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Biomedical subjects

S S Easter

Publications and source records attributed to S S Easter.

At least 37 records · Page 2Linked to original sources
PubMed1992

Local regeneration in the retina of the goldfish.

We have studied regeneration of the retina in the goldfish as a model of regenerative neurogenesis in the central nervous system. Using a transscleral surgical approach, we excised small patches of retina that were replaced over several weeks by regeneration. Lesioned retinas from three groups of animals were studied to characterize, respectively, the qualitative changes of the retina and surrounding tissues during regeneration, the concomitant cellular proliferation, and the quantitative relationship between regenerated and intact retina. The qualitative and quantitative analyses were done on retinas prepared using standard methods for light microscopy. The planimetric density of regenerated and intact retinal neurons was computed in a group of animals in which the normal planimetric density ranged from high to low. Cell proliferation was investigated by making intraocular injections of 5-bromo-2'-deoxyuridine (BUdr) at various survival times to label proliferating cells and processing retinal sections for BUdr immunocytochemistry. The qualitative analysis showed that the surgery created a gap in the existing retina that was replaced with new retina over the subsequent weeks. The BUdr-labeling experiments demonstrated that the excised retina was replaced by regeneration of new neurons. Neuroepithial-like cells clustered on the wound margin and migrated centripetally, appositionally adding new retina to the old. The quantitative analysis showed that the planimetric density of the regenerated neurons approximated that of the intact ones.

Animals↗
PubMed1992

Acquisition of regional and cellular identities in the developing zebrafish nervous system.

In the past year, several new techniques have been used with great success in the study of nervous system development in the zebrafish. Perhaps the most exciting results have come from experiments in which single identified cells or small groups of cells have been transplanted between embryos in order to examine cell determination and the site of action of genetic mutations.

Animals↗
PubMed1992

Axonogenesis and morphogenesis in the embryonic zebrafish brain.

We have examined early neuronal differentiation and axonogenesis in the fore- and midbrain of zebrafish embryos to address general issues of early vertebrate brain development. AChE expression and HNK-1 antibody immunoreactivity were used as markers for differentiated neurons and axons, respectively. The pattern of neuronal differentiation followed a stereotyped sequence. AChE-positive cells first appeared between 14 and 16 hr in three small, isolated, bilaterally symmetrical clusters on the surface of the brain. The three clusters--the dorsorostral, ventrorostral, and ventrocaudal clusters--proved to be the progenitors of the telencephalon, ventral diencephalon, and mesencephalic tegmentum, respectively. With further development, more cells were added to these three clusters, and new clusters appeared in the anlage of the epiphysis (18 hr) and in the pituitary and dorsal mesencephalon (by 24 hr). Subsequently, as more neurons differentiated, the gaps of unlabeled cells were reduced; by 48 hr, the cluster boundaries were indistinguishable. Axonogenesis also followed a stereotyped sequence. The first HNK-1-labeled processes arose from the first three clusters of AChE-positive cells and connected the clusters. The earliest axonal growth cones appeared at 16 hr, directed caudally from two to three neurons of the ventrocaudal cluster and pioneering the ventral longitudinal tract. By 18 hr, the tract of the postoptic commissure was initiated by growth cones directed caudally from the ventrorostral cluster toward the ventrocaudal cluster. By 20 hr, axons from the dorsorostral cluster projected ventrally to form the supraoptic tract. The other dorsoventral tracts (the dorsoventral diencephalic tract and the tract of the posterior commissure) became evident between 20 and 24 hr. These observations provide a continuous record of the topological distortions involved in the conversion of the tubular embryonic brain into the contorted adult form. The telencephalon, ventral diencephalon, and hypothalamus originate from the same rostrocaudal level of the neural tube. The pattern of differentiation demonstrated that the early development of the rostral neural tube occurs simultaneously in several independent centers, similar to the overtly segmental development of the hindbrain.

Acetylcholinesterase↗
PubMed1992

Retinal growth in foveated teleosts: nasotemporal asymmetry keeps the fovea in temporal retina.

Fish retinas continue to grow throughout life by adding neurons at the margin, with the result that cells born at a peripheral site are steadily displaced toward the center of the enlarging retina. This presents a functional problem for fish with specialized temporal areas such as a fovea--how to reconcile continual growth with the maintenance of a temporal location for the fovea. One possibility is that the retina grows asymmetrically, with most new retina added nasally, relatively little temporally. I have tested this hypothesis by evaluating retinal growth in marine teleosts from 15 families, both foveated and unfoveated. The pattern of growth was revealed by exploiting the fact that each new generation of ganglion cells sends its axons into the optic nerve as a cohort; small grains of the carbocyanine dye 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine were applied to various sites in the cross section of the optic nerves of adults, and the retrogradely labeled cell bodies in the retina were visualized in whole-mounts. The labeled cells lay in annuli, each one a generation of ganglion cells. Representatives of seven of the families showed clearly asymmetric growth: the labeled annuli were close together on the temporal side and more distant nasally, the embryonic fissure curved from its ventral origin toward the temporal side, and in six of these families, labeled fibers from temporal retina skirted the fovea. Members of the other eight families, without specialized areas, had more symmetric retinal growth: labeled annuli were equally spaced on all sides, the embryonic fissure was vertical, and there were no skirting fibers. The following hypothesis is supported: the retina grows asymmetrically, and maintains the area for acute vision oriented toward the anterior field.

Animals↗
PubMed1991

A pioneering growth cone in the embryonic zebrafish brain.

During development of the nervous system, growth cones navigate very precisely to their appropriate, often distant, targets. In insects, the task of establishing the earliest pathways is accomplished by a small number of neurons, termed pioneers. These neurons have axons that lay down an early scaffold, which provides a substrate for many later-developing axons. Here we show that a similar type of cell exists in the embryonic vertebrate brain. Using light- and electron-microscopic techniques we have examined the formation of one of the earliest tracts in the zebrafish brain. We find that it is pioneered at a precise time by the growth cone of a single neuron present in a predictable location. These observations show a fundamental similarity in the establishment of axonal pathways in the central nervous systems of both invertebrates and vertebrates.

Animals↗
PubMed1991

Stereotyped pathway selection by growth cones of early epiphysial neurons in the embryonic zebrafish.

In this report we have examined the development of one of the earliest projections in the embryonic zebrafish brain, that from the epiphysis. Epiphysial axons and growth cones were labelled anterogradely in whole-mounted brains, using either the carbocyanine dye, diI, or horseradish peroxidase (HRP). Some embryos were also either stained with anti-acetylated tubulin or HNK-1 antibodies to reveal other axons in the brain, or were secondarily sectioned for light and electron microscopy. The epiphysial axons have a very specific projection pattern and virtually all axons grow precisely to their target regions without error. The first epiphysial growth cone extends ventrally from the epiphysis into the dorsoventral diencephalic tract at 19-20 h post-fertilisation (h PF). Several hours later, it turns rostrally to grow alongside axons in the tract of the postoptic commissure. The morphology of the leading growth cone changes in predictable ways at different locations along its pathway and these changes correlate with differences in the local environment that it encounters. In contrast to other published descriptions of other developing systems, the epiphysial growth cone is no more complex either when pioneering a pathway, or when encountering divergent axonal pathways. Indeed, it is most complex (i.e. has the greatest number of processes) when it first starts to follow the tract of the postoptic commissure. The presence and selective retention of filopodia within other axonal pathways suggests that growth cones have access to these pathways but do not select them. These observations support the notion that local guidance cues exist within the early scaffold of brain tracts. Subsequent epiphysial axons form a tight fascicle within the dorsoventral diencephalic tract, but abruptly defasciculate from each other upon turning rostrally into the tract of the postoptic commissure. Epiphysial growth cones that enter this tract at abnormal locations still turn in the appropriate direction. Therefore, guidance cues are not restricted solely to the normal intersections but may be distributed along the length of the tracts. The epiphysial growth cones and axons have very characteristic spatial relations to other axons in the tracts of the developing brain. They are restricted to the dorsal region of the tract of the postoptic commissure and the rostral region of the postoptic commissure. At early developmental stages, the epiphysial axons are the only axons within the dorsoventral diencephalic tract and they are located very superficially within the neuroepithelium. At later stages, they are displaced to deeper regions of the neuropil by non-epiphysial axons.

Animals↗
PubMed1990

The development of a simple scaffold of axon tracts in the brain of the embryonic zebrafish, Brachydanio rerio.

We have examined neuronal differentiation and the formation of axon tracts in the embryonic forebrain and midbrain of the zebrafish, between 1 and 2 days postfertilisation. Axons were visualised with three techniques; immunocytochemistry (using HNK-1 and antiacetylated tubulin antibodies) and horseradish peroxidase (HRP) labelling in whole-mounted brains, and transmission electron microscopy. Differentiation was monitored by histochemical staining for acetylcholinesterase (AChE). These independent methods demonstrated that a simple grid of tracts and commissures forms the initial axon scaffold of the brain. At 1 day, the olfactory nerve, four commissures, their associated tracts and three other non-commissural tracts are present. By 2 days, these tracts and commissures have all greatly enlarged and, in addition, the optic nerve and tract, and three new commissures and their associated tracts have been added. Small applications of HRP at various sites revealed the origins and projections of some of these earliest axons. Retrogradely labelled cell bodies originated from regions that were also positive for AChE activity. At 1 day, HRP-labelled axons were traced: (1) from the olfactory placode through the olfactory nerve to the dorsal telencephalon; (2) from the telencephalon into the tract of the anterior commissure and also to the postoptic region of the diencephalon; (3) from the hindbrain through the ventral midbrain and diencephalon to the postoptic commissure; (4) from the dorsal diencephalon (in or near the epiphysis) to the tract of the postoptic commissure; (5) from ventral and rostral midbrain through the posterior commissure. Three new projections were demonstrated at 2 days: (1) from the retina through the tract of the postoptic commissure to the tectum; (2) from the telencephalon to the contralateral diencephalon; and (3) from the telencephalon to the ventral flexure. These results show that at 1 day, the zebrafish brain is impressively simple, with a few small, well-separated tracts but by 2 days the brain is already considerably more complex. Most of the additional axons added onto pre-existent tracts rather than pioneered new ones supporting the notion that other axons play a crucial role in the guidance of early central nervous system (CNS) axons.

Acetylcholinesterase↗
PubMed1989

Is the capacity for optic nerve regeneration related to continued retinal ganglion cell production in the frog?

In the central nervous system of fish and frogs, some, but not all, axons can regenerate. Retinal ganglion cells are among those that can. The retinae of fish and frogs produce new retinal neurons, including ganglion cells, for months or years after hatching. We have evaluated the hypothesis that retinal axonal regeneration is obligatorily linked to continued production of new ganglion cells. We used bromodeoxyuridine immunocytochemistry to assess retinal neurogenesis in juvenile, yearling, and 10 year old Xenopus laevis. Retinal ganglion cell genesis was vigorous in the marginal retina of the juveniles, but in the yearlings and the 10 year olds, no new ganglion cells were produced there. Cellular proliferation in the central retina was evident at all three ages, but none of the cells produced centrally were in the ganglion cell layer. Regeneration was examined in vivo by cutting one optic nerve and then, weeks later, injecting the eye with tritiated proline. Autoradiographs of brain sections showed that the optic nerves of all three ages regenerated. Regeneration in vitro was assessed using retinal explants from frogs of all three ages. In all cases, the cultures produced neurites, with some age-specific differences in the patterns of outgrowth. We conclude that retinal axonal regeneration is not linked obligatorily to maintained neurogenesis.

Journal Article↗
PubMed1989

The development of the Xenopus retinofugal pathway: optic fibers join a pre-existing tract.

The developing optic nerve and tract have received considerable attention in recent years, but the cellular and subcellular microenvironment of the growing axons has not been described. In the belief that such a description is essential (though certainly not sufficient) for an understanding of pathway formation, we have examined the normal development of the retinofugal projection of Xenopus laevis. Optic fibers were labeled anterogradely at the retina with horseradish peroxidase (HRP) or the carbocyanine dye, DiI, at stages 32 to postmetamorphosis. The brains were examined both as whole mounts and in sections, light- and electron-microscopically, with the emphasis on tracts associated with the route of the optic fibers. At stage 32, two ventral commissures were present, the anterior and postoptic. They were immediately subjacent to the pia. All tracts and even isolated axons were in similarly superficial locations. The first deep pathway (separated from the pia by cell nuclei) was seen at stage 46; it was a dorsal commissure, probably the posterior. The first retinal axons passed from the optic stalk into the ventral part of the diencephalon, where they coursed along the rostral edge of the postoptic commissure, and maintained this position, relative to the other fibers in the tract of the commissure, throughout the remainder of their contralateral trajectory. They reached the presumptive thalamic and tectal termination sites and arborized. Subsequent optic axons followed this same route, thus enlarging the optic pathway relative to the more slowly growing nonoptic part of the commissure and its tract. Electron microscopy revealed, as early as stage 35, specialized contacts between cellular processes in the neuropil. These contacts had the form of symmetric membranous thickenings; some were associated with vesicles and were presumed to be synapses. We conclude that the early forebrain and midbrain have only two ventral commissural pathways, and most axons that grow out after these pathways have formed add to them rather than establish new tracts. The optic axons travel a stereotyped pathway alongside a pre-existing tract associated with the postoptic commissure. The possibility that optic fiber outgrowth is normally influenced by pre-existing tracts is discussed in relation to recent experimental investigations of fiber growth from ectopic eyes.

Animals↗
PubMed1988

Regenerated optic fibers in goldfish reestablish a crude sectoral order in the visual pathway.

The goldfish optic pathway is regenerated after an optic nerve crush. We have examined the axonal topography of the regenerated pathway by labeling, with horseradish peroxidase (HRP), axons originating from retinal sectors or annuli. The positions of the labeled axons in the cross section of the pathway were compared to the normal and related to the factors that may influence axonal pathfinding. The positions of retinal axons in the cross section of the normal pathway are predictable from the retinal addresses of the ganglion cells described by the polar coordinates r (the distance from the optic disc) and theta (the sectoral or clockface position). The two coordinates map orthogonally onto the cross section of the pathway; r varies monotonically along one axis; theta varies along a perpendicular axis. The normal r-order, present in the nonregenerated stump of the experimental nerve, was severely degraded and perhaps lost entirely in the regenerated optic nerve, tract, and brachia. Sectoral order was also lost as the axons passed the crush site, but it was reestablished, albeit crudely, in the regenerated tract and brachia where axons tended to occupy positions appropriate to their dorsal, ventral, nasal, and temporal retinal origins. The exit sequence of the regenerated axons from the stratum opticum into the tectal neuropil was normal: temporal first, nasal last. These results suggest that the regenerating fibers followed some theta-specific cue located in the nonaxonal environment. It seems likely that the original axons probably followed the same cue. In contrast, the absence of r-order suggests that there is no r-specific cue for the regenerates to follow. It seems likely that the original r-order was a consequence of nonspecific influences--the orderly spatiotemporal growth of the retina and the existence of a permissive region for axonal growth.

Animals↗
PubMed1988

Axons added to the regenerated visual pathway of goldfish establish a normal fiber topography along the age-axis.

Throughout a goldfish's life, new generations of ganglion cells are added on the retinal margin and their axons extend centrally to occupy predictable positions in the retinotectal pathway, adjacent to their predecessors and subjacent to the pia. The stacking of successive generations of axons defines the age-axis of the pathway. This study examined whether an ordered array of predecessor axons is a prerequisite for the patterned growth of new axons. One optic nerve was crushed intraorbitally and the fish was injected with 3H-thymidine to label the proliferating cells on the retinal margin. The ring of 3H-thymidine-labeled cells separated retina that was present at the time of nerve crush (inside the ring) from new retina added afterward (outside). After a period of 14-16 months postcrush, both tectal lobes received two punctate applications of horseradish peroxidase (HRP), one in the central and the other in peripheral tectum, to retrogradely label contralateral retinal ganglion cell bodies and their axons. The pattern of HRP labeling from the control tectum confirmed earlier work: axons on the central tectum had somata in the central retina, and axons on the peripheral tectum had somata in the peripheral retina. The labeled cells and axons were both in predictable patterns. The somata that were backfilled from applications to the center of the experimental tectum lay inside the radioactive ring and had therefore regenerated their axons. The patterns of their labeled axons in the optic pathway and of their somata in the retina were typical of the regenerated condition as described in earlier studies. The somata backfilled from the periphery of the experimental tectum were outside the radioactive ring and had been added after the optic nerve crush. The patterns of their labeled axons and somata were comparable to the normal pattern. These observations indicate that new axons do not depend on an ordered array of predecessors to reestablish normal order along the age-axis of the pathway.

Aging↗
PubMed1987

Evidence for centripetally shifting terminals on the tectum of postmetamorphic Rana pipiens.

In larval frogs the retina and tectum grow in topologically dissimilar patterns: new cells are added as peripheral annuli in the retina and as caudal crescents in the tectum. Retinotopy is maintained by the continual caudalward shifting of the terminals of the optic axons. After metamorphosis the pattern of growth changes. The retina continues to add new ganglion cells peripherally, but there is no neurogenesis in the tectum. To maintain retinotopy in postmetamorphic frogs, the terminals of the optic axons must continually shift toward the central tectum. We tested the proposal of centripetally shifting axons by making punctate injections of horseradish peroxidase (HRP) in the tectum of adult Rana pipiens and observing the patterns of filled cells in the contralateral retina, as was done in the goldfish (Easter and Stuermer, '84). Punctate applications of HRP in the tectum should be taken up: 1) by fascicles, and label a partial anulus of cells, 2) by terminals, and label a cluster of cells in the corresponding retinotopic site, and 3) by the extrafascicular axonal segments, and label a band of cells connecting the partial annulus to the cluster. If the terminals have shifted centripetally, the band of cells labeled through their extrafascicular segments should have a spoke-like orientation, with the center of the retina as the hub. As the tectal site moves from rostral to caudal, this band of cells should move, pendulum-like, from temporal to nasal retina. In general, the patterns of HRP-filled retinal cells we observed were consistent with our predictions. In addition, HRP taken up by the oldest (rostral) tectal axons produced more complex patterns of filled cells that indicated that these axons had shifted both caudally before metamorphosis and centripetally after.

Animals↗
PubMed1986

Map of retinal position onto the cross section of the optic pathway of goldfish.

The position of a retinal cell is defined by the two polar coordinates: r, the distance from the optic disc, and theta, the angular (or clock-face) position. Axons of similar theta value were labeled by the punctate application of horseradish peroxidase (HRP) to optic axons in the retina, and axons of similar r-value were labeled by the application of this same marker to a tectal fascicle. Labeled axons were traced in serial transverse sections of the optic pathway from the retina to the tectum to learn the map of the retinal surface onto the cross section of the pathway. Retinas were flat-mounted and treated for HRP to show the retinal origins of the labeled axons. Axons of similar r were clustered together, and the fraction of the pathway's cross-sectional area occupied by the cluster was about the same as the fraction of the retinal area occupied by the group of labeled somata. Axons of similar theta were also clustered, but the fraction of the cross-sectional area they occupied was larger than the fraction of retinal area occupied by their somata. The geometry of the clusters of labeled axons depended on the proximodistal location in the pathway. Near the retina both were strip-shaped, but the location and orientation of the strip varied. Both an r-strip and a theta-strip were labeled in some pathways by dual applications of HRP; the two strips were mutually orthogonal at all levels. Each of r and theta mapped onto a separate axis. The axons from most peripheral retina (largest r) were everywhere adjacent to the pia, and axons of progressively more central retina (smaller r) were progressively more separated from the pia (except in the nerve, where the secondary fasciculation complicates the geometry by wrapping old axons in new pia). The map of the circular variable, theta, onto a line, required a discontinuity, the location of which differed, depending on the proximodistal level. From the retina to the chiasm, the discontinuity was at the ventral retinal radius (i.e., the right retinal clock-face positions were ordered 6-9-12-3-6 o'clock across the line); just central to the chiasm, the fibers reordered to put the discontinuity at the nasal radius (clock-face positions ordered 3-6-9-12-3); at the brachial bifurcation, the 3-6-9 half turned dorsally, the 9-12-3 half, ventrally.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1986

The myopic eye of the Black Moor goldfish.

Optical and anatomical methods were used to establish the refractive state of the eye of the Black Moor variety of goldfish. The eye is strongly myopic. The refractive error in 12 eyes from 6 fish ranged from 89 to 268 D.

Animals↗
PubMed1986

Retinal ganglion cells in goldfish: a qualitative classification into four morphological types, and a quantitative study of the development of one of them.

In this paper we describe the dendritic morphology of ganglion cells that have been retrogradely stained with HRP taken up by the cut optic nerve. This technique produces an extensive Golgi-like filling of the cells. From their appearance in the retinal whole-mount, they were classified as four types, according to the sizes of the soma and dendritic field, the thickness of the primary dendrites, and the density of the arbors. Each type was subdivided according to the level(s) of stratification of the dendrites within the inner and outer plexiform layer(s) to yield a total of 15 subtypes (four for three types, three for the other). The retina of the goldfish grows by a balloon-like expansion, and by the addition of new neurons, in annuli, at the margin. Therefore, a similar cell type may be examined at a variety of stages of development in the same retina, as well as in the retinae from fish of various ages. We have used a computer-assisted microscope to do so, quantitatively, for one large and easily identified subtype. In small fish (ca. 4 cm long), the number of dendritic branch points, the total dendritic length, and the dendritic field sizes of these cells are constant inside a central zone extending to 70-80% of the retinal radius. The magnitudes of all three numeric descriptors decrease closer to the margin. In large fish (ca. 14 cm long), the central zone extends to more than 90% of the retinal radius, and the same pattern holds. The area of the dendritic fields and the total dendritic lengths are both greater in the central zone of the large fish than in the small, but the number of branches is the same in both. This suggests that once a cell has achieved the "mature" number of dendritic branches, further growth is interstitial. A comparison of dendritic morphologies across the retina shows that the pattern of dendritic outgrowth in peripheral retina is initially directed parallel to the margin, and, later, toward the margin. This suggests that dendritic growth is impeded by the dendrites present in more central retina and proceeds preferentially where they are absent. Cells of the same age are at different distances from the optic disk in the small and large retinae. In some cases, they have quite different dendritic morphologies. This implies that dendritic development depends not only on the age and subtype of the cell, but on extrinsic factors as well.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1985

The changing view of neural specificity.

The generation of specific patterns of neuronal connections has usually been regarded as a central problem in neurobiology. The prevailing view for many years has been that these connections are established by complementary recognition molecules on the pre- and postsynaptic cells (the chemoaffinity theory). Experimental results obtained in the past decade, however, indicate that the view that axon guidance and synaptogenesis proceed according to restrictive chemical markers is too narrow. Although a more rigid plan may prevail in some invertebrates, the formation of specific connections in vertebrates also involves competition between axon terminals, trophic feedback between pre- and postsynaptic cells, and modification of connections by functional activity.

Animals↗
PubMed1984

A comparison of the normal and regenerated retinotectal pathways of goldfish.

This is a light and electron microscopic study of the retinotectal pathway: intact and after regeneration of the optic nerve. The spatiotemporal pattern of axonal outgrowth and termination was studied with the methods of proline autoradiography, horseradish peroxidase (HRP) labeling, and fiber degeneration. The spatial order of optic fibers in the normal and regenerated pathways was assessed by labeling small groups intraretinally with HRP and then tracing them to the tectum. The labeled fibers occupied a greater fraction of the cross section of the regenerated than the normal optic tract. At the brachial bifurcation, roughly 20% of the regenerated fibers chose the incorrect brachium vs. less than 1% of the normals. In tectum, the regenerated optic fibers reestablished fascicles in stratum opticum, but they were less orderly than in the normals. The retinal origins of the fibers in the fascicles were established by labeling individual fascicles with HRP and then, following retrograde transport, finding labeled ganglion cells in whole-mounted retinas. Labeled cells were more widely scattered over the previously axotomized retinas than over the normal ones. A similar result was obtained when HRP was applied in the tectal synaptic layer. All of these results indicate that the pathway of the regenerated optic fibers is less well ordered than the intact pathway. Both autoradiography and HRP showed that the regenerating optic fibers invaded the tectum from the rostral end, and advanced from rostral to caudal and from peripheral to central tectum, along a front roughly perpendicular to the tectal fascicles. Synapses of retinal origin were noted electron microscopically in the tectum at the same sites where autoradiography indicated that the fibers had arrived. No retinal terminals were seen where grain densities were at background levels. Fiber ingrowth and synaptogenesis apparently occurred simultaneously. The synapses were initially smaller and sparser than in normals, but were in the normal tectal strata and contacted the same classes of post synaptic elements as in normals.

Animals↗