Eckart Zeiske

Taste bud development in the zebrafish, Danio rerio

Taste buds are chemosensory endorgans consisting of modified epithelial cells. Fish and other vertebrates use their taste bud cells to sample potential food, either selecting or rejecting substances according to their edibility. The adult gustatory system in fish has been studied thoroughly, including regeneration experiments. Taste buds occur in the epithelia of the lips, the mouth cavity, the oropharyngeal cavity, and also in the skin of the barbels, the head, and sometimes even all over the body surface. Despite its importance for feeding, little is known about the ontogeny of the fish taste system. We examined the development of taste buds in the zebrafish on the light microscopical and the scanning and transmission electron microscopical levels. Taste buds develop later than the olfactory organ and the solitary chemosensory cells, two other chemosensory systems in aquatic vertebrates. The first few taste bud primordia are visible within the epithelia of lips and gill arches 3 to 4 days after fertilization, and the first few taste buds with open receptor areas appear on the lips and simultaneously on the gill arches 4–5 days after fertilization, which coincides with the onset of feeding. Taste buds in the mouth cavity, on the head, and on the barbels are formed later in development. As seen in other fish, zebrafish taste buds contain elongate dark and light cells, termed according to their electron density. Dark cells with a cell apex of many short microvilli appear first, followed by the light cells with one large microvillus. In addition, the zebrafish has a third fusiform cell type, which appears last. This cell type is low in electron density and has a brush‐like apical ending with several small microvilli. This cell type has not been described previously. Furthermore, in zebrafish, the ontogenetic processes of taste bud formation differ from regenerative processes described in the literature.

Early development of the olfactory organ in sturgeons of the genus Acipenser: a comparative and electron microscopic study

Formation and morphology of the olfactory organ of vertebrates has been intensely studied in some taxa for more than a century. As a functionally important and complex sensory organ, its ontogenetic development has often been a matter of debate on higher-level craniate evolution. However, sufficient knowledge of structure and development of the olfactory organ in the crucial taxa needed for a serious phylogenetic reasoning is generally not available. This study aims at this essential primary data source, the detailed structure, morphogenesis, and character definition of the olfactory organ in more basal clades of jawed vertebrates (Gnathostomata). Sturgeon fishes (Acipenseriformes) as recent basal actinopterygians are expected to provide insight into archaic characters and character combinations in bony fishes. Thus, the development of the olfactory placodes of the sterlet, Acipenser ruthenus, and the Siberian sturgeon, Acipenser baerii, was followed histologically, by semi-thin serial sections, and by scanning and transmission electron microscopy. Except for the timing, virtually no differences were observed between the two species. The olfactory placodes become two-layered early in embryonic development. Both the superficial epidermal and the subepidermal layer can easily be distinguished and their development followed by ultrastructural properties. There are three different types of receptor cells: ciliated, microvillous, and crypt cells. The development of the ciliated and the less abundant microvillous receptor cells from the subepidermal layer of the placode is demonstrated. The non-sensory cells of the differentiated olfactory epithelium, i.e. ciliated non-sensory cells and supporting cells, exclusively derive from the superficial epidermal layer. In this respect, acipenserids clearly demonstrate close resemblance to the morphogenetic process found in the tetrapod Xenopus (Anura). The only other adequately described mode found in the actinopterygian zebrafish (Danio rerio), is considered a derived character. In this case, all cells of the differentiated olfactory epithelium derive from one placodal cell layer. The mode of formation of the nasal sac and its ventilatory openings found in the acipenserids examined here, represents a widespread and probably a plesiomorphic condition of osteognathostomes. In both species, differentiation of the basic cellular composition of the olfactory epithelium is far advanced at the time of onset of extrinsic feeding.

Ultrastructural studies on the epithelia of the olfactory organ of cyprinodonts (teleostei, cyprinodontoidea)

SummaryThe epithelia of the olfactory organ of two cyprinodontoid fish species were studied both by transmission and scanning electron microscopy. The relatively flat floor of the organ is covered by sensory and nonsensory epithelia. The latter is distributed in the form of bands or ridges separating distinct areas of sensory epithelium. Differences between the olfactory organs of the two species investigated related only to the topography and quantitative distribution of the epithelia. Their ultrastructural features are very similar. The nonsensory stratified squamous epithelium contains numerous goblet cells and surface cells provided with microridges. A hypothetical function of the microridges is discussed. The sensory epithelium consists mainly of basal, supporting, and two types of sensory cells, i.e., ciliated and microvillous receptor cells. The cilia exhibit a predominant 9+0 microtubule pattern. Both epithelia are covered by a mucus layer in which all surface structures seem to be embedded. The possible nature, origin, and movement mechanisms of the mucus are discussed.

Development of the olfactory organ in the rainbow fish Nematocentris maccullochi (Atheriniformes, Melanotaeniidae)

SummaryThe development of the olfactory organ in the rainbow fish, Nematocentris maccullochi, was studied using scanning and transmission electron microscopy; it was compared with the developmental process in other teleosts, especially in the closely related atherinids and cyprinodonts. The formation of the nares parallels that in atherinids, salmonids, cyprinids and heterosomats, but differs from that found in cyprinodonts. Another ontogenetic feature in which the olfactory organs of the rainbow fish and also of atherinids differ from those of cyprinodonts, is the occurrence of transitory kinociliary cells which disappear during the postlarval period. The divergent evolutionary pathways are discussed with reference to experimental investigations. During development, ciliated and microvillous receptor cell types occur. At the primary larval stage ciliated receptor neurons are exclusively present. At a later stage the microvillous type develops and becomes equal in frequency. Thus, the microvillous receptor represents a separate type of olfactory neuron and is not a progenitor of the ciliated receptor cell.

Morphologische und morphometrische untersuchungen am geruchsorgan oviparer zahnkarpfen (Pisces)

Summary1.The olfactory organs of 37 species of oviparous cyprinodonts belonging to the sub-families Rivulinae, Aphaniinae, Fundulinae, Cyprinodontinae, Procatopodinae and Orestiatinae are studied.2.The organs consist of a usually flat olfactory chamber and an accessory sac. The olfactory chamber is lined with the olfactory mucosa and has two openings: an anterior incurrent nostril and a valvular posterior excurrent nostril.3.Some of the species differ conspicuously in the olfactory chamber and its openings and in the topographic position of the accessory sac.4.Most of the sensory epithelium, which is part of the olfactory mucosa, is separated into distinct areas by more or less prominent ridges. These ridges are compared with secondary lamellae which are found on olfactory folds of some other fish species for structure, ontogeny and supposed function.5.Four types of arrangement of the olfactory mucosa can be distinguished: whereas there are no significant differences in five of the sub-families, in the rivulins three types are found. These are geographically separately distributed.6.The relationship of the different systematic units is discussed in relation to the structural variation in the different types of olfactory organs.Zusammenfassung1.Es wird das Geruchsorgan von 37 Arten oviparer Zahnkarpfen (Familie Cyprinodontidae) untersucht, die 21 Gattungen aus den Unterfamilien Rivulinae, Aphaniinae, Fundulinae, Cyprinodontinae, Procatopodinae und Orestiatinae angehören.2.Die Geruchsorgane bestehen aus einer mit Riechschleimhaut ausgekleideten, meist niedrigen Riechhöhle Bowie einem akzessorischen Ventilationssack. Die Riechhöhle besitzt zwei Öffnungen, eine Einström- und eine mit einem Klappenventil versehene Ausströmoffnung.3.In der Form der Riechöffnungen, in der Ausgestaltung der Riechhöhle und in der topographischen Lage des akzessorischen Ventilationssackes gibt es zwischen einigen Arten auffallende Unterschiede.4.Das Sinnesepithel, Bestandteil der Riechschleimhaut, ist zumeist von Wülsten durchsetzt, die mehr oder weniger stark räumlich hervortreten. Sie werden in ihrem Aufbau und ihrer Ontogenese Bowie in ihrer mutmaßlichen Funktion mit Sekundärfalten verglichen, die auf Riechfalten in den Geruchsorganen mancher anderer Fishhe vorkommen.5.In der Art der Anordnung der Riechschleimhaut werden vier Typen unterschieden. Während bei Vertretern aus fünf Unterfamilien die Art der Anordnung einheitlich ist, werden allein bei den rivulinen Zahnkarpfen drei Typen gefunden, die in ihrem Vorkommen geographisch getrennt sind.6.Die unterschiedlichen Merkmalsausprägungen werden im Zusammenhang mit Fragen der verwandtschaftlichen Beziehungen zwischen den einzelnen systematischen Gruppen diskutiert.

Morphologische untersuchungen am geruchsorgan von zahnkarpfen (Pisces, Cyprinodontoidea)

Summary1.The olfactory organs of 30 teleost species of four cyprinodontoid families (Cyprinodontidae, Jenynsiidae, Anablepidae, Poeciliidae) are studied and the type most commonly found is described.2.The olfactory epithelium was found to be confined to the planar lower surface of the olfactory chamber and to contain sensory and indifferent epithelium. The sensory epithelium is separated into distinct areas by ridges of indifferent cells.3.These ridges grow in towards the center of the sensory epithelium from the sides of the olfactory chamber in a different way from that in which olfactory lamellae are formed. These ridges are similar to the formations found in the olfactory chamber in some fish species, but are not thought to be rudimentary lamellae.4.The planar surface of the olfactory epithelium is considered to be an adaptation to special morphological factors (flat head and olfactory chamber).Zusammenfassung1.DasGeruchsorgan von 30 Teleosteerarten aus vier Familien der Zahnkarpfen (Cyprinodontidae, Jenynsiidae, Anablepidae und Poeciliidae) wird untersucht and der am häufigsten vertretene Bautyp beschrieben.2.Die auf einem flachen Riechhöhlenboden angeordnete Riechschleimhaut besteht aus Sinnesepithel, das durch hervortretende Leisten aus indifferentem Epithel in zahlreiehe Riechepithelfelder aufgeteilt ist.3.Diese Leisten entstehen nicht wie Riechfalten, sondern wachsen vom Rand der Riechschleimhaut nach innen und zerteilen das anfangs zusammenhängende Sinnesepithel. Sie ähneln Bildungen im Geruchsraum anderer Fishhe, sind aber nicht als Riechfaltenrudimente aufzufassen.4.Die flache Anordnung dieser Riechschleimhaut wird als eine speziellen morphologischen Verhältnissen (flache Schädelform und flashes Riechhöhlenlumen) angepaßte Auskleidung der Riechhöhle aufgefaßt.

Cell Proliferation and Differentiation in the Olfactory Organ of the Embryonic and Larval Zebrafish, Brachydanio rerio

The olfactory organ of the zebrafish, Brachydanio rerio, a standard model in fish embryology, has been studied by light and electron microscopy [1], but little is known about the proliferation of placodal cells during the ontogeny of this organ. The aim of the present study was to show the proliferation sites in relation to the developmental stages of the olfactory organ of zebrafish. Eggs and larvae were obtained from our breeding colony, where adult fish are kept at a 14-h light/10-h dark cycle at a temperature of 26.5°C. The age of the animals studied ranged from 24 h after fertilization (AF) to hatching (3–4 days AF). We used standard methods of light and scanning electron microscopy to visualize the development of the olfactory organ, and we applied immunocytochemical methods to show bromodeoxyuridine (BrdU) incorporation and proliferating cell nuclear antigen (PCNA) immunoreactivity of proliferating cells. In addition to paraffin embedding, some animals were embedded in Quetol 651, an epoxy resin of low viscosity, since only thin sections give a precise idea of the placode in very young embryos. Scanning electron microscopic pictures of the head of the embryonic zebrafish at 24h AF show that the epidermis covering the olfactory placode is still completely closed. Within the placode, the cells in the rostromedial region differentiate first. About 30–32h AF, the epidermal cells separate to form the olfactory pit. Pools of round, undifferentiated cells in the rostral and caudal ends of the placode remain covered by the epidermis, even after hatching. Twenty-four h AF, tests with antibodies against BrdU or PCNA show stained nuclei all over the placode. There is no distinct proliferation center. Thirty-four to 36h AF, the distribution of immunoreactive nuclei is different for the antibodies against PCNA and BrdU. BrdU-ir is seen in the basal region and occasionally in the upper part of the placode. In Quetol sections, however, PCNA-ir is seen in almost all nuclei, ranging from weak to strong staining. After hatching (3–4 days AF), the labeled nuclei show the same distribution for both antibodies. In the rostral and caudal end of the placode, nearly all nuclei are labeled by both antibodies in all stages examined. These findings confirm that, apart from the basal cells, there are regions of cells that retain their capacity to proliferate during the embryonic and larval development of the olfactory organ. The difference in the staining pattern of BrdU and PCNA may be caused by the long half-life of PCNA, so that PCNA could be immunologically detectable in cells that have already left the cell cycle. Moreover, Quetol sections are much clearer than paraffin sections, thus making small amounts of PCNA visible, which amounts escape detection in paraffin sections.

Contents Vol. 73, 2009

304 29th Annual Meeting of the J.B. Johnston Club and 21st Annual Karger Workshop Chicago, Ill., October 15–16, 2009 311 Author Index Vol. 73, 2009 312 Subject Index Vol. 73, 2009