Stephen S. Easter

Neuroanatomy of the Zebrafish Brain: A Topological Atlas

This is a very impressive atlas of the brain of the adult zebrafish, and I know of no comparable atlas on any fish brain. The data are a large number of clear and well-labeled double illustrations (a photomicrograph and an interpretive line drawing) of transverse, sagittal and horizontal sections. Each is accompanied by a small sketch of the whole brain with the plane of that particular section indicated. Every illustration has its own table of abbreviations, which is a great help to the forgetful reader. Furthermore, the back of the book contains an alphabetical list of structures given by both their Latin and English names, with their abbreviations and the page numbers on which they are either mentioned or illustrated. The accompanying text (a concise 26 pages) is divided into three chapters: these provide a link to the embryonic brain, an overview of the adult brain, and a comparative evaluation of the zebrafish brain relative to those of other fish. My only criticism is that the text omits references to particular illustrations; hence, the reader who wants to see a picture of what is described in the text must look in the back of the book, search for the name of the structure, and then go to the appropriate page, rather than finding the page directly.The illustrations, which are the heart of the book, can be taken as authoritative. Mario Wullimann, the senior author, is an able comparative neuroanatomist who specializes in fish. Therefore, the reader can be quite sure that his interpretations are not obviously wrong, and are thus arguably correct. Such a guarded conclusion is necessary because the amount of research on fish brains is minuscule compared with mammals, for example, so there are plenty of issues that simply have not been addressed thoroughly enough to produce consensus. Moreover, fish neurobiologists often take the comparative approach, which is to use taxonomic differences as a source of variation to be studied usefully in its own right; they have not adopted a standard fish, whereas mammalian neurobiologists have made the rat a standard mammal. However, for some research topics, one fish is probably just as good as another, and one of the benefits of an integrative book such as this is that it provides a reason for a researcher to choose the zebrafish as a subject.Of course, most researchers studying the zebrafish brain study embryos and larvae, not the adult, and the young brains look distressingly unlike those in the adults. While some structures, such as the eye and the nose and the nerves that issue from them, can be identified unequivocally both early and late in life, others are not so easy. For example, the most prominent longitudinal tract in the embryonic fore-brain is the ‘tract of the postoptic commissure’1xWilson, S.W. et al. Development. 1990; 108: 121–145PubMedSee all References; one would suppose that its equivalent in the adult is the ‘postoptic commissure’, but we learn from the text that this commissure subsumes two others (the ‘minor’ and the ‘transverse commissures’), and neither text nor illustrations indicate where the fibers in these commissures originate and terminate. This is not intended as a criticism of the authors, who are admirably conservative (if they do not know the origins and terminations, they do not guess), but it does illustrate the difficulty of direct comparisons. The adult minor and transverse commissures cross the midline near one another, and the tracts that issue from each follow different paths that are more easily discerned in fish other than the zebrafish. The embryonic postoptic commissure was given that name because it lies just behind the optic stalk. In the absence of more information, it is impossible to say whether the axons in the embryonic postoptic commissure are the pioneers of the adult transverse or minor commissures or both.Another source of uncertainty is the use of similar names to describe different structures. For example, the ‘lateral longitudinal fascicle’ is described in the atlas as the fishs version of the mammalian lateral lemniscus, a tract of mostly ascending fibers; however, a descending sensory tract formed initially by the trigeminal afferents has also been called the lateral longitudinal fascicle in embryos2xMetcalfe, W.K., Mendelson, B., and Kimmel, C.B. J. Comp. Neurol. 1986; 251: 147–159Crossref | PubMed | Scopus (214)See all References. The developmental neurobiologist whose own work focuses on a particular structure, and who hopes to learn from this atlas the ultimate disposition of that structure in the adult, will frequently be disappointed, and not only because of ambiguous terminology. Structural comparisons are complicated by the post-hatching growth that zebrafish undergo for many months, during which time they become approximately 1000-fold bigger, acquire new neurons, augment old tracts, add new ones, and sculpt the previously tubular brain into a shape resembling a shepherds crook with lumps3xRoss, L.S., Parrett, T., and Easter, S.S. Jr. J. Neurosci. 1992; 12: 467–482PubMedSee all References. The relation of the adult and embryonic brains is probably best approximated by the neuromeric model of Puelles and collaborators4xPuelles, L. and Rubenstein, J.L.R. Trends Neurosci. 1993; 16: 472–479Abstract | Full Text PDF | PubMed | Scopus (634)See all References, but the detailed relationship is only speculative in the absence of fate maps.This atlas provides the previously lacking end point that will be needed for such fate maps. I hope that everyone who works on the embryonic fish brain will use the atlas, and that all future discussion of structures will share it as a common point of reference.

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.

The role of the optic tectum in various visually mediated behaviors of goldfish

Five visually mediated behaviors were assessed following ablation of one or both lobes of the optic tectum in goldfish. Three of the behaviors disappeared following tectal ablations: optomotor response (swimming with the stripes in a rotating striped drum), food pellet localization and shadow-induced deceleration of respiration. Two of the behaviors persisted following tectal ablation: optokinetic nystagmus (movement of the eyes with the stripes in a rotating striped drum) and dorsal light reflex (tilting of the vertical axis toward the brighter of two laterally placed lights). The unexpected result that lesioned fish tracked the stripes with their eyes, but did not swim after them as normal fish did, suggests that the tectum serves a pre-motor function in addition to its sensory role. In addition, the results demonstrate that selected behaviors can be used to establish whether functional tectal or non-tectal connections are made by regenerating goldfish optic nerves.

Degenerative and regenerative changes in the trochlear nerve of goldfish.

SummaryThe 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.1.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.2.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.3.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.4.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.5.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 sprouts 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.6.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.7.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. The number of remyelinated axons remained constant at least until 107 days, the longest time considered, and none was observed to degenerate, whereas some axons that were not remyelinated appeared to degenerate.8.Although each basal lamina in the distal stump often surrounded several regenerating axons during the first 2 weeks post-lesion, each remyelinated axon became individually surrounded by a basal lamina, collagen fibres and extracellular space between 13 and 107 days, thereby increasing the number of basal laminae in the distal stump.9.Regenerated axonal terminals in the superior oblique were first observed 8 days after crush. The number of synapses increased progressively between 8 and 107 days, at which time they were as numerous as in unlesioned animals.

Growth of the adult goldfish eye—I: Optics

Abstract We have measured the optical and retinal fields of goldfish eyes; the animals ranged from 6 to 20 cm in body length. Both fields are spherically symmetric, invariant with the size of the eye, and equal (retinal field = 185.3° ± 4.1°, optical field = 183.6° ± 2.7°, means ± S.D.). They are tilted with respect to one another by a few (

The Morphogenesis of the Zebrafish Eye, Including a Fate Map of the Optic Vesicle

We have examined the morphogenesis of the zebrafish eye, from the flat optic vesicle at 16 hours post fertilization (hpf) to the functional hemispheric eye at 72 hpf. We have produced three‐dimensional reconstructions from semithin sections, measured volumes and areas, and produced a fate map by labeling clusters of cells at 14–15 hpf and finding them in the 24 hpf eye cup. Both volume and area increased sevenfold, with different schedules. Initially (16–33 hpf), area increased but volume remained constant; later (33–72 hpf) both increased. When the volume remained constant, the presumptive pigmented epithelium (PE) shrank and the presumptive neural retina (NR) enlarged. The fate map revealed that during 14–24 hpf cells changed layers, moving from the PE into the NR, probably through involution around the margin of the eye. The transformation of the flat epithelial layers of the vesicle into their cup‐shaped counterparts in the eye was also accompanied by cellular rearrangements; most cells in a cluster labeled in the vesicle remained neighbors in the eye cup, but occasionally they were separated widely. This description of normal zebrafish eye development provides explanations for some mutant phenotypes and for the effects of altered retinoic acid. Dev Dyn;218:175–188.

Postnatal neurogenesis and changing connections

Abstract It is generally accepted that the proper function of a nervous system depends on the ordered set of connections between nerve cells. If new neurons are added to a system which is already functioning, then some new connections must be formed and perhaps some old ones removed. In this review, I describe some cases of postnatal neurogenesis and their functional consequences, including the alteration of pre-existing synaptic connections. Most of the examples are taken from the visual systems of various animals.

Modulation of cell proliferation in the embryonic retina of zebrafish (Danio rerio)

We describe light‐microscopically the development of the embryonic zebrafish eye with particular attention to cell number, cell proliferation, and cell death. The period from 16 to 36 hr post fertilization (hpf) comprises two phases; during the first (16–27 hpf) the optic vesicle becomes the eye cup, and during the second (27–36 hpf) the eye cup begins to differentiate into the neural retina and pigmented epithelium. All cells in the eye primordium are proliferative prior to 28 hpf, and the length of the cell cycle has been estimated to be 10 hr at 24–28 hpf (Nawrocki, 1985 ). Our cell counts are consistent with that estimate at that age, but not at earlier ages. A 10‐hr cell cycle predicts that the cell number should increase by 7% per hr, but during 16–24 hpf the cell number increased by only 1.5% per hr. Despite the low rate of increase, all cells labeled with bromo‐deoxyuridine, so all were proliferative. We considered three possible explanations for the nearly‐constant cell number in the first phase: proliferation balanced by cell emigration from the eye, proliferation balanced by cell death, and low proliferation caused by a transient prolongation of the cell cycle. We excluded the first two, and found direct support for the third. Previous examinations of the cell cycle length in vertebrate central nervous system have concluded that it increases monotonically, in contrast to the modulation that we have shown. Modulation of the cell cycle length is well‐known in flies, but it is generally effected by a prolonged arrest at one phase, in contrast to the general deceleration that we have shown.

Pursuit eye movements in goldfish (Carassius auratus)

Abstract Pursuit eye movements made by goldfish were investigated with an optical technique in which the horizontal orientations of both eyes were measured automatically. Moving targets were provided by: (1) a striped drum which rotated about the vertical axis concentrically with the animals head, and (2) tangent screens on either side. Movement seen by either eye alone caused both to move, but the response was greater when both viewed the drum. The angular velocities of the eyes were always less than that of the drum. The ocular velocity depended upon the velocity, area, and contrast of the target, over wide ranges, and upon the state of adaptation and the recent history of the visual system. Evidence is offered supporting the hypothesis that the pursuit movements are controlled by directionally-selective movement-sensitive retinal ganglion cells.

The zebrafish eye: developmental and genetic analysis.

In this review, we have attempted to cover all the major points of zebrafish eye development, and have found that, for the most part, it has much in common with other eyes, in both vertebrates and the fly. In addition to the confirmation and extension of earlier studies, however, the work on zebrafish has provided some new insights that should be assessed for their applicability to the development of other vertebrates. Among these are the modulated cellular proliferation in the optic vesicle, the complex spatiotemporal pattern of central retinal neurogenesis, the emergence of spatial order among the photoreceptors, the genetic controls of cell fates, and the genetic mechanisms underlying retinal stratification. Substantial though it is, this contribution will grow rapidly in the next few years as the advances of zebrafish genetics are accelerated by progress of genomics, especially the zebrafish genome project.