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S S Easter

Publications and source records attributed to S S Easter.

At least 55 records · Page 3Linked to original sources
PubMed1984

Degenerative and regenerative changes in the trochlear nerve of goldfish.

The features of unlesioned and lesioned trochlear nerves of goldfish have been examined electron microscopically. Lesioned nerves were studied between 1 and 107 days after cutting or crushing the nerve. Unlesioned nerves contained, on average, 77 myelinated axons and 19 unmyelinated axons. The latter were found in 1-2 fascicles per nerve. A basal lamina surrounded each myelinated axon and fascicle of unmyelinated axons. The numbers of myelinated axons, fascicles of unmyelinated axons and basal laminae varied by less than 5% over the intraorbital extramuscular segment of the nerve. Following interruption of the nerve, by either cutting or crushing, all of the axons and their myelin sheaths began to degenerate by 4 days in the distal nerve-stump. Both abnormally electron-dense and electron-lucent axons were observed. Both Schwann cells and macrophages appeared to phagocytose the myelin sheaths. Following a lesion, the Schwann cells and their basal laminae persisted in the distal nerve-stump. In crushed nerves, the basal laminae surrounding myelinated axons formed 97%, on average, of the Schwann tubes in the distal stump. The perimeters of the basal laminae were of similar size to those in the proximal stump, at least for the first 8 days after crush. In crushed nerves, single myelinated axons in the proximal nerve-stump gave rise to multiple sprouts, some of which reached the site of crush by 2 days, the distal stump by 4 days and the superior oblique muscle by 8 days. The regeneration of the unmyelinated axons was not examined. In both crushed and transected nerves, nearly all of the sprouts in the proximal and distal stumps were found within the basal laminae of Schwann cells, even though the spouts were disorganized in the transected region where there were no basal laminae. The growth cones of the regenerating axons were always found apposed to the inner surface of the basal laminae, which may have provided an adhesive substrate that directed their growth. Terminal sprouts from the ends of myelinated axons in the proximal stump accounted for the majority of the regenerating axons in the distal stump, as only a few collateral sprouts were found in the proximal stump, and only a small amount of axonal branching was found within the distal stump itself. The largest axons in the distal stump were remyelinated first, and the number of remyelinated axons increased progressively between 8 and 31 days after crush, at which time there were about twice as many as in unlesioned nerves.(ABSTRACT TRUNCATED AT 400 WORDS)

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PubMed1984

Rules of order in the retinotectal fascicles of goldfish.

Individual fascicles of retinal axons were labeled in the goldfish tectum with horseradish peroxidase (HRP). The contralateral retina was later processed for HRP histochemistry to mark the cells that had axons in the fascicles. Labeled cells were found in a partial half anulus in ventral hemiretina, centered on the optic disk. The distance of the partial anulus from the disk depended on which tectal fascicle had been labeled; the more rostrocentral the fascicle, the smaller was the annular radius. The angular subtense of the partial anulus with respect to the disk depended on where (along its tectal course) the fascicle had been labeled; the more rostral the label site, the longer was the angular subtense. These results were interpreted in the context of retinotectal growth, and it was inferred that the axons followed two rules: (1) grow in along the edge of the tectum and (2) exit and terminate in order, axons from temporal retina first, nasal retina last. These rules would produce a retinotopic projection in peripheral tectum, but they require that some of the terminals already in place must shift as the tectum grows.

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PubMed1984

An evaluation of the hypothesis of shifting terminals in goldfish optic tectum.

It has been proposed that the terminal arbors of retinal ganglion cells continually shift positions in the optic tectum as a natural consequence of growth in larval amphibians and teleost fish. Here, we develop a detailed anatomical model of this process, based on recent observations in goldfish. The model predicts some unusual structural features of optic axons in the tectum. It was tested in two ways by use of the tracer, horseradish peroxidase (HRP). In the first, optic axons in the tectum were visualized by anterogradely transported HRP applied in the nerve or retina. As predicted, the axons had three parts: (1) fascicular segment (in the stratum opticum), (2) extrafascicular segment (in the synaptic neuropil), and (3) terminal arbor (also in the neuropil). The extrafascicular segment extended up to 1 mm caudally from the fascicle through which it entered the tectum. At its terminus, the arbor was restricted to a single lamina in which it branched extensively over an elliptical area with major axes between 100 and 200 micron long. In the second test, retinal ganglion cell bodies were labeled by retrogradely transported HRP applied to the tectum in a manner intended to label all axons and terminals in the vicinity. The pattern and locations of labeled cells varied, depending on where the HRP was applied, in detailed accord with the model's predictions. The close agreement between the model and the observations supports the hypothesis of shifting terminals.

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PubMed1984

Growth-related order of the retinal fiber layer in goldfish.

The retinal fiber layer and the juxtaretinal portion of the optic nerve of goldfish have been studied with light and electron microscopy in order to determine whether the age-related order of fibers in the nerve originates in the retina. In the retina, no patent spaces (channels) were noted. The fibers ran in fascicles and consisted of two classes: nonmyelinated fibers, which ran superficially (close to the vitreal surface), and "myelinated" fibers, which ran more deeply and were loosely wrapped by processes presumed to be glial. The myelinated fibers were larger and presumably older. The nonmyelinated fibers are believed to be the young ones, from the peripheral, more recently generated, ganglion cells, for the following reasons. (1) Their size and cytoskeletal elements were typical of young axons. (2) They were the only axons in peripheral retina. (3) They were continuous with the nonmyelinated fibers in the nerve, previously shown to be the young ones. (4) When retinal axons were cut peripherally, the degenerating axons were in the superficial part of the fiber layer. (5) Growth cones, presumably from the newest ganglion cells, were always observed at the most superficial position in the fiber layer, in direct contact with the basal lamina of the inner limiting membrane superficially and nonmyelinated fibers deeply. The nonmyelinated fibers always clustered together in the retinal fiber layer and occupied the most central portion in the cross-section of the optic nerve head. Thus, the age-related organization of fibers in the nerve is established in the retina. These results are discussed in the context of growth, with the aim of evaluating the relative importance of four factors that might influence the intraretinal course of the growth cone. Its interactions with other fibers and with the basal lamina of the inner limiting membrane seem to be more important than interactions with the glial end feet or guidance into open, preformed channels.

Aging↗
PubMed1983

Postembryonic growth of the optic tectum in goldfish. I. Location of germinal cells and numbers of neurons produced.

The growth and morphology of the optic tectum of adult goldfish were studied with light and electron microscopy and with thymidine radioautography. The tectum is roughly hemispheric in shape, with a smaller radius of curvature rostrally than caudally. A narrow region containing proliferating cells (the germinal zone) is found along two-thirds of the rim of the tectal hemisphere but is absent rostrally, adjacent to the tectal region which receives input from the rostral visual field. New cells generated in the germinal zone are added to the tectum appositionally in crescent-shaped increments; these was no evidence of migration of new cells into the rostral region which lacks a germinal zone. Some of the new cells added to the adult tectum were shown to be neurons on the basis of cytological and ultrastructural features. Counts of tectal neurons likewise demonstrated that new cells were added with growth of the tectum; large goldfish (25 cm long) had 27% more tectal neurons than did small fish (4 cm long). Spreading apart of existing cells also contributed to overall growth of the tectum. These results confirm and extend those of R. L. Meyer ((1978) Exp. Neurol. 59: 99-111). The topological dissimilarity of the patterns of growth of retina (which adds cells appositionally around its entire perimeter) and tectum supports the suggestion that retinotectal terminals must continually move (Gaze R. M., M. J. Keating, A. Ostberg, and S. H. Chung (1979) J. Embryol. Exp. Morphol. 53: 103-143). Our estimates of cell numbers and tectal areas lead to predictions about the directions and magnitudes of these displacements.

Aging↗
PubMed1983

Postembryonic growth of the optic tectum in goldfish. II. Modulation of cell proliferation by retinal fiber input.

The proliferation of cells in the germinal zone of the optic tectum of adult goldfish was studied following unilateral optic nerve crush or removal of one eye. Dividing germinal cells were labeled with [3H]thymidine, which was injected at various times (0 to 30 days) following surgery; fish were sacrificed after short (48 hr) survival times. The numbers of labeled nuclei in the tectal germinal zones were compared on the two sides (intact and denervated). We show that permanent removal of optic input (by enucleation) resulted in a sustained depression of [3H]thymidine incorporation in the tectal germinal zone on the denervated compared to the intact side. Temporary denervation (by optic nerve crush) initially had a similar effect; however, upon reinnervation of the tectum by regenerating optic fibers, proliferation was enhanced on the experimental side compared to the intact side. Because cells in the germinal zone are known to produce new tectal cells, neurons as well as glia, in the normal growing adult brain (Raymond, P. A. and S. S. Easter, Jr. (1983) J. Neurosci. 3: 1077-1091), some of the proliferating cells may have been generating neurons. This inference is supported by the observation that in two fish whose right eye had been removed more than 2 years earlier, there were fewer neurons in the denervated tectum than in the intact tectum. Thus, it is likely that the observed decrease in incorporation of [3H]thymidine by cells in the germinal zone of the denervated optic tectum resulted in a slower rate of addition of new tectal cells on the affected side. We conclude that cytogenesis in the germinal zone of the growing optic tectum of adult goldfish is regulated by optic fiber input. This mechanism may be important in matching the rates of growth of retina and tectum in the normal brain of the growing adult fish.

Aging↗
PubMed1982

Fish vision and the detection of planktonic prey.

Planktivorous sunfish of various sizes were studied to ascertain whether growth-related changes in the retina are related to the ability to capture small planktonic crustaceans. Behaviorally, the larger fish detected and captured crustaceans that subtended smaller visual angles. Histological examination of the retinas revealed that the distance between cones, measured in minutes of visual angle, decreased as the animals grew, suggesting that the larger retinas could resolve smaller objects. These correlated behavioral and anatomical results suggest that improved visual resolution contributes to improved predation. This finding provides a selective advantage for the continuous retinal growth noted in many fish.

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PubMed1981

The growth and organization of the optic nerve and tract in juvenile and adult goldfish.

The optic nerves, tracts, and tecta of goldfish, 1 to 5 years old, have been studied anatomically using light and electron microscopy, horseradish peroxidase (HRP), and tritiated proline radioautography. The aims were to document an earlier inference that fibers are added to the nerve continually and to describe the growth and organization of the pathway. (1) The numbers of optic fibers were counted in electron micrographs of the nerve. There were about 120,000, 165,000 and 180,000 in 1-, 3-, and 5-year-old fish, respectively. (2) In young fish, there are a few thousand nonmyelinated fibers which exit the retina together and cluster together in the nerve and tract. When the axons of only the newest (peripheral) ganglion cells were cut intraretinally, fibers in and around the bundles of nonmyelinated fibers degenerated. The nonmyelinated fibers are, therefore, the new ones. (3) Fibers from ventral or dorsal hemiretinas were backfilled selectively with HRP introduced into one of the brachia of the optic tract. Behind the optic papilla, where the cross-section of the optic nerve was trapezoidal, the new fibers were found in a strip along the narrow base of the two flanking zones. Closer to the brain, the fibers from the two hemiretinas intermingled before being segregated again at the origin of the brachia. (4) Small groups of ganglion cells were labeled by intraretinal injection of HRP and their fibers were traced in sections of the nerve and tract. The labeled fibers were clustered, but the positions of the fibers in the cross-section of the nerve were defined less precisely than the positions of the somata in the retina. (5) Hemisection of the nerve in the orbit, followed by intraocular injection of tritiated proline, produced radioautographs with an unlabeled annular zone of tectum. Since the retina projects topographically to the tectum, the severed fibers must have originated from an annular region of the retina. We infer that new fibers are added to the nerve continually and that the retinal origins of fibers are correlated with their positions in the cross-section of the nerve. These rules of order change with distance from the retina; the strict order at the optic papilla changes gradually to an equally strict but different, order at the level of the brachia.

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PubMed1980

Order in the optic nerve of goldfish.

A small amount of horseradish peroxidase, injected into the goldfish optic nerve and transported into the retina, filled an annulus of ganglion cells. Since the retina grew by annular addition of cells, this result shows that axons from cells of similar age clustered together in the nerve.

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PubMed1979

Retinal synaptic arrays: continuing development in the adult goldfish.

We report a light- and electron-microscopic examination of the inner plexiform layer of the central retina of young (c. 1 year) and old (3-4 year) goldfish. There were no new neurons added to this region during the growth period. Nonetheless, there were substantially more synapses (per cell, per mm2, or per degree 2) in the older retinas. This result is discussed in the contexts of retina function and neural development.

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PubMed1979

The growth and development of the superior oblique muscle and trochlear nerve in juvenile and adult goldfish.

Superior oblique muscle/trochlear nerve pairs from goldfish of various ages (1-5 years) have been examined light and electron microscopically. The muscle grows by enlargement (longitudinally and transversely) of individual fibers, and by addition of new ones at the rate of about 250/year. The nerve grows by enlargement of fibers, but few and perhaps no new axons are added. The somata enlarge, and the neuromuscular synapses become much more numerous. The ratio of muscle fibers to nerve fibers increases from about 5 in the young to about 16 in the old fish.

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PubMed1978

Independent biaxial reorganization of the retinotectal projection: a reassessment.

It has been previously suggested that the retinotectal projection can reorganize independently along two orthogonal tectal axes. This possibility was reexamined by removing roughly a quarter of the retina and slightly less than a quarter of the tectum. In the tectal case, the unseated fibers arborized rostral to the ablation, but not lateral to it, and the projection shifted irrespective of tectal axes to maintain topographic order and a roughly uniform representation of retinal areas. In the retinal case, expansion into the denervated quadrant was only from the rostral, never from the medial or lateral directions. Analysis of the movements of fiber arbors shows that they respond to local competition for tectal space rather than following tectal axes.

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PubMed1978

Expansion of the half retinal projection to the tectum in goldfish: an electrophysiological and anatomical study.

The topographical retino-tectal projection of goldfish was electrophysiologically mapped at various intervals after surgical removal of the nasal half of the retina and pigment epithelium. The remaining projection was initially restricted to the appropriate rostral half of the tectum, even if the nerve was crushed and allowed to regenerate. But later, after 137 days or more, it showed a progressive expansion onto the foreign caudal half of the tectum. The magnification factor, the number of micrometers of tectum per degree in the visual field, doubled in the rostro-caudal but not in the medio-lateral direction. Analysis of the sequence of the expansion showed that a few fibers originally projecting nearest the denervated area were the first to spread over it. Then, progressively more fibers moved caudally until a nearly uniform representation of the half retina was established on the tectum. Radioautography also demonstrated that retinal fiber terminals had invaded the caudal tectum. The retinae of these fish were also examined histologically. The density of ganglion cells had not increased, but they consistently showed the axonal reaction. This was not found to be associated with any initial surgical trauma, but rather with the movement of their fiber terminals within the tectum. Frozen sections through half retinal and normal eyes, were cut and photographed for comparison of ocular geometry. Operated eyes were normal except for a slight but consistent loss of ocular volume. Analysis of the optical geometry showed that recording with fish in air produced two effects: Myopia (10 degrees blur circle, or less) and enlargement of the visual field by 15 percent to 20 percent.

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