Differential central distribution of optic nerve components in the rat

Brain Research, 116 (1976) 83-100

83

© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

D I F F E R E N T I A L C E N T R A L D I S T R I B U T I O N OF OPTIC N E R V E COMPON E N T S IN T H E RAT

J. S. LUND, F. L. REMINGTON and R. D. LUND Departments of Ophthalmology and Biological Structure, University of Washington School of Medicine, Seattle, Wash. 98195 (U.S.A.)

(Accepted March 5th, 1976)

SUMMARY The distribution of Fink-Heimer positive degeneration and neurofibrillar proliferation has been examined in the dorsal lateral geniculate nucleus (dLGN) and superior colliculus of albino and pigmented rats following enucleation between postnatal days 12 and 30 and as adults. With survival times of 6 h to 5 days following enucleation, the location of maximum degenerative reaction stained by the FinkHeimer method changes with increasing survival time in both regions. In the d L G N contralateral to the eye removal the earliest degeneration appears in a lamina occupying the medioventral extent of the nucleus and is rapidly removed; later degeneration fills a central lamina of the nucleus with a patch of degeneration extending to the outer surface of the nucleus at the mediodorsal margin. The latest occurring degeneration fills the outermost lamina and is still obvious when degeneration is largely dispersed from the inner and central laminae; this outer lamina shows an early filamentous degenerative reaction in the adult. The uncrossed optic pathway occupies a portion of the central lamina of the nucleus, and in albino animals it shows a rapid degeneration and dispersion similar to the innermost lamina on the crossed side; in the pigmented animals degeneration of the uncrossed projection starts as early as that of the albino but persists as long as the degeneration of the central lamina on the crossed side. The degeneration time of the sprouted, uncrossed pathway resulting from unilateral enucleation at birth is similar in albino and pigmented rats and resembles in timing the normal uncrossed pathway of pigmented rats. These results suggest that there are at least 3 different fiber populations in the rat optic nerve with different distribution in the dLGN. The uncrossed optic pathway of albino and pigmented rats appears to differ in fiber composition; this may relate to aberrations in mapping of uncrossed projections in the albino.

84 INTRODUCTION Recent physiological studies of the properties of mammalian optic nerve fibers, particularly in the cat but also in the rat, have shown the presence of at least 3 major functional groups of fibers separable on the basis of their differing conduction velocities 7,12,16,a4-46. The differences in conduction velocity have been attributed to the nerve fibers having different diameters, the fastest conduction occurring in the largest diameter fibers and the slowest in the finest fibers26,42,47. A correlation has also been suggested between ganglion cell size, axon diameter and physiological properties, the largest ganglion cells being presumed to give rise to the largest axons and the smallest cells to give rise to the finest axons. This hypothesis has gained support from the findings,a9 of direct correlation between the largest ganglion cells in the cat retina (alpha cells2) and 'Y' type (brisk-transient) physiological responses. It has also been shown physiologically in the cat that these different types of optic nerve fibers have different central distributions. All 3 fiber groups project to the dorsal lateral geniculate nucleus (dLGN) but with the 'W' (slowest) population distributing only to the C laminae6,9,17,1s,23,24,51, and only the 'Y' and 'W' components projecting to the superior colliculus18,22,23. The retrograde transport of horseradish peroxidase (HRP) in the cat has provided a correlation between ganglion cell size and their central projection that tends to confirm the projection outlined physiologically28. In the rat, physiological data suggest 3 types of fiber project to both dLGN and superior colliculus 44,46. Since, physiologically, fibers with different conduction velocities seem to be concerned with different aspects of visual information, it would be of great value to try and distinguish them anatomically in order to map their central distributions and synaptic relationships more exactly. The present study therefore explores the possibility that in the rat different populations of optic nerve fibers may be separable on the basis of their degenerative reaction following enucleation both in terms of the time taken to degenerate and in the presence or absence of a marked neurofibrillar hypertrophy. In addition, the albino and pigmented rat have been compared to see if any difference in degenerative sequence exists between the two strains which are known to differ in their central optic projections z7. Previous studies 19,30,4°,43 in the cat land hamster have described changes in distribution of degeneration with different survival time following enucleation, and the present study examines in more detail such changes in the rat dLGN and superior colliculus. A preliminary report of the results of this work has been made elsewhere 33. METHODS In this study, degeneration resulting from monocular enucleation, stained by the Fink-Heimer method I (see ref. 13), was followed in the dLGN and superior colliculus after survival times from 6 h to 5 days. The pattern of degeneration was studied in adult (12) and infant rats (14 litters, averaging 12 offspring per litter, ages 12-30 days) of both albino (Sprague-Dawley) and pigmented (Long Evans) strains. Infant animals were used since degeneration debris as stained by the Fink-Heimer

85 technique clears rapidly and completely in the young animal and the changing distribution of degeneration is more clearly evident than in animals over 25 days of age where degeneration debris, probably largely myelin, remains stainable for long periods of time. Animals were perfused with 4 ~o buffered paraformaldehyde, the brains sunk in 30~o sucrose in the same fixative, and frozen sectioned at 25#m or 35/~m. Every third section was stained by the Fink-Heimer technique. In the adult brains further section series were stained by an unsuppressed Nauta-Gygax method37 and the dLGN and superior colliculus examined with the light microscope for evidence of neurofibrillar hypertrophy compared to completely normal brains stained by the same technique. For comparison, the central optic projections were also demonstrated by autoradiographic tracing techniques. [3H]Proline (15-35/~Ci at 10/~Ci/#l) was injected into one eye of albino and pigmented rats at ages 10 days, 12 days, 14 days, 20 days and adult. In each case 24 h elapsed between injection and perfusion. The animals were perfused as before and a 1:3 frozen section series mounted and coated with NTB2 emulsion. Exposure time was approximately 3 weeks. In addition to normal animals, a series of rats was prepared with monocular enucleation at birth and the central projection of the remaining eye was examined by the same techniques and at similar ages to the normal series outlined above. RESULTS Optic projection to the contralateral dLG N The rat dLGN shows no clearly evident cellular lamination such as is seen in the cat and monkey dLGN. Optic fibers enter the nucleus ventrolaterally and travel through the nucleus tangential to the surface. Autoradiographic tracing of the optic projection to the contralateral dLGN shows silver grain covering the entire nucleus with the exception of a distinct gap in the pigmented animals where the uncrossed optic projection occurs (Fig. 1A). Some clumping of the grain occurs in the most superficial region of the nucleus. In the albino animals, only the most anterior portion of the uncrossed projection region remains clear of grain, more posteriorly a light projection from the contralateral eye appears to occupy the region of ipsilateral projection (Fig. 1C). The same distribution was seen in all ages of animals examined. Fink-Heimer stainable degeneration first appears in the innermost part of the nucleus extending from medial to ventrolateral margins of the nucleus (Fig. 2, dLGN, stage A; Fig. 3A). With longer survival times (see Table I) after eye removal, the degeneration gradually spreads towards the surface of the nucleus (Fig. 2, dLGN, stage B) and a band of dense degeneration forms at an intermediate depth in the nucleus (Fig. 2, dLGN, stage C; Fig. 3B) which medially includes the region of the ipsilateral projection; a dense patch of degeneration extends from this band up to the surface at the most dorsomedial margin of the nucleus in the region of ipsilateral visual field representation37. As the intermediate band of degeneration becomes prominent, the outer region of the nucleus starts to fill with extremely fine degeneration particles and in the young animal the degeneration clears from the inner part of the nucleus. With longer survival times the outermost region of the nucleus becomes densely

86

Fig. 1. A: autoradiographic labeling of optic projection to the contralateral dLGN of a pigmented rat (age 16 days). Transverse section through anterior third of nucleus, medial to right. Gap (indicated by arrow) in projection is filled by unlabeled ipsilateral optic projection. The ipsilateral projection is shown in B, medial to the left. C: autoradiographic labeling of optic projection to the contralateral d L G N of an albino rat (age 12 days). Transverse section through anterior third of nucleus, medial to right. Little indication is seen of position of unlabeled ipsilateral optic projection. Ipsilateral projection is shown in D, medial to the left. E: degenerating sprouted ipsilateral optic projection to d L G N in pigmented rat following removal of remaining eye at 12 days of age from an animal which had the other eye removed at birth (survival time 6 h). Fink-Heimer method. Transverse section through anterior third of nucleus medial to the left. F: degenerating sprouted ipsilateral optic projection to d L G N in albino rat following removal of remaining eye at 12 days of age, from an animal which had the other eye removed at birth (survival time 8 h). Transverse section through anterior third of nucleus. Medial to the left. Fink-Heimer method. All figures are dark-field photomicrographs, × 40.

I

2

3

4

5

SC

dLGN

Fig. 2. Diagram representing changes in position of Fink-Heimer positive degeneration with progressively longer survival times in the superior colliculus (SC) and dorsal lateral geniculate nucleus (dLGN) contralateral to eye removal. Dots represent sparse degeneration; widely spaced cross hatching, moderate degeneration; closely spaced cross hatching, heavy degeneration; black, very heavy degeneration. The time sequence for all stages is shown in Table I. The diagrams for SC and dLGN are aligned with each other to correlate in time so that the sequential degeneration can be compared between the two regions. Further description is given in text.

Fig. 3. Successive stages of Fink-Heimer positive degeneration in the d L G N contralateral to eye removal stages as outlined in Fig. 2. A: stage A, earliest stainable degeneration occupying the innermost (ventromedial) portion of the nucleus from medial to ventrolateral boundaries. Very sparse degeneration is seen in the rest of the nucleus. Pigmented rat, aged 12 days, 6 h survival. B: stage C, degeneration concentrated in an intermediate band reaching from medial to ventrolateral boundaries, rising to the surface at the medial edge in the region of ipsilateral field representation. Albino rat, aged 25 days on enucleation, 24 h survival. C: stage D, degeneration concentrated in the outer region of the nucleus; the innermost region is almost clear of degeneration; degeneration in the intermediate band is thinning and two clear patches can be seen where the intact uncrossed projection is localized. Albino rat, aged 16 days on enucleation, 24 h survival. D: stage E, final stage of degeneration when granules are concentrated in the outermost region of the nucleus. Degeneration has cleared from the innermost region and has almost gone from the intermediate band. Albino rat, aged 16 days on enucleation, 36 h survival. All photomicrographs taken with dark-field illumination, × 40.

88 TABLE I Optimal survival times, at different ages in albino (A) and pigmented (P) rats, showing the progression of degeneration in the dLGN (stages A-E) and superior colliculus (stages 1-5) illustrated in Figs. 2, 3 and 5, as stained by the Fink-Heimer method. Optimal survival times for degeneration stages in contralateral dLGN (h) Age of rat (days}

Stage A Stage B Stage C Stage D Stage E . . . . . . . . . . . . . . . . . . . . . . . . . . .

12

6

12 P

16

8

16

20-25 Adult

Age of rat (days)

12 16 20-25 Adult

15 24

12 A 24 P 22 A 24 P 22-24

15-22 48

24-36

36-48

24-36

36-48

48 5 (days)

72

Optimal survival times for degeneration stages in contralateral superior colliculus (h) Stage 1 Stage 2 Stage 3 Stage 4 Stage 5

8

6 15 15

12 16-20 24 4-5 (days)

24-48 A 48-72 P (Filamentous hypertrophy in outer lamina)

24 36 48

48 48 72

Age of rat (days)

Persistence of degeneration of ipsilateral projection to dLG N (h)

12 16 20-25 Adult

12 A (optimal 6), 24 P 24 A (optimal 8), 36 P 36 A (optimal 15), 48 P Uncrossed projection staining lightly, 24, very heavy by 48 h.

Age of rat (days)

Persistence of degeneration of ipsilateral sprouted projection in young animals (h) In ipsilateral dLG N In ipsilateral superior colliculus

12 16 20-25

17 A, 24 P 36 48

6 8 24

filled with d e g e n e r a t i o n (Fig. 2, d L G N , stage D ; Fig. 3C); the d e g e n e r a t i o n in the inter m e d i a t e b a n d thins a n d only a few scattered grains r e m a i n in the i n n e r m o s t p o r t i o n o f the nucleus in the y o u n g animals. F i n a l l y only an o u t e r b a n d in the nucleus f r o m medial to v e n t r o l a t e r a l b o r d e r s has dense degeneration, f o r m i n g large clumps, the inner two-thirds o f the nucleus c o n t a i n i n g only very occasional coarse degenerative particles (Fig. 2, d L G N , stage 5; Fig. 3D). The same p a t t e r n o f d i s t r i b u t i o n o f de-

89

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.

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,~

AIb. ~ . ~ . ipsi.

.~

.~,

,,~

"~

cross.

Pig:

=psi.

Pig. sprout AIb. ~ ipsi. \

~-A ,,~

s p r o u t ~

Fig. 4. Diagram comparing the persistence of Fink-Heimer positive degeneration in the contralateral and ipsilateral dLGN of pigmented and albino rats, aged 12-25 days. Normal animals are compared with those with one eye removed at birth. The top row shows changes over time in the dLGN of the pigmented animals and the albino crossed projection is essentially similar. The diagrams are aligned in vertical rows of similar survival times (see Table I and Fig. 2 for specific timing of each stage at each age). Note that (i) the persistence of staining in the uncrossed projection of albinos resembles the timing of degeneration in the innermost region of the contralateral dLGN; however, the uncrossed projection in the pigmented strain resembles the timing of degeneration in the innermost and intermediate regions of the contralateral nucleus; (ii) the different distribution of the sprouted pathway (after removal of the remaining eye in animals with one eye removed at birth) in the ipsilateral dLGN of the albino and pigmented animals, and the increased persistence of this pathway compared to normal in the albino (see also Fig. 1E and F). generation is seen in both albino and pigmented animals o f all ages with the pigmented animals taking slightly longer survival times than the albinos to demonstrate the same pattern (Table I).

Optic projection to the ipsilateral dLGN A u t o r a d i o g r a p h i c tracing shows the uncrossed p a t h w a y o f albino and pigmented rats to distribute as demonstrated previously by degenerative techniques ~7. A t the level o f the anterior third o f the nucleus, representative sections are shown in Fig. 1B and 1D for the albino and pigmented animals. Using the F i n k - H e i m e r technique, pigmented and albino animals differ markedly at 12-25 days o f age in the degree o f persistence o f degeneration in the un-

90

Fig. 5. Optic projection to the contralateral superior colliculus. A : autoradiographic labeling of optic projection. Pigmented rat, 12 days old. B-F: Fink-Heimer positive degeneration seen withp rogressively longer survival times after enucleation at 12-25 days of age in both albino and pigmented animals. B: stage l, the earliest Fink-Heimer degeneration is concentrated in the deepest third of the stratum griseum superficiale (SGS) with slight encroachment into the stratum opticum (SO). Albino animal, aged 16 days, 8 h survival. C: stage 2, degeneration grains become most numerous at surface and lowermost third of SGS with somewhat lighter degeneration in the intermediate SGS. Pigmented animal, 12 days old, 6 h survival. D: stage 3, evenly distributed heavy degeneration throughout SGS. Albino animal, 25 days old on enucleation, 24 h survival. E: stage 4, clearing of degeneration from the deeper SGS. Albino animal, 22 days old on enucleation, 60 h survival. F: stage 5, heavy degeneration with clumping of granules in upper SGS with the surface zone and deeper SGS almost free of stainable degeneration. Pigmented animal, I6 days old on enucleation, 48 h survival. All photomicrographs taken with dark-field illumination of transverse sections at mid-collicular level. × 40.

91 crossed pathway to the dLGN (see Table I). In the albino, the degeneration of the uncrossed projection is at its densest when the contralateral degeneration fills only the innermost third of the nucleus and it rapidly diminishes in intensity as the intermediate band fills with degeneration in the contralateral nucleus. In the pigmented animals, the uncrossed degeneration appears as early as that in the albinos but remains clearly stainable until the outermost band of degeneration begins to become prominent. The persistence of the crossed and uncrossed degeneration in the two strains is compared in Fig. 4 and in Table I.

Projection to the contralateral superior colliculus Autoradiographic tracing of the optic projection to the superior colliculus gives an even distribution of grain throughout the stratum griseum superficiale (SGS) (Fig. 5A). The earliest degeneration stainable by the Fink-Heimer technique appears as a band in the deepest one-third of the SGS and adjacent stratum opticum with a light scatter of granules more superficially (Fig. 2, SC, stage 1 ; Fig. 5B). With longer survival times, degeneration granules increase in number both in the deepest third and also in a narrow zone close to the surface to give over a brief period of time a heavier concentration of degeneration in these regions than in the rest of the SGS (Fig. 2, SC, stage 2; Fig. 5C). The degeneration then rapidly fills the SGS with an even density of degeneration grain (Fig. 2, SC, stage 3; Fig. 5D). The degeneration thins in the deeper SGS (Fig. 2, SC, stage 4; Fig. 5E) and finally also in a narrow zone close to the surface, leaving a band of very heavy, clumped degeneration in the upper SGD (Fig. 2, SC, stage 5; Fig. 5F). Ipsilateral projection to the superior colliculus The ipsilateral projection to the superior colliculus is extremely sparse, particularly in the albino animals. In the pigmented animals, autoradiographic tracing shows clumps of grain anterolaterally in the stratum opticum. Fine degeneration is seen in the stratum opticum in the colliculus ipsilateral to the eye removal, and in the young animals this is evident only at short survival times when degeneration is prominent deep in the SGS of the contralateral superior colliculus. Projection of remaining eye following monocular enucleation at birth Projection to contralateral dLGN. Autoradiographic tracing shows the dLGN contralateral to the remaining eye to be evenly filled with grain. Fink-Heimer stained degeneration after removal of the remaining eye follows the same patterning over time as described for the normal animals, and in both albino and pigmented animals no gap or thinning in degeneration is seen in the region of the intermediate lamina which would correspond to the ipsilateral pathway of the eye removed at birth. This indicates that the position of the uncrossed projection has been filled by crossed axons from the remaining eye with a degeneration time resembling that of the crossed projection to the intermediate lamina. Projection to the ipsilateral dLGN. In the dLGN of the albino animals ipsilateral

92 to the second eye removal, the uncrossed optic pathway changes its projection from its normal restricted location in the intermediate band to occupy the outermost region of the nucleus starting a short distance from the medial border (approximately at the mid-line field representation) and extending to the ventrolateral and posterior borders of the nucleus (Fig. 1F); no trace remains of the two concentrated patches of degeneration seen in the normal animal (Fig. 1D). Autoradiographic tracing and degeneration show a similar distribution. The ipsilateral pathway also expands its area of distribution in the pigmented animals, occupying a larger area than normal ventrally and laterally in the nucleus. However, the expanded pathway in the pigmented animals does not extend right to the outer surface of the nucleus as in the albino, nor does it extend to the ventrolateral edge of the nucleus. Unlike the pathway in the albino animals, it does occupy a large part of the inner region of the nucleus (Fig. 1E). In the albino and pigmented animals the onset and duration of stainable degeneration in the sprouted ipsilateral projection is similar and of about the same persistence as the degenerating uncrossed pathway in normal pigmented animals (Table I; Fig. 4). Therefore, degeneration from the albino sprouted pathway remains stainable for longer than does the normal albino uncrossed pathway (Fig. 4). Projection to superior colliculus following enucleation at birth. Following eye removal at birth, there is an increased ipsilateral projection from the remaining eye to the superior colliculus, covering the whole SGS in a topographically ordered manner a6. In the animals 12-25 days of age, the sprouted pathway to the ipsilateral colliculus can be demonstrated only at very short survival times, the degeneration being rapidly removed. This rapid progression of degeneration is seen in both pigmented and albino animals and is far briefer than the period of persistence of the expanded ipsilateral pathway in the d L G N (see Table I). Degeneration patterns in animals older than 25 days Around 25 days of age, the degree of myelination of optic nerve fibers in the superior colliculus and d L G N increases markedly. The degeneration debris of these myelinated fibers persists much longer than the unmyelinated axons of earlier ages. This persistent debris partly obscures the markedly different distributions of degeneration seen with different survival times. However, with short survival times even in the adult, stages 1 and 3 of Fig. 2 are clearly evident in the colliculus as are stages A - D (Fig. 2) in the dLGN. The later stages of degeneration occur in the adult animal but persistent degeneration masks the boundaries of the regions showing the latest phases. Neurofibrillar degeneration in the adult rat Neurofibrillar proliferation during degeneration (seen as an increase in the number of fine ring formations under oil immersion objectives after staining with the unsuppressed Nauta-Gygax technique as compared to normal tissue) is clearly seen in the d L G N and superior colliculus of albino and pigmented adult rats (Fig. 6). This fibrillar reaction is not seen in the juvenile animals using the light microscope

93

Fig. 6. Neurofibrillar rings in the outer lamina of the rat dorsal lateral geniculate nucleus following enucleation of the contralateral eye. Adult pigmented rat; survival of 3 days following enucleation. x 2400. but there is some indication of neurofilamentous proliferation using the electron microscope. In the colliculus contralateral to the eye removal the neurofibrillar hypertrophy in the adult is pronounced 24 h after eye removal in the albino and 48 h after eye removal in the pigmented animals. The increase in fibrillar rings is seen from surface to stratum opticum but is most pronounced in the upper two-thirds of the SGD. In the d L G N contralateral to the eye removal the neurofibrillar proliferation is seen only in the outermost third of the nucleus from medial to ventrolateral boundaries and lapping around the posterior border of the nucleus ~5 (Fig. 6). The reaction is seen most intensely in the albino from 24 h to 3 days following eye removal and in the pigmented animals between 2 and 4 days survival. This intense fibrillar reaction therefore precedes the Fink-Heiner positive degenerative reaction in the outer lamina of the nucleus (see Table I). The uncrossed projection in the two strains of rat does not show a marked neurofibriUar hypertrophy although, due to the presence of scattered fibrillar rings in the normal animals, it is uncertain whether or not a slight increase in numbers of rings may occur in the region of the uncrossed projection (see also ref. 10). If the remaining eye is removed from adult rats monocularly enucleated at birth, the distribution of fibrillar proliferation is as described above contralateral to the adult eye removal. There is no evidence of increased fibrillar rings in the region

94 formerly occupied by the uncrossed pathway of the neonatally removed eye. Ipsilateral to the adult eye removal in albino animals there is a fibrillar proliferation in the outer lamina coincident in area with the Fink-Heimer positive degeneration. In pigmented animals the ipsilateral projection shows fibrillar hypertrophy where the Fink-Heimer degeneration encroaches on the outer lamina. However, such neurofibrillar change is not obvious in the rest of the uncrossed pathway distribution, either in the normal region of uncrossed projection or its extended distribution ventromedially and ventrolaterally. DISCUSSION Following eye removal, Fink-Heimer stainable degeneration in the rat d L G N and superior colliculus changes its distribution as successively longer survival times are allowed. Contralateral to the eye removal in both regions, the degeneration appears to concentrate as 3 laminae over time. There are a number of possible explanations for these changes in location of degeneration. One is that they represent a progressive degeneration along the optic axons within each nucleus travelling from preterminal axon to terminal contact region. However, in the d L G N at least, the optic axons run mainly parallel to the degeneration bands rather than vertically through them, and so this explanation seems unlikely. A second possibility is that since the patterns are most distinct in young animals they might reflect maturational changes; however, the same basic patterns are also evident in the adult, so this explanation also seems unsatisfactory. The most likely explanation is that they relate to some fundamental organizational pattern intrinsic to each region, and that the degeneration of optic nerve fibers to each division progresses at different rates or in a different manner. Certain findings of the present study suggest that degeneration rate relates to fiber diameter. For example, the younger the animal is, the smaller the average diameter of the optic axons 3s and the sooner the degeneration appears. A second example is the presence of an early neurofibriltar hypertrophy in nerve terminals that later become Fink-Heimer positive; this may be characteristic of large diameter axons since the number of neurofilaments normally present in an axon is in direct proportion to the axon diameter15, 50. Such a correlation between fine fiber and rapid degeneration, and coarse fiber with slower degeneration has been made in previous studies (see reviews of Joseph 27 and Lubinska31). However, a number of investigations have come to the opposite conclusion, that fine fibers take longer to degenerate than those with larger diameter. One problem with these studies is the methods used to assess degenerative change. One widely quoted investigation, that of Van Crevel and Verhaart 4s, studied degeneration rate in the cat optic nerve and came to the conclusion that fine fibers take longest to degenerate. However, they used a method -the H~iggqvist modification of Alzheimer-Mann - - which is difficult to interpret and they obtained a low estimate for the total number of axons in the optic nerve compared to other investigatorsl,3,11,zL It is probable that the finest axons are not resolvable by the light microscope. Moreover, if neurofilamentous hypertrophy can be considered a degenerative phenomenon, it is perhaps unwise to speak of degenera-

95

Fig. 7. Flat mount of albino rat retina, stained for neurofibrils, showing an example of the largest variety of ganglion cell. Such cells are rich in neurofibrils in both soma and dendrites, unlike the medium or small ganglion cells. × 248. tion starting earlier in one population of axons than another without specifying the precise nature of the degenerative change. There is also uncertainty as to whether or not nerve terminals have the same temporal sequence of degeneration as their parent axons. It is possible that nerve terminals may show degenerative changes quite out of phase with those in their axons some distance from the synaptic junction; furthermore, the degenerative changes shown by axon terminals may be dependent on their postsynaptic relationships or glial wrapping of the normal pre- and post-synaptic elements rather than on their actual size. Studies on the fiber composition of the rat optic nerve show axon diameter varies from less than 0.6 to 8.0/~m 14,39. If the larger diameter fibers are the slowest to show F i n k - H e i m e r positivity, it might be concluded that in the rat d L G N the finest optic nerve fibers terminate in the innermost lamina, the medium sized fibers terminate in the intermediate lamina, and the largest fibers terminate in the outer lamina. The uncrossed projection to the d L G N in both albino and pigmented animals would by this reasoning have a fine fiber component, and in the pigmented rat also have a medium diameter population which is absent in normal albino rats. It could well be that some crossed fiber populations supply at least two laminae since during the progression of degeneration through the nucleus, all 3 laminae may show degeneration debris simultaneously; however, there appears to be some unique relationship of

96 optic nerve fibers to each lamina since at any one time the degeneration will be concentrated in only one lamina. It has been shown previously that the outer region of the dLGN differs in its synaptic arrays from the rest of the nucleus in having optic nerve terminals encapsulated by glia in complex glomerulae, and it is these terminals that show neurofilamentous hyperplasia in the adulP 0,35. In considering the origin of these terminals, a possible candidate would seem to be the largest ganglion cells of the retina. The somata and dendrites of these ganglion cells are normally filled with neurofibrils (see Fig. 7), a feature not detected in the medium or small ganglion cells. This possible correlation may be well worth further investigation since the largest ganglion cells in the cat (alpha cells2) are also strongly filamentous40 and they have been correlated directly with Y cells which have fast conducting (i.e. large diameter) axons4L The progressive degenerative changes that occur in the superior colliculus also appear to indicate 3 laminae. The deeper half of the SGS shows the earliest and middle period Fink-Heimer positivity, the upper half of the SGS shows the middle and late periods of Fink-Heimer positivity plus the bulk of the early neurofilamentous proliferation, and the narrow surface zone lacks the latest degenerative stage but shows a particularly rich granulation during the middle period of degeneration. The early degenerative phase in the deeper half of the SGS has also been observed in the hamster43. Golgi studies29 show a different neuronal organization in the deep and upper halves of the SGS and during electron microscopic investigation of the synaptology of the SGS, compared to the deeper half. This deeper region of the SGS also receives the bulk of the projection from the visual cortex. Again, if the rate of becoming Fink-Heimer positive indicates fibers diameter, it could be suggested that the 3 fiber components of the optic projection to the superior colliculus, which have been separated physiologically using conduction velocity, terminate differentially, one in the deeper half of the SGS, one throughout all the laminae, and one in the upper half of the SGS exclusive of the most superficial region. In considering the sprouted ipsilateral projection to dLGN and colliculus in the rat following eye removal at birth, it is clear that the projection to the dLGN differs in its nature from that to the superior colliculus, since the dLGN has a Fink-Heimer positivity far exceeding in duration that shown by the projection to the superior colliculus. Also of interest is the different distribution of the sprouted pathway to the dLGN in albino and pigmented rats. The sprouted pathway in the albino occupies the outer lamina except for its most medial part which represents the ipsilateral visual field, and does not extend into the deeper part of the nucleus. In the pigmented rat the sprouted ipsilateral pathway extends deeper in the nucleus and expands its projection only into the outer lamina where it overlies the normal position of the uncrossed projection. While these different distributions could simply represent a dispersion from the normal position of the uncrossed projection in the two species as appears to be the case in the cat 2o, this seems unlikely in at least the albino rat, where an equal spread deep and superficial to the normal position of the uncrossed projection might be expected but is not found. The increased duration of FinkHeimer positivity in the albino sprouted projection to the dLGN compared to the

97 normal suggests that perhaps an additional fiber component has been added to the sprouted pathway and the distinct neurofibrillar hypertrophy in the outer lamina of the dLGN ipsilateral to the final eye removal as an adult would suggest that at least the component responsible for these terminals has been increased beyond its normal ipsilateral contribution. In the dLGN of the rat the 3 laminae defined by degenerative techniques lie parallel to the outer surface of the nucleus and the outer two curve ventrally at the posterior end of the nucleus. This orientation allows that each lamina will have a complete map of the visual field projecting to that nucleus 87. The intermediate band of degeneration curves up to the surface of the nucleus at its medial edge and is superimposed in this region by the later degeneration of the outer lamina. This area of overlap occurs in the region of visual field map which crosses the vertical midline and represents a compression of the ipsilateral visual field as viewed by the contralateral eye37. In the cat, the uncrossed projection to lamina A1 takes a shorter time to become Fink-Heimer positive than does the crossed optic projection to lamina A19,40. However, a small portion of ipsilateral lamina A1 close to the vertical midline also shows late degeneration resembling that of the crossed projection to lamina A a0. Richards and Kali140 propose that this late degeneration originates from nasal retina since it resembles the crossed projection. It is perhaps more likely that this projection does arise from temporal retina adjacent to the midline but that it is composed of a different fiber population than that projecting throughout lamina A1. In the rat also, the overlap of territory of fibers of different degeneration times occurs only in the region of temporal retinal projection between fibers arising from the same (in this case contralateral) eye. It is of interest that in the Siamese cat it is this same medial region of lamina A1 that shows projections aberrations2~; lamina A appears to curve downward and join with the medial region of A1 and the visual field then maps across the midline into m121,34. While the studies on cat21,34 investigating degeneration rates of optic fibers to each division of the dL G N show a difference in timing of degenerative change in lamina A and A1 resembling the changes described in the present study using the rat, physiological data on the distributions of optic nerve fibers with different conduction properties within the dLGN show no difference in input to the A and A1 laminae TM. Furthermore, we suggest on the basis of the present study that fine fibers may degenerate earlier than those of large diameter, but the C laminae of the cat, which receive a fine fiber ('W') input in addition to 'X' and 'Y' inputsg, 51, apparently show degenerative change later than the A and A~ laminaelL It is at present very hard to reconcile these different impressions of optic fiber organization unless the W component had already degenerated by the earliest time studied (3 days)lL Studies on the retinae of rat, cat and monkey show distinct ganglion cell populations of large, medium and small cells4,5,2L Retrograde transport of horseradish peroxidase in each species has indicated that particular sizes of ganglion cells may differ in their central projections from one species to another 5,2s,~2, and no common pattern relating ganglion cell size with a particular destination has yet emerged. It remains to be seen what relationship these connections may have with the type of degenerative reactions described here for the rat and previously in the cat.

98 In s u m m a r y , it is proposed from the results of the present study that in the raf optic nerve there are 3 s u b p o p u l a t i o n s of axons which can be differentiated on the basis o f degeneration rate a n d character. These s u b p o p u l a t i o n s have differential l a m i n a r distributions in d L G N a n d superior colliculus, differ in their c o n t r i b u t i o n to uncrossed optic projections in a l b i n o a n d pigmented animals, a n d behave differently after experimental m a n i p u l a t i o n . While 3 optic axon p o p u l a t i o n s projecting to these nuclei have been identified in previous physiological studies, it is clear that a more direct correlation is needed between a n a t o m i c a l a n d physiological data to address the p r o b l e m of parallel processing in the visual system. ACKNOWLEDGEMENTS We would like to t h a n k J. S. Foltz a n d W. L. Cartwright for photographic assistance a n d E. E. P a t t o n for secretarial help. This research was supported by N I H G r a n t s EY01086 a n d EY00596.

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