles in retinal photoreceptors is generalized to the uertebrare retina. However, reports on the ultrastructure of photoreceptor terminals in retinas of agnath chordates or cyclostomes (i.e. lampreys and hagfishes) are few (Holmberg, 1969, 1970, 1971) and none of these reports has involved the receptor terminals in the lamprey retina. In studying the ultrastructure of photoreceptor terminals in the river lamprey (Lamnpetrapuviutilis) and in different hagfish species (Eptatretus burgeri, E. stouti, Paramyxine atami, Myxine glutinosu) we have found that the synaptic ribbons in the river lamprey are comparable with corresponding organelles in photoreceptor terminals of gnathostomes, whereas the synpatic organelles in hagfish. if present, markedly differ from the synaptic ribbons found in the gnathostome retina.
chordates Pnram.~ glutinosa. but also hagfishes,
HatIfish All the retinas were obtained from adult animals. about 35-7Ocm in length. In Eptarrrrrrs burger?, E. srorrri and Paramyxine atami the eyes (04-l+ mm in diameter) are situated under the skin. in Myxinr glutinosa the eyes (about 05 mm in diameter) are located beneath overlying muscle layers and skin (cf. Femholm and Holmberg 1975). The eyes were therefore dissected out by way of a dorsolateral approach after removing overlying skin (and muscle layers in the case of Myxine). We have used two eyes of E. brrrgeri. four eyes of E. stouti, two eyes of P. atami and three eyes of M. glurinosa. One eye of E. storrri was fixed for 2 hr in a cold solution of formaldehyde-glutaraldehyde (Karnovsky, 1965) followed by postfixation for 2 hr in a cold 2:; solution of osmium tetroxide buffered at pH 75 with 02~ cacodylate buffer. Remaining eyes (10) were fixed for l-2 hr in a cold 1 or 2Y0 solution of osmium tetroxide buffered at pH 744.2 with 2.5% sodium bicarbonate in 01 Y hydrochloric acid or with B2 M cacodylate buffer. All the retinas were dehydrated in graded ethanol series and embedded in Epon. Sections were contrasted for 3 5 min with uranyl acetate in water or in methanol followed by l-3min with lead citrate and examined with a Zeiss EM9-S2 electron microscope.
RESULT.5
Lampetra fluviatilis Electron dense synaptic ribbons, approx 60-70mn thick. of varying length and surrounded by a single MATERIALS AND IMETHODS layer of synaptic vesicles. 50-60 nm dia. are located at River lamprey (Lampetra Ruviatilis) the photoreceptor cell terminals. In sections the ribAdult animals were enucleated, the iris and lens bons appear straight or curved (Figs. 1 and 2) but removed. and the eye cups immersed in the fixative solu- in one case a plate-like ribbon was observed (Fig. tion. Six retinas were fixed for I hr in a solution of Zo/, 3). The ribbons are frequently directed perpendicuosmium tetroxide buffered at pH 7.4-7.5 with 25% sodium larly towards synaptic junctions between the receptor bicarbonate in @l N hydroctiloric acid (Wood aid Luft_ cell and postsynaptic processes. In most cases two 1965) or with @07 M s-collidine buffer (Bennet and Luft. postsynaptic processes are seen at each ribbon 1959). Five retinas were fixed for 2 hr in a cold solution synapse. but the probable occurrence of one to three of 47; glutaraldehyde buffered at pH 7.4 with veronal-aceprocesses is occasionally indicated in the sections. The tate buffer followed bv oostfixation with LX osmium origin of these processes has been traced to horizontal tetroxide solution in tie &ne buffer. Four r&nas were and bipolar cells, but lack of structural characteristics fixed with mixtures of osmium tetroxide and glutaraldehyde solutions (Simionescu. Simionescu and Palade. 1972). makes it almost impossible to determine to which cell 237
type a particular profile belongs in one section. .Arcidensities are seen at the tip of some ribbons (Fig. I). the synaptic cleft is slightly widened and the posts>-naptic membranes are thickened. The photoreceptor cells are oi two types.long and short. with regard to their inner segments. There are, hovvever. no significant morphological differences between the two types of receptor cells with regard to the synaptic organization at the receptor terminals. form
E-ptatrstus arrd Paramyxine At the receptor cell terminals of E. h~grri (Fig. 1). E. sror~ri (Fig. 5) and P. CKCZM~ spherical synaptic organelles, about 0.3 pm in diameter. are located adjacent to the receptor cell membrane. These organelles. commonly referred to as synaptic bodies. display an electron translucent core and an electron dense rim. They are surrounded by synaptic vesicles. 40-j0nm dia. The fusion of a synaptic vesicle with the receptor cell membrane is shown in Fig. 3. perhaps indicating the release of a transmitter substance into the int~rcei~LI~arcleft. The synaptic bodies are located adjacent to one postsynaptic process or at the boundary between two processes. Mysins giutinosa At the receptor terminals of .~f~.~v.ui~tu glutinosa clusters of synaptic vesicles occur adjacent to the receptor cell membrane contiguous to postsynaptic processes entering into invaginations of the receptor terminal (Fig. 6). Afthough membrane densities are seen at several places. indicating possible synaptic sites. synaptic bodies do not occur at the receptor terminals. Di5WSSIOS
The appearance of the synaptic ribbons in retinal photoreceptor cells is strikingly the same in all species of gnathostome chordates so far examined and in the river lamprey. Although we have investigated only one species of lamprey, our observations show that the occurrence of this organelle is not restricted to the gnathostome retina, but includes also one of the two agnath groups, the lampreys. As regards the other agnath group, the hagfishes. Eptatretus burgeri. E. stouti and Paramyine afami (family Eptatretidae) have instead evolved spherical synaptic bodies. whereas ~~f~~i~e Q~~I~~~~o~~ (family Myxinidaef lacks such organelles. In this connection it may be pointed out that the occurrence of synaptic bodies in hagiishes also coincides with the degenerative stage of the eye (HoImberg, 1970, 1971; Fernholm and Holmberg, 1975). From a phylogenetic point of view the occurrence of synaptic ribbons in lampreys but synaptic bodies in hagfishes may suggest that the lampreys in this respect are closer related to ~thostome chordates than to hagfishes. despite the fact that lampreys and hagfrshes are grouped together systematically on the basis of several other morphological and phylogenetical criteria (cf. Fernhelm. 1969, 1972). The synaptic bodies in Ept~~etu~ and Pffr~~~~~e are located adjacent to one or two postsynaptic processes. In the latter case this organization resembles the -‘dyad” in the inner plexiform layer of the gnath-
ostome retina. Here. bipolar cells sstabltsh ~~n;lpt~~ ribbon contacts %ith luo posts!,nap!ic procrssss. fn .Il~.~lltt,. the jtnaptrc sites cJn onfh br rax3gni7~d h> ths clttsters ot synaptic t&ties near densiurs of the receptor ceil membrane. Jnd each 5).naptic sir,- is associated with only one postsynaptic process. The postsynaptic processes have few or most often no vesicles in their cytoplasm. but it still remains to determine the cell type from which these processes originate. In the lamprey. the postsynaptic processes originate from horizontal and bipolar cells. The synaptic organization at each ribbon s!napsc differs. hon-ever, from that commonly found in gnathostomes. in which tu’o horizontal cell processes are located laterally to the synaptic ribbon and one or more bipolar cell dendrites are positioned centrally. In the lamprey. usuatly two processes are seen at each ribbon synapse. an organization which again rescmbles the “dyad” in the inner plexiform layer of gnathostomes.The”dyad”organization found in the receptor terminals in the lamprey and in &~~~~t’r~s and Pal-~tn!_ri~re suggests a less complicated synaptic site at which only two (or one in .tf~~ine) postsynaptic processes receive impulses instead of thrss or more as in rods and cones of the gnathostomss. In summary. the results of the present investigation show that the presence of synaptic bodies m the retinal receptor cells of Eprarretus hurgeri. E. srouri and ParamyGe trtami is a unique feature in hagfishes and which IS not found in cones and rods of gnathostome chordates. This difference probably reflects the early phylogenetical separation of the hagfishes both from the lampreys and the gnathostome chordates. It is evident, that vve cannot generalize the occurrence of synaptic ribbons to the rertehrate retina. Although synaptic ribbons and sy-naptic bodies probably are homologous organelles and have similar function. a distinction must be made between the occurrence of synaptic ribbons in the gnathostome chordates and in one of the agnath subdivisions. the Lampreys. whereas spherical synaptic bodies occur only in the other agnath subdivision. the hagfishes. Acitna~led~enrrrrrs-This study was supported by grants from Helge Ax:son Johnsons Stiftelse. the Swedish Academy of Sciences (Hjalmar Theels fond. Regnells ghvomedel) and the Swedish Natural Science Research Council (grant 2124). We are also indebted to Professor Richard M. Eakin. Department of Zoology, University of Cahfornia. Berkeley. and Dr. Bo Fernholm for provision of material. and to Miss Ylva Lilliemarck and hfrs. Vanja Lundin for able technical assistance. REFERESCES
Bennet H. S. and Luft J. H. (1959) s-coilidine as a basis for buffering fixatives. J. &o&s. biothrm. C_~tol.6. 11-S
11-k De Robertis E. and Franchi C. M. (1956) Electron microscope observations on synaptic vesicles in synapses of retinal rods and cones. J. hiophys. biochern. Cytol. 2. TO?318. Dowling J. E. (1970) Organization of vertebrate retinas. The Jonas M. Friedenwald Memorial Lecture. Inresrrr ~p~~r~.9. 655480. Fcrnholm B. (1969) A cytopharmacoiogicaf study of the .LIxuinr adenoh!pophysis. Grn. cornp. Erdocr. 13. 3% 356.
Figs. 1-3. Lampetra flrtciutilis. Figure 1: longitudinal section through a photoreceptor terminal (R). Five synaptic ribbons are directed towards postsynaptic processes. x 37,500. Figure 2: curved synaptic ribbon in a photoreceptor terminal (R). p: postsynaptic process, g: glial cell process. x 24,000. Figure 3: two serial sections of a receptor terminal (R). p: postsynaptic processes. In (a) a plate-like ribbon is sectioned medially (arrow) and in the following section, (b) its cluster of synaptic vesicles is seen (arrow). x 24,000. The scales correspond to 1 pm.
[fici~g page 238
Fig. 4. Eprafrcius brrrgeri. TUT synaptic bodies are seen close to the receptor cell CR) membrane which surrounds a postsqnapitc process (p) in rhe receptor cell terminal. The synaptic bodies have an electron translucent core and an electron dense rim. Orderly arranged synaptic vesicles surround the synaptic bodies, and the fusion of a vesicle with the receptor cell membrane is seen at the arrow. -,. 37,000. The scale corresponds to O-5 pm. Fig. 5. Ep!arcrus sforrti. Three synaptic bodies are seen close to the receptor cell (R) membrane surrounding a postsynaptic process (p). These bodies are sectioned through the electron dense rim. Inset: synaptic bodies sectioned at different levels. ,’ 37,000. The scale corresponds to 0.5 Frn.
Fig. 6. MyMre glttrinosa. Two postsynaptic processes fp) are seen in a receptor cell terminal (Rj. Clusters of synaptic vesicles occur in the receptor cell cytoplasm adjacent to the inraginated ceil membrane. Synaptic sites with membrane densities are indicated with arrows, Notice the absence of synaptic bodies. :.: 20,000. The scale corresponds to I sm.
Retinal synaptic organelles in cyclostomes Fernholm 8. (1972) The ultrastructure of the adenohypophysis in ,Myxinr glutinosa. Z. Zrl[forsch. 132. 151-472. Frrnholm B. and Holmberg K. (1975) The eyes in three genera of hag&h (Eptaretus. Paramy.uine and .Mvxine)---a case of degenerative evolution. i’ision Rrs. 13. 253-259. Holmberg K. (1969) Hagfish eye: ultrastructure of retinal cells. Acta zool.. Srockh. SO. 179-183. Holmberg K. (1970) The hagfish retina: fme structure of retinal cells in Myxine ghtinosa. L.. with special reference to receptor and epithelial cells. Z. Zrllforsch. 111. 519-538. Holmberg K. (1971) The hagfish retina: electron microscopic study comparing receptor and epithelial cells in the Pacific hagftsh Polistorrema srotrti. with those of the Atlantic hagfish, &Iyxine glrrtinosa. Z. Zellforsch. 121. 249-269. Karnovsky M. J. (1965) A formaldehyde-glutaraldehyde
239
fixative of high osmolality for use in electron microscopy. 1. Cell. Biol. 27. 137X. Simionescu N., Simionescu M. and Palade G. E. (1972) Permeability of intestinal capillaries. Pathwa! followed by dextrans and glycogens. J. Cell Biol. 13. 365-392. Sjastrand F. S. (1953) The ultrastructure of the retinal rod synapses of the guinea pig eye. J. appl. Phxs. 24. 1412 (Abstract). SjGstrand F. S. (1958) Ultrastructure of retinal rod synapses of the guinea pig eye as revealed by three dimensional reconstructions from serial sections. J. Ulrrasrruct. Res. 2. 1X-170. Stell W. K. (1972) The morphological organization of the vertebrate retina. In Handbook of Sensory Physioloy) (Edited by Fuortes M. G. F.). Vol. VII.‘? (Physiolom of Photoreceptor Organs), pp. I I l-213. Springer, Berlci. Wood R. L. and Luft J. H. (1965) The influence of buhr systems on fixation with osmium tetroxide. J. L’lrrastrrrct. Res. 12. 2235.
Rrs. Vol. 16. pp. 137 IO 29,
Pcrgamon
Press
19i6. Pnnred
tn Great
Bnrain
FINE STRUCTURE OF RETINAL SYNAPTIC ORGANELLES IN LAMPREY AND HAGFISH PHOTORECEPTORS KAJ HOLMBERGand PETER &MAN Department of Zoology, University of Stockholm. Sweden (Received 20 April 1975) Abstract- --Synaptic ribbons comparable with those in retinal photoreceutors of gnathostome occur in ~hc‘lamprey (Larrll,cr,.tr.Pltciurilis)-The hagfishes Eptatrrt~rshuryrri. E. scouri and inr arami have instead spherical synaptic bodies. These organelles are absent in Myxine The results show that synaptic ribbons do not only occur in the gnathostome chordates in one of the agnath subdivisions, the lampreys while the other agnath subdivision. the instead have synaptic bodies.
INTRODL’CTION
Electron dense synaptic ribbons, originally described by Sjiistrand (1953. 1958) and De Robertis and Franchi (1956). are characteristic organelles in cones and rods in the retina of gnathostome chordates (i.e. fishes, amphibians, reptiles, birds and mammals). Since the appearance of these synaptic ribbons and their location in the receptor terminals is strikingly the same in all gnathostome species so far examined (for references see Dowling. 1970; Steli, 1972). it is common to see in the literature that the occurrence of these organelles in retinal photoreceptors is generalized to the uertebrare retina. However, reports on the ultrastructure of photoreceptor terminals in retinas of agnath chordates or cyclostomes (i.e. lampreys and hagfishes) are few (Holmberg, 1969, 1970, 1971) and none of these reports has involved the receptor terminals in the lamprey retina. In studying the ultrastructure of photoreceptor terminals in the river lamprey (Lamnpetrapuviutilis) and in different hagfish species (Eptatretus burgeri, E. stouti, Paramyxine atami, Myxine glutinosu) we have found that the synaptic ribbons in the river lamprey are comparable with corresponding organelles in photoreceptor terminals of gnathostomes, whereas the synpatic organelles in hagfish. if present, markedly differ from the synaptic ribbons found in the gnathostome retina.
chordates Pnram.~ glutinosa. but also hagfishes,
HatIfish All the retinas were obtained from adult animals. about 35-7Ocm in length. In Eptarrrrrrs burger?, E. srorrri and Paramyxine atami the eyes (04-l+ mm in diameter) are situated under the skin. in Myxinr glutinosa the eyes (about 05 mm in diameter) are located beneath overlying muscle layers and skin (cf. Femholm and Holmberg 1975). The eyes were therefore dissected out by way of a dorsolateral approach after removing overlying skin (and muscle layers in the case of Myxine). We have used two eyes of E. brrrgeri. four eyes of E. stouti, two eyes of P. atami and three eyes of M. glurinosa. One eye of E. storrri was fixed for 2 hr in a cold solution of formaldehyde-glutaraldehyde (Karnovsky, 1965) followed by postfixation for 2 hr in a cold 2:; solution of osmium tetroxide buffered at pH 75 with 02~ cacodylate buffer. Remaining eyes (10) were fixed for l-2 hr in a cold 1 or 2Y0 solution of osmium tetroxide buffered at pH 744.2 with 2.5% sodium bicarbonate in 01 Y hydrochloric acid or with B2 M cacodylate buffer. All the retinas were dehydrated in graded ethanol series and embedded in Epon. Sections were contrasted for 3 5 min with uranyl acetate in water or in methanol followed by l-3min with lead citrate and examined with a Zeiss EM9-S2 electron microscope.
RESULT.5
Lampetra fluviatilis Electron dense synaptic ribbons, approx 60-70mn thick. of varying length and surrounded by a single MATERIALS AND IMETHODS layer of synaptic vesicles. 50-60 nm dia. are located at River lamprey (Lampetra Ruviatilis) the photoreceptor cell terminals. In sections the ribAdult animals were enucleated, the iris and lens bons appear straight or curved (Figs. 1 and 2) but removed. and the eye cups immersed in the fixative solu- in one case a plate-like ribbon was observed (Fig. tion. Six retinas were fixed for I hr in a solution of Zo/, 3). The ribbons are frequently directed perpendicuosmium tetroxide buffered at pH 7.4-7.5 with 25% sodium larly towards synaptic junctions between the receptor bicarbonate in @l N hydroctiloric acid (Wood aid Luft_ cell and postsynaptic processes. In most cases two 1965) or with @07 M s-collidine buffer (Bennet and Luft. postsynaptic processes are seen at each ribbon 1959). Five retinas were fixed for 2 hr in a cold solution synapse. but the probable occurrence of one to three of 47; glutaraldehyde buffered at pH 7.4 with veronal-aceprocesses is occasionally indicated in the sections. The tate buffer followed bv oostfixation with LX osmium origin of these processes has been traced to horizontal tetroxide solution in tie &ne buffer. Four r&nas were and bipolar cells, but lack of structural characteristics fixed with mixtures of osmium tetroxide and glutaraldehyde solutions (Simionescu. Simionescu and Palade. 1972). makes it almost impossible to determine to which cell 237
type a particular profile belongs in one section. .Arcidensities are seen at the tip of some ribbons (Fig. I). the synaptic cleft is slightly widened and the posts>-naptic membranes are thickened. The photoreceptor cells are oi two types.long and short. with regard to their inner segments. There are, hovvever. no significant morphological differences between the two types of receptor cells with regard to the synaptic organization at the receptor terminals. form
E-ptatrstus arrd Paramyxine At the receptor cell terminals of E. h~grri (Fig. 1). E. sror~ri (Fig. 5) and P. CKCZM~ spherical synaptic organelles, about 0.3 pm in diameter. are located adjacent to the receptor cell membrane. These organelles. commonly referred to as synaptic bodies. display an electron translucent core and an electron dense rim. They are surrounded by synaptic vesicles. 40-j0nm dia. The fusion of a synaptic vesicle with the receptor cell membrane is shown in Fig. 3. perhaps indicating the release of a transmitter substance into the int~rcei~LI~arcleft. The synaptic bodies are located adjacent to one postsynaptic process or at the boundary between two processes. Mysins giutinosa At the receptor terminals of .~f~.~v.ui~tu glutinosa clusters of synaptic vesicles occur adjacent to the receptor cell membrane contiguous to postsynaptic processes entering into invaginations of the receptor terminal (Fig. 6). Afthough membrane densities are seen at several places. indicating possible synaptic sites. synaptic bodies do not occur at the receptor terminals. Di5WSSIOS
The appearance of the synaptic ribbons in retinal photoreceptor cells is strikingly the same in all species of gnathostome chordates so far examined and in the river lamprey. Although we have investigated only one species of lamprey, our observations show that the occurrence of this organelle is not restricted to the gnathostome retina, but includes also one of the two agnath groups, the lampreys. As regards the other agnath group, the hagfishes. Eptatretus burgeri. E. stouti and Paramyine afami (family Eptatretidae) have instead evolved spherical synaptic bodies. whereas ~~f~~i~e Q~~I~~~~o~~ (family Myxinidaef lacks such organelles. In this connection it may be pointed out that the occurrence of synaptic bodies in hagiishes also coincides with the degenerative stage of the eye (HoImberg, 1970, 1971; Fernholm and Holmberg, 1975). From a phylogenetic point of view the occurrence of synaptic ribbons in lampreys but synaptic bodies in hagfishes may suggest that the lampreys in this respect are closer related to ~thostome chordates than to hagfishes. despite the fact that lampreys and hagfrshes are grouped together systematically on the basis of several other morphological and phylogenetical criteria (cf. Fernhelm. 1969, 1972). The synaptic bodies in Ept~~etu~ and Pffr~~~~~e are located adjacent to one or two postsynaptic processes. In the latter case this organization resembles the -‘dyad” in the inner plexiform layer of the gnath-
ostome retina. Here. bipolar cells sstabltsh ~~n;lpt~~ ribbon contacts %ith luo posts!,nap!ic procrssss. fn .Il~.~lltt,. the jtnaptrc sites cJn onfh br rax3gni7~d h> ths clttsters ot synaptic t&ties near densiurs of the receptor ceil membrane. Jnd each 5).naptic sir,- is associated with only one postsynaptic process. The postsynaptic processes have few or most often no vesicles in their cytoplasm. but it still remains to determine the cell type from which these processes originate. In the lamprey. the postsynaptic processes originate from horizontal and bipolar cells. The synaptic organization at each ribbon s!napsc differs. hon-ever, from that commonly found in gnathostomes. in which tu’o horizontal cell processes are located laterally to the synaptic ribbon and one or more bipolar cell dendrites are positioned centrally. In the lamprey. usuatly two processes are seen at each ribbon synapse. an organization which again rescmbles the “dyad” in the inner plexiform layer of gnathostomes.The”dyad”organization found in the receptor terminals in the lamprey and in &~~~~t’r~s and Pal-~tn!_ri~re suggests a less complicated synaptic site at which only two (or one in .tf~~ine) postsynaptic processes receive impulses instead of thrss or more as in rods and cones of the gnathostomss. In summary. the results of the present investigation show that the presence of synaptic bodies m the retinal receptor cells of Eprarretus hurgeri. E. srouri and ParamyGe trtami is a unique feature in hagfishes and which IS not found in cones and rods of gnathostome chordates. This difference probably reflects the early phylogenetical separation of the hagfishes both from the lampreys and the gnathostome chordates. It is evident, that vve cannot generalize the occurrence of synaptic ribbons to the rertehrate retina. Although synaptic ribbons and sy-naptic bodies probably are homologous organelles and have similar function. a distinction must be made between the occurrence of synaptic ribbons in the gnathostome chordates and in one of the agnath subdivisions. the Lampreys. whereas spherical synaptic bodies occur only in the other agnath subdivision. the hagfishes. Acitna~led~enrrrrrs-This study was supported by grants from Helge Ax:son Johnsons Stiftelse. the Swedish Academy of Sciences (Hjalmar Theels fond. Regnells ghvomedel) and the Swedish Natural Science Research Council (grant 2124). We are also indebted to Professor Richard M. Eakin. Department of Zoology, University of Cahfornia. Berkeley. and Dr. Bo Fernholm for provision of material. and to Miss Ylva Lilliemarck and hfrs. Vanja Lundin for able technical assistance. REFERESCES
Bennet H. S. and Luft J. H. (1959) s-coilidine as a basis for buffering fixatives. J. &o&s. biothrm. C_~tol.6. 11-S
11-k De Robertis E. and Franchi C. M. (1956) Electron microscope observations on synaptic vesicles in synapses of retinal rods and cones. J. hiophys. biochern. Cytol. 2. TO?318. Dowling J. E. (1970) Organization of vertebrate retinas. The Jonas M. Friedenwald Memorial Lecture. Inresrrr ~p~~r~.9. 655480. Fcrnholm B. (1969) A cytopharmacoiogicaf study of the .LIxuinr adenoh!pophysis. Grn. cornp. Erdocr. 13. 3% 356.
Figs. 1-3. Lampetra flrtciutilis. Figure 1: longitudinal section through a photoreceptor terminal (R). Five synaptic ribbons are directed towards postsynaptic processes. x 37,500. Figure 2: curved synaptic ribbon in a photoreceptor terminal (R). p: postsynaptic process, g: glial cell process. x 24,000. Figure 3: two serial sections of a receptor terminal (R). p: postsynaptic processes. In (a) a plate-like ribbon is sectioned medially (arrow) and in the following section, (b) its cluster of synaptic vesicles is seen (arrow). x 24,000. The scales correspond to 1 pm.
[fici~g page 238
Fig. 4. Eprafrcius brrrgeri. TUT synaptic bodies are seen close to the receptor cell CR) membrane which surrounds a postsqnapitc process (p) in rhe receptor cell terminal. The synaptic bodies have an electron translucent core and an electron dense rim. Orderly arranged synaptic vesicles surround the synaptic bodies, and the fusion of a vesicle with the receptor cell membrane is seen at the arrow. -,. 37,000. The scale corresponds to O-5 pm. Fig. 5. Ep!arcrus sforrti. Three synaptic bodies are seen close to the receptor cell (R) membrane surrounding a postsynaptic process (p). These bodies are sectioned through the electron dense rim. Inset: synaptic bodies sectioned at different levels. ,’ 37,000. The scale corresponds to 0.5 Frn.
Fig. 6. MyMre glttrinosa. Two postsynaptic processes fp) are seen in a receptor cell terminal (Rj. Clusters of synaptic vesicles occur in the receptor cell cytoplasm adjacent to the inraginated ceil membrane. Synaptic sites with membrane densities are indicated with arrows, Notice the absence of synaptic bodies. :.: 20,000. The scale corresponds to I sm.
Retinal synaptic organelles in cyclostomes Fernholm 8. (1972) The ultrastructure of the adenohypophysis in ,Myxinr glutinosa. Z. Zrl[forsch. 132. 151-472. Frrnholm B. and Holmberg K. (1975) The eyes in three genera of hag&h (Eptaretus. Paramy.uine and .Mvxine)---a case of degenerative evolution. i’ision Rrs. 13. 253-259. Holmberg K. (1969) Hagfish eye: ultrastructure of retinal cells. Acta zool.. Srockh. SO. 179-183. Holmberg K. (1970) The hagfish retina: fme structure of retinal cells in Myxine ghtinosa. L.. with special reference to receptor and epithelial cells. Z. Zrllforsch. 111. 519-538. Holmberg K. (1971) The hagfish retina: electron microscopic study comparing receptor and epithelial cells in the Pacific hagftsh Polistorrema srotrti. with those of the Atlantic hagfish, &Iyxine glrrtinosa. Z. Zellforsch. 121. 249-269. Karnovsky M. J. (1965) A formaldehyde-glutaraldehyde
239
fixative of high osmolality for use in electron microscopy. 1. Cell. Biol. 27. 137X. Simionescu N., Simionescu M. and Palade G. E. (1972) Permeability of intestinal capillaries. Pathwa! followed by dextrans and glycogens. J. Cell Biol. 13. 365-392. Sjastrand F. S. (1953) The ultrastructure of the retinal rod synapses of the guinea pig eye. J. appl. Phxs. 24. 1412 (Abstract). SjGstrand F. S. (1958) Ultrastructure of retinal rod synapses of the guinea pig eye as revealed by three dimensional reconstructions from serial sections. J. Ulrrasrruct. Res. 2. 1X-170. Stell W. K. (1972) The morphological organization of the vertebrate retina. In Handbook of Sensory Physioloy) (Edited by Fuortes M. G. F.). Vol. VII.‘? (Physiolom of Photoreceptor Organs), pp. I I l-213. Springer, Berlci. Wood R. L. and Luft J. H. (1965) The influence of buhr systems on fixation with osmium tetroxide. J. L’lrrastrrrct. Res. 12. 2235.