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 fish's 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 shepherd's 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.